How to read a pigment label when the color name doesn’t tell you what’s inside

Pick up two tubes of “Yellow Ochre” from two different manufacturers. Same color name, same general appearance in the tube. Turn them over and look at the small print — the Colour Index code, usually printed near the ingredient list or pigment information. One says PY 43. The other says PY 42.

That single-digit difference is the whole story. PY 43 is natural yellow iron oxide — goethite, a mineral with a specific geological origin. PY 42 is its synthetic counterpart — a precipitated iron oxide, chemically similar but structurally different, manufactured rather than mined. The color name on the front of the tube does not distinguish between them. The Colour Index code on the back does.

Color names describe a category. Colour Index codes describe a composition.

“Ochre,” “sienna,” “umber,” “terre verte” — these are historical color-family names. They describe a recognizable range of hue and character that painters have used for centuries. What they do not do, and were never designed to do, is guarantee what mineral is actually in the tube. A color name is a category claim. A Colour Index code is a composition claim. Conflating the two is the single most common misunderstanding painters have about earth pigments.

This distinction matters more for earth pigments than for almost any other pigment family, because the earth family has both a long natural-mineral tradition and a long synthetic-equivalent tradition running in parallel — and modern manufacturing has made the synthetic versions cheaper, more consistent, and easier to source at scale. Most paint companies do not hide which version they use. They print it. Most painters have just never been taught to look.

The Colour Index, briefly

The Colour Index International is a joint reference maintained by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists, used across every industry that manufactures pigments — far beyond fine art. Every pigment, natural or synthetic, organic or inorganic, has a Colour Index generic name: a letter prefix indicating color (PY for Pigment Yellow, PR for Pigment Red, PBr for Pigment Brown, PG for Pigment Green) followed by a number.

The number identifies a specific chemical class — not a specific manufacturer, not a specific hue, and not necessarily a single material. Two tubes can carry the identical Colour Index code and still look different from each other, because the code identifies the mineral family, while particle size, source deposit, and processing all affect the final color. The code is the floor of what you know about a pigment, not the whole picture — but it is reliable, standardized, and printed on virtually every professional-grade paint tube sold today.

The earth family, by code

A few codes in this table — PBr 7 in particular — do not by themselves resolve the natural-versus-synthetic question, because the same Colour Index designation legitimately covers both versions. Where the code is ambiguous, a manufacturer that discloses composition openly, with a published pigment list or safety data sheet, is the more reliable source than the code alone.

What “Hue” means on a label

When a paint name includes “Hue” — “Burnt Sienna Hue,” for example — this is the industry’s standard way of disclosing a substitution. It means the color matches the named pigment, but the pigment inside is something else: usually a less expensive or more consistent material formulated to approximate the original. This is not deceptive. It is, in fact, the opposite — an explicit label telling you that what’s inside is a stand-in. The practice exists across all pigment families, not only in earths. The Colour Index line on the same label tells you exactly what the stand-in is.

Green earth: the clearest case

Green earth is where the gap between the color name and the actual composition is widest in the earth family, and it’s worth a closer look because the underlying research is unusually well documented.

The natural pigment is composed of celadonite or glauconite — clay minerals with iron in layered silicate structures, found in altered volcanic rock (celadonite) or marine sedimentary deposits (glauconite). Both carry the Colour Index code PG 23. The historical material is low in tinting strength and muted in chroma by nature — qualities that made it useful for flesh-tone underpainting in egg tempera and early oil technique, where a small amount of green modulates the warmth of skin tones beneath.

Two peer-reviewed analytical studies have examined what’s actually inside commercial tubes labeled “Green Earth” or “Terre Verte.” Ospitali and colleagues (2008, Journal of Raman Spectroscopy) and Fanost and colleagues (2020, Colloids and Surfaces A) together analyzed more than a dozen commercial products using Raman spectroscopy, FTIR, XRD, SEM-EDS, and Mössbauer spectroscopy — complementary techniques that identify mineral composition directly. The findings were consistent across both studies: some commercial products contain genuine celadonite or glauconite. Others contain phthalocyanine green (PG 7) — a synthetic organic pigment with no mineralogical relationship to celadonite or glauconite at all — sometimes blended with mineral fillers to approximate the historical color and texture.

This is not a story about any single company doing something wrong. The pigment industry’s largest customers are construction, plastics, cosmetics, and industrial coatings — fine artists are a small fraction of total demand, and substitutions, blends, and fillers are standard practice across pigment manufacturing generally, for reasons of cost and supply consistency that have nothing to do with deceiving painters. The point for the working painter is simply this: the color name “Green Earth” is not a guarantee of composition. The Colour Index code is. PG 23 means the genuine mineral. PG 7 means a synthetic substitute. Both are legitimate pigments — they are not the same pigment.

What to do with this

Reading the Colour Index is a five-second habit, not a research project. Turn the tube over, find the code, and know what the prefix-and-number combination means for the pigment family you’re buying. For earth pigments specifically:

• PY 43 vs. PY 42 — natural versus synthetic yellow iron oxide
• PR 102 vs. PR 101 — natural versus synthetic red iron oxide
• PBr 7 — could be either; check for a disclosed source if it matters to your work
• PG 23 vs. PG 7 — genuine green earth versus phthalocyanine substitute

Neither version of any of these pigments is the “correct” one in any absolute sense. Synthetic transparent iron oxides are often more transparent, more consistent from batch to batch, and less expensive — real advantages for certain applications, particularly glazing. Natural earth pigments carry mineralogical complexity — particle size variation, accessory minerals, surface character — that many painters find produces a different, more textured result in body color and underpainting. Which one belongs in a given passage is a working decision, not a purity test. The Colour Index is what lets you make that decision deliberately instead of by accident.

Rublev Colours pigments and oil paints disclose Colour Index designations on every product page and label. Natural earth pigments sourced from documented deposits are available at naturalpigments.com.

A longer companion review with citations to the peer-reviewed literature, brand-by-brand Colour Index verification across roughly ninety oil paints from seventeen manufacturers, and a full bibliography is available on the sister site: Natural Earth Pigments in Oil Paint: Why the Color Name Doesn't Tell You What's Inside.

What conservation science found in commercial green earth pigments — and what it means for ours

When Tatiana and I first traveled to visit our Italian earth pigment supplier in 2007, we were starting a business relationship. We visited their factory in Verona, observed their operations, and came away satisfied that we were working with a serious producer. We were not looking for adulteration. It did not occur to us to look. Like most painters, like most paint manufacturers, and — as it later turned out — like museum conservators who had been using the same commercial pigments as reference materials for decades, we assumed that a pigment sold as green earth was green earth.

Our product descriptions said as much: natural mineral pigment. Independent analysis has shown that description does not tell the whole story. This article explains what that analysis found, why it happens, and what we currently believe is in these pigments.

George and Tatiana O’Hanlon in Italy, 2007 — the year Natural Pigments established its relationship with European earth pigment suppliers.

What a Conservation Scientist Found

In 2019, Gregory Dale Smith of the Indianapolis Museum of Art and Heidi Kastenholz of Butler University published a paper in the Journal of Cultural Heritage that began as a response to an unrelated study. Researchers at the University of Malta had purchased Rublev Verona Green Earth from Natural Pigments, used it in a series of crystal engineering experiments intended to improve the pigment’s opacity for conservation inpainting, and characterized it using X-ray fluorescence, infrared spectroscopy, and X-ray diffraction. Their analyses indicated a mixed mineral pigment, and they proceeded accordingly.

Before publishing, Smith contacted us directly to let us know what he and Kastenholz were finding, and asked whether we could supply samples of our other green earth pigments for further analysis. We did, including material from our Armenian sources. We cooperated because we understood this kind of color adjustment to be common practice across the pigment trade, not a problem specific to our supplier or our products.

Smith and Kastenholz looked at the same pigment using Raman microspectroscopy — a technique that can identify specific molecular structures at the particle level. What they found was not what the label said.

Rublev Colours Verona Green Earth under reflected light microscopy at 20× magnification. The dominant warm yellow-ochre matrix consists of mineral earth components. The scattered vivid teal-blue particles are copper phthalocyanine green (PG7), a synthetic organic pigment identified by Raman microspectroscopy. G.D. Smith, Indianapolis Museum of Art.

Raman spectra of Rublev Colours Verona Green Earth archived and purchased samples, dating from 2007, 2009, and 2019, compared against a phthalocyanine green (PG7) reference spectrum. Trace colors correspond to the sample year; peak positions in both Verona lot samples and the PG7 reference confirm the presence of phthalocyanine green pigment. G.D. Smith, Indianapolis Museum of Art.

The bright green crystallites dispersed throughout the pigment were neither celadonite nor glauconite. They were copper phthalocyanine green — PG7, a synthetic organic pigment introduced commercially in the 1930s. Under magnification, PG7 appears as scattered grains within a much larger field of mineral material — a minor presence by volume, not a dominant one. But synthetic organic pigments like PG7 carry far more tinting strength than mineral earths, so a small amount goes a long way toward shaping the color you see. The relationship is similar to a Cabernet Sauvignon containing a smaller portion of Merlot: the dominant material gives the body and structure, while the smaller addition measurably changes the final character. Here, the mineral component gives the pigment its body, particle structure, and much of its handling behavior in oil; the phthalo green addition shifts the color toward a more saturated, consistent green than the mineral alone would produce. Based on this analysis, we believe PG7 is present as a minor but real colorant in Verona Green Earth, as we currently source it.

They also tested other green earths from Natural Pigments’ line, from Kremer Pigments, and from Williamsburg. The full results are in their published paper, alongside the analytical data we are publishing with this article. The findings were broad. Kremer’s Verona Green Earth, labeled “genuine, pure,” contained PG7 in the sample tested. Williamsburg’s Italian Terra Verte, in the sample tested, also contained PG7. Williamsburg’s French Terre Verte, in the sample tested, contained cobalt compounds — PB36 and PG19 — which surprised the Williamsburg formulators, who believed they were using authentic natural green earth and had been told as much by their own European supplier. We mention this not to single anyone out, but because it matters for understanding the scope of the issue: this is not a problem unique to one supplier. In Natural Pigments’ own line, Antica Green Earth from Prun, Italy, was found to contain PG7 as well, mixed with yellow ochre. We believe this combination is present in our Antica Green Earth as we currently source it, and that it accounts for this pigment’s more olive tone relative to Verona. Nicosia Green Earth, from Cyprus, was found to contain cobalt compounds — PB36 and PG7 — and we believe these are present in the Nicosia Green Earth we currently source.

Two products in our line returned clean results. No synthetic colorants were detected in the Tavush Green Earth or Tavush Transparent Green Earth samples analyzed, both of which were sourced from Armenia. Based on those results, we believe these remain genuine mineral green earth pigments.

Why It was Missed For So Long

The analytical methods most commonly used to characterize pigments — XRF, FTIR, and XRD — are not well suited to detecting small quantities of synthetic organic pigments dispersed in a mineral matrix. PG7 produces only a trace copper signal by XRF, and its infrared absorption is masked by the stronger bands from the surrounding minerals. It took Raman microspectroscopy — specifically, the ability to focus on individual bright green particles and analyze them selectively — to identify what was actually there.

The same gap explains a related problem Smith documents: the widely used Raman reference library compiled by Bell and colleagues in 1997 includes a terre verte entry whose spectrum is, in fact, a mixture of anatase, phthalocyanine green, gypsum, and calcite — not genuine green earth. The library was built partly from commercial pigment samples; if those samples were already affected by the same problem, the reference spectra were compromised at the source. Conservation researchers have been working with that reference for decades without knowing it.

Why It Happens At All

This is not a new problem, and it did not begin with synthetic organic chemistry. Pigment making has never been primarily about preserving a specific mineral’s identity; it has been about producing reliable color. Historically, colormen blended, extended, and substituted materials constantly — toning one earth with another, adding lake pigments to deepen a hue, mixing sources to standardize a batch — because the customer wanted a consistent, useful color, not a certified mineral specimen. The term "green earth" or "terre verte" has always described a color and a general category of material rather than a single, fixed mineral composition. What has changed is not the underlying practice but the tools available to detect it, and the expectation — mostly recent, mostly driven by conservation science and historical reconstruction painters — that the name on the label should correspond to a specific, verifiable mineral.

The historical green earth source at Verona was long considered among the finest terre verte in the world, mined commercially through the first half of the twentieth century. Access declined over subsequent decades; an earthquake in 1922 disrupted the most productive deposits. What replaced large-scale mining were smaller, scattered excavations — geologically inferior material from fragmented sources, difficult even for our supplier to trace back to a single origin.

The largest-volume markets for iron oxide earth pigments — architectural coatings, mulch coloring, and similar industrial uses — are indifferent to the distinction between natural and synthetic material. Those markets require consistency, which mine-sourced material cannot reliably guarantee on its own. A synthetic correction that brings variable batches to a uniform color strength serves as a quality improvement from a buyer’s perspective and is not disclosed further down the supply chain to artists’ colormen, because for an industrial customer there is nothing to disclose. The material simply meets the specification.

This is, as far as we can tell, standard practice across the trade, not a failure specific to any one supplier. Williamsburg’s Sarah Sands described a related dynamic in a Just Paint article on undisclosed substitution in ochres, siennas, and umbers — including a later, candid correction of her own where she acknowledged that even major suppliers use small synthetic additions to adjust natural earth pigments for color consistency, a practice she noted has been common in the trade for a long time. That context explains how these adjustments enter the supply chain. It does not mean painters should be left without disclosure when a pigment is sold under a natural-earth name. We are not aware of any supplier in this industry, including conservation-focused ones, that tests and discloses synthetic content in green earth pigments as a matter of course. We have chosen to do so where we know or believe it to be the case.

George O’Hanlon at a pigment excavation site in Saint-Amand-en-Puisaye, Burgundy, France, May 2023, during a trip to visit earth pigment suppliers across Europe. Sites like this one illustrate the small-scale, fragmented sourcing that characterizes much of the natural earth pigment trade today.

What We Currently Believe is in Our Green Earth Pigments

Verona Green Earth (Mt. Baldo, Italy): We believe this pigment is based on genuine mineral celadonite and glauconite, with copper phthalocyanine green (PG7) present as a minor addition that shapes the final color, as indicated by Raman microspectroscopy.

Antica Green Earth (Prun, Italy): We believe this pigment contains PG7 mixed with yellow ochre, alongside genuine mineral glauconite, based on the same analysis. This mixture is the source of Antica’s more olive tone relative to Verona.

Nicosia Green Earth (Cyprus): We believe this pigment contains cobalt blue-greenish (PB36) and PG7, alongside genuine mineral glauconite, based on the same analysis.

Tavush Green Earth and Tavush Transparent Green Earth (Armenia): No synthetic colorants were detected. We believe these remain genuine, unadulterated mineral pigments.

We are reporting what we currently believe to be true, based on the best analysis available to us. We do not test every lot we sell, and we do not claim to. Few, if any, suppliers in this trade do. What we can offer is this: when we know something, or are reasonably confident of it, we will say so.

What This Means for Painters Using These Pigments

The synthetic pigments identified are generally regarded as durable artists’ materials. Phthalocyanine green (PG7) carries an ASTM Lightfastness rating of I, the same top rating given to genuine celadonite and glauconite, and it is chemically stable and non-reactive in oil. The cobalt compounds found in Nicosia Green Earth carry the same lightfastness rating. Their presence does not indicate a known inherent fading or paint-film instability problem. The issue here is one of composition and accuracy of description, not a known conservation hazard.

What it does change is the accuracy of any claim about mineral authenticity. A painter using these pigments for historical reconstruction, conservation retouching, or because they specifically wanted the optical properties of genuine celadonite or glauconite should know that this pigment includes a synthetic color adjustment, since that may matter for that kind of work.

PG7 has considerably higher tinting strength than celadonite or glauconite, so even a minor addition can noticeably shift the color toward a more saturated, consistent green than the mineral alone would produce. Because PG7 is present only as scattered grains within the mineral matrix, we would not expect it to meaningfully change this pigment’s oil absorption, rheology, or behavior in mixtures — those properties are governed by the mineral component, which makes up the bulk of the material. The difference to expect is in color, not handling.

If the specific colorant content of a green earth pigment is critical to your work — for conservation retouching, historical reconstruction, or any application where you need to know precisely what you are using — contact us. We will do what we can to help you find the right material for that purpose, whether that means Tavush Green Earth, Tavush Transparent Green Earth, or further information about a specific product.

The Broader Picture

Smith’s paper examined fifteen commercial green earth products from four suppliers. Eight of those fifteen contained synthetic colorants not disclosed on their labels. This tells us that the issue is not confined to a single supplier or country of origin. It is, as far as the evidence shows, how this part of the trade generally operates: pigment names like “green earth” function as a color and category description more than a guarantee of mineral content, and most suppliers do not test for or disclose the difference.

For painters, the practical implication is straightforward: a label that says “natural green earth” or names a geographic source is not, by itself, proof of what is in the tube or jar. We are telling you what we currently believe is in our pigments, based on the best analysis available to us. We believe this level of disclosure remains uncommon in the artist pigment trade, including among suppliers serving conservation-minded customers. We think that’s worth knowing, even if it isn’t the norm.

The two products in our line confirmed clean in testing, Tavush Green Earth and Tavush Transparent Green Earth, remain in the line. Their lower tinting strength compared to the other green earths in our catalog is not a shortcoming; it is one of the visual signatures of genuine, mineral celadonite-type material, and a reminder of how historical green earth actually behaves in use.

Questions We Expect Painters to Ask

Is this pigment unsafe to use? The synthetic pigments identified do not indicate a known paint-film stability problem or an unusual hazard beyond normal dry pigment precautions. As with all dry pigments, avoid inhaling dust. For Nicosia Green Earth, the cobalt-containing pigments identified make dust control especially important.

Will paintings I have already made with these pigments fade or change? PG7 and the cobalt compounds identified all carry top ASTM lightfastness ratings. We have no evidence suggesting a paint-film stability problem from these synthetic additions.

Why didn't Natural Pigments know sooner? The standard analytical methods used to characterize commercial pigments are not well-suited to detecting small quantities of synthetic organic colorants in a mineral matrix. That gap is common across the trade, not specific to us.

Is this just a Natural Pigments problem? No. The available evidence, including testing of products from other suppliers, indicates this is common practice across the green earth trade. We are disclosing what we currently believe about our own products because we think painters are better served knowing it, not because we believe we are unusual in this respect.

Is Tavush Green Earth genuine? No synthetic colorants were detected in the samples analyzed, and we believe it to be a genuine mineral pigment.

What if the colorant content matters for my work? Contact us. If you need to know precisely what’s in the pigment you’re using, we’ll help you find the right material, which may mean Tavush Green Earth or Tavush Transparent Green Earth.

References

Smith, G.D. and Kastenholz, H. (2019). "A note on the use of crystal engineering to increase the opacity of terra verde inpainting pigment: Adulteration of green earths." Journal of Cultural Heritage. Available online October 17, 2019. DOI: 10.1016/j.culher.2019.10.004

Micallef, D., Spiteri, R., Baisch, U., and Vella-Zarb, L. (2019). “Terra verde: overcoming the problem of transparency by crystal engineering.” Journal of Cultural Heritage 38, 1–7.

Fanost, A., Gimat, A., de Viguerie, L., et al. (2020). “Revisiting the identification of commercial and historical green earth pigments.” Colloids and Surfaces A 584, 124035.

Uebele, C.L. (1913). Paint Making and Color Grinding: A Practical Treatise for Paint Manufacturers and Factory Managers. Chicago.

Auf der Mauer, M. (2024). “Availability Issues with Natural Earth Pigments.” Just Paint, Golden Artist Colors. justpaint.org.

Sands, S. (2017). “Sorry, Wrong Umber — Part I.” Just Paint, Golden Artist Colors. justpaint.org.

Grissom, C.A. (2012). “Green Earth.” In R.L. Feller (ed.), Artists’ Pigments: A Handbook of Their History and Characteristics, Vol. 1. Archetype Publications, London, pp. 141–168.

Cáceres-Rivero, C., Tupa-Quispe, A.L., Torres-Casas, R., and Bedregal, P. (2022). “Identification of adulterants in artistic earth pigments using a multi-technique approach.” Results in Chemistry 4, 100561. DOI: 10.1016/j.rechem.2022.10056

George O’Hanlon is the technical director of Natural Pigments. The full Raman analysis data for each affected product, including spectral comparisons, is published on the respective pigment product pages linked above.

I met Georg Kremer several times over the years — first in his New York store decades ago, when I was beginning to take seriously the questions that would eventually become my own life’s work. We were both chemists, though by different routes, and we had both been drawn to the same problem from different ends of it: that some of the materials that built the great paintings of the European tradition had simply become unavailable to anyone who wanted to use them today.

Georg got there first, and he got there by a more difficult path. In the early 1970s, when he was a student at Tübingen, a conservator friend asked him whether he could reproduce a particular blue — a color that 16th- and 17th-century painters had used for skies and mountains, that Vermeer had layered into some of his early paintings, and that had become commercially unavailable around 1910. Georg dug through archives, found the recipe, and made it. The pigment was smalt, a glassy cobalt blue. It became the first Kremer pigment, and it set the pattern for the next fifty years of his work.

My own path to the same field was later and smaller. I came to chemistry through industry rather than university — training in the 1980s while employed by chemical companies, no degree, but enough working knowledge to read a structure and run a reaction. I came to historical pigments through painting. And I came to the materials that would eventually define Natural Pigments through my wife Tatiana Zaytseva, who is the company’s co-founder, and without whom none of this would exist. I met Tatiana on my first trip to Russia in 2000. We were married in 2001. Through her, I met Alexander Grigoriev, a Russian geologist whose knowledge of mineral deposits across the former Soviet Union became the source for the pigments I would carry back, in suitcases, on the early trips: cinnabar and orpiment from Ukraine, iron oxides and volkonskoite and malachite from the Urals, glauconite and celadonite from the Baltic states, lapis from Lake Baikal, lapis from Badakhshan via the routes that ran through Russia. The first such trip was in 2001. We did this several times before tightening airport restrictions made it impossible to bring sealed bags of mineral powder onto international flights.

In 2003, I tried these pigments in oil paint. I had been an oil painter before, and I knew what oil paint was supposed to feel like. These pigments did not feel like that. They had different rheology, different drying behavior, different relationships with the oil. I added the standard stabilizers a paint maker is supposed to add, and the unique behavior disappeared. I went looking for published research on the rheology of old masters’ paints and found only two articles. The conclusion was unavoidable: the modern stabilizers added to commercial oil paints had eliminated the unique rheological behavior of natural mineral pigments in oil — and the field had stopped studying it. The decision that followed — to make oil paint without those stabilizers, and to find out what these pigments actually did in oil — became the founding question of Natural Pigments.

What Georg built from his beginning, and what Tatiana and I built from ours, are not the same thing. He has been doing this work for over fifty years; we have been doing it for twenty-five. His company in Aichstetten supplies painters and conservators around the world; we are a small operation in northern California, with a more limited range. But the instinct is the same instinct, and the obligation is the same obligation: to bring back materials that had become unavailable, and to make them available to people who want to use them seriously.

What I want to write about here is what was lost when the New York store closed at the end of November 2025, and where to look now.

What Kremer NYC offered

The New York store carried materials across roughly ten broad categories: historical and modern pigments, dyes and plant colors, fillers and ground materials, paints and binders and adhesives, solvents and chemicals, ready-to-use paints, gold leaf and gilding products, painting surfaces and papers, brushes, and the assorted tools that serious painting and pigment work require. The range was extensive but not exhaustive — even Kremer didn’t carry every specialty material a serious painter or conservator might need. The table below maps each of those categories to the U.S.-based suppliers I think a displaced Kremer customer should know about, including Natural Pigments, where we genuinely fit, as well as other suppliers where they’re stronger.

I have limited the recommendations to U.S.-based suppliers. Kremer Pigmente in Germany continues to operate, and serious customers can still order internationally, but the tariffs and shipping costs that contributed to the New York store’s closure apply to direct orders from Germany. The U.S. focus is the practical one for U.S. customers in 2026.

Where to look now

The table is current as of the article’s publication. Suppliers’ ranges and reputations change; the recommendations above reflect what we know in May 2026.

No single supplier replaces what Kremer NYC was. The displaced customer’s path forward is to learn the landscape, build relationships with two or three suppliers whose ranges together cover your needs, and be patient — the supply chain for serious materials has always required some patience, and the closure of the New York storefront makes that more true, not less.

A different kind of resource

Among the things that were lost when Kremer NYC closed was the possibility of a certain kind of conversation. Georg himself, when he was in the store, was that conversation — a chemist who could explain the structural difference between a natural azurite and a synthetic one, or why a particular oil-grinding technique mattered for a particular pigment. But Georg has long lived and worked in Germany, and the depth of explanation he could offer in person was rare on the New York shop floor. The day-to-day staff was helpful, often very helpful, but the technical conversation that the field’s most serious customers wanted was harder to find at retail.

This is one of the gaps in the displaced customer’s situation that I want to address, because it is one that Natural Pigments aims to fill.

The work that put Tatiana and me into a position to do this began in 2006. That year I joined the ASTM D01.57 subcommittee — Artist Paints and Related Materials —the group that maintains the artist materials standards, including ASTM D 4302 — where I met Joy Luke, Mark Gottsegen, and Ross Merrill, the chief of conservation at the National Gallery of Art. Ross had taken an interest in the stabilizer-free oil paints I had been making in my garage in 2005, and he invited me to give a lecture at the National Gallery. Tatiana and I expected an audience of perhaps forty serious people. The Gallery put us in the East Building Small Auditorium, which seats about 160, and the room was full, with people standing in the back and in the aisles — close to two hundred attendees in all. Most were artists. I came home from that trip with a different sense of the scale of interest in what we were doing, and a list of conservators who would become long-term collaborators — including Marion Mecklenburg at the Smithsonian, whose 28-year study on zinc oxide in oil paint became the foundation of the technical work I have been writing about ever since.

In 2007, we traveled to England. We received a personal tour from Ian Garrett at Winsor & Newton. We consulted the archives of several nineteenth-century artist material manufacturers whose companies are now defunct. From those archives, we recovered methods that we have since incorporated into our production at Natural Pigments. The same trip took us to Tate Britain, where we met Leslie Carlyle, then head of conservation, and Joyce Townsend. Leslie had been deeply involved in the HART project — the research program that systematically reconstructed nineteenth-century painting materials — and was the author of The Artist’s Assistant, the standard reference on British artist material practice in that period. We have remained close colleagues since then. As I write this, I am working with Leslie to make the HART research publicly available on a website for the first time.

What Tatiana and I have built at Natural Pigments grows from that lineage. By the time the New York store closed, Natural Pigments had become the second-largest historical pigment supplier in the United States, after Kremer itself. We make our oil paints in Willits, California, and ship from there, with fulfillment partners in Canada and Germany. We have been teaching technical material to painters since 2013, when we began the Painting Best Practices seminar program; over a thousand painters have now taken those courses, and the Painting Best Practices community on Facebook has grown to 30,000 members. In 2024, we moved the educational program to a membership site with live monthly events. We are currently expanding production of historically reconstructed pigments — including Stack Process Flake White, which we have been making since 2007 — and we are restoring British watercolor formulations from the first half of the nineteenth century, drawing on Ackermann and Winsor & Newton sources, for our Rublev Colours watercolor line. We are developing new wax-resin materials in cooperation with the conservation community.

This is what we can be for a displaced Kremer customer. Not a replacement for what was lost. A different kind of resource for the parts of the work where we genuinely fit.

A closing acknowledgment

There is an account, in a 2019 profile of Georg Kremer in Ursula magazine, of the painter Jack Whitten’s studio in Queens after his death. Whitten, who died in January 2018, had spent his later years obsessively experimenting with Kremer pigments — the iridescent and pearlescent ones that gave his Quantum Wall paintings their strange, shifting surfaces. After he died, his daughter placed some of his ashes in a plain brown box and put it on the shelf, among the jars of pigment. “I put Dad with the pigments,” she said. “I think he’d be happy there.”

I think about that detail. It captures something true about what a place like Kremer NYC was for the people who depended on it. Not just a store. A part of the work itself.

The store is gone, and there is no replacement for what it was. Kremer Pigmente in Germany continues — Georg’s son David is now co-managing director, and the work that began with smalt blue in the early 1970s continues to evolve. There are other suppliers, including Natural Pigments, who carry parts of what Kremer offered. And there is the small specialist community of painters, conservators, and craftspeople who depend on these materials and who, by continuing to buy seriously from the makers who continue to do this work seriously, keep the field from collapsing into mass-market alternatives.

That last point is the one I want to leave you with. Where you spend your money in the next year matters. Spend it where the work is being done well, and the work continues. Spend it carelessly, and a field that has already lost a great deal loses more.

— George O’Hanlon, Technical Director, Natural Pigments

And why the label can’t tell you

In 2007, two scientists at the Smithsonian’s Museum Conservation Institute published the results of a 28-year study on how oil paint ages. The headline finding has been quoted in conservation journals for nearly two decades, but it has barely reached painters. It deserves to.

A 14-year-old film of zinc white oil paint, when stretched, will elongate about three-tenths of one percent before it snaps. A 14-year-old film of lead white oil paint will stretch nearly fifteen times further — about 4.5 percent — before failing. The two paints look the same on the palette. They feel the same in the brush. They cover the same. But after fourteen years on a stretched canvas, one of them has become something close to glass: rigid, brittle, prone to crack along the lines of canvas weave or to delaminate from whatever sits beneath it. The other still flexes.

This is not an obscure finding. It has been replicated in the laboratory of at least one major paint manufacturer. It has been documented in the deterioration of named twentieth-century paintings — works by de Kooning, Pollock, Fontana, O’Keeffe — held in major museum collections. The mechanism is understood; we’ll come to it. But the question worth holding in mind first is a different one.

If you bought a tube of “Titanium White” last week, do you know whether it contains zinc oxide?

The answer is almost certainly no, and the reason is not that the manufacturer is hiding something. The reason is that the standard governing what goes on the label was never designed to tell you.

What ASTM D 4302 Requires

Most major artists’ oil paints sold in the United States and Europe comply with a standard called ASTM D 4302 — the Standard Specification for Artists’ Oil, Resin-Oil, and Alkyd Paints. Compliance is voluntary, but it’s nearly universal among reputable manufacturers, and the symbol or wording appears on most professional-grade paint labels.

The standard is a serious document. It governs which pigments may be used (only those meeting strict lightfastness requirements), how those pigments must be identified on the label (by Common Name and Colour Index designation — “Titanium White, PW6”), what the lightfastness rating must say, how the oil must be described, and how the paint must perform on tests for fineness of dispersion, drying time, and consistency. It is the reason a tube labeled “Cadmium Red” sold by a reputable manufacturer is, in fact, made with a cadmium pigment that meets a defined performance standard.

What the standard does not govern is everything else.

Read carefully, and you’ll find a few clauses that quietly acknowledge the gap. Drier compounds — organometallic salts of cobalt, manganese, zirconium, lead — “may be used in minimal amounts” (clause 6.4). “Minimal amounts of inert pigments may be used to produce desirable working qualities” (clause 6.5). The standard permits these things. It does not require them to be disclosed.

And the additives most commonly found in modern oil paint — aluminum stearate, zinc stearate, beeswax, hydrogenated castor oil — aren’t mentioned at all. The standard is silent on them. A manufacturer can fully comply with ASTM D 4302 even if every tube in the line contains aluminum stearate at one or two percent and beeswax at another one or two percent, because those additives fall outside the standard’s scope. The label says “PW6, alkali-refined linseed oil.” That’s true. It’s also incomplete.

This is not a loophole. The standard does what it was designed to do — guarantee pigment identity and lightfastness — and it does it well. The problem is that painters have come to read the label as a complete ingredient list, when it has never been one. A label that complies with ASTM D 4302 tells you about pigments. It does not tell you about everything in the tube.

What Else Is in the Tube

To understand why this matters, it helps to know what the additives do, why they’re there, and what happens to them over time.

Stearates — most often aluminum stearate, sometimes zinc — have been added to commercial artists’ oil paint since approximately 1920. They are rheology modifiers: they keep the pigment from separating from the oil during storage, give the paint a buttery, non-stringy consistency, and prevent the paint from collapsing into a puddle when squeezed onto a palette. From a manufacturing standpoint, they are nearly indispensable. Without them, paint must be ground more carefully, packaged more carefully, and used more quickly. Conservation studies have documented the presence of stearates in the formulations of most major oil paint brands, in samples ranging from the 1940s to the early 2020s.

Waxes — beeswax, paraffin, hydrogenated castor oil (often called “castor wax”) — serve a similar purpose. They thicken the paint, prevent oil-pigment separation, and adjust the way the paint releases from the brush. Castor wax is identifiable in the laboratory by a characteristic chemical signature, 12-hydroxystearic acid; it has been documented in the formulations of several major brands and is present in tested colors from at least the early 2000s to the present. Beeswax has been documented as well, in concentrations typically around one to two percent.

Driers — siccatives — are organometallic compounds that accelerate the chemistry of oil drying. Cobalt, manganese, and zirconium are common. They significantly speed surface drying, which is useful both for the painter (less waiting between layers) and for the manufacturer (less risk of undried paint on the studio floor). Metal-based driers have been used in oil painting practice for centuries; the specific organometallic compounds common in modern commercial paint are a more recent development, and questions about their long-term effects remain open in the conservation literature.

Fillers — calcium carbonate (chalk), magnesium carbonate, barium sulfate, kaolin (china clay) — are inert mineral powders added to bulk the paint and adjust its handling properties. They are documented in many major brands, particularly in colors where the pigment itself is expensive (cadmiums, cobalts) or has unusual oil absorption.

None of these materials is, by itself, evidence of bad paint. Some painters reading this will recognize the additives in their own homemade paint mediums. Wax and oil mediums go back to the Renaissance. The question is not whether additives exist — they have always existed — but what they do over decades, and whether the painter knows they are there.

What Conservation Science Has Documented

The literature distinguishes between different additives, different mechanisms, and different consequences. A blanket statement that “additives are bad” would be wrong, and worse, it would obscure what’s actually known.

Metal soaps: not all are damaging

When fatty acids in the oil react with metal ions from pigments, driers, or stearates, they form compounds called metal soaps. These have been the subject of intense conservation research for the past twenty-five years, since they were first identified in the 1990s as the cause of crater-like protrusions on the surface of Rembrandt’s Anatomy Lesson of Dr. Nicolaes Tulp. A 2021 critical review by Izzo and colleagues in ChemistryOpen reports that up to seventy percent of oil paintings in conserved collections show metal soaps in some form.

But metal soap formation is not always damaging. A 2017 review by Cotte and colleagues in Studies in Conservation takes the title “Lead soaps in paintings: friends or foes?” and answers carefully: sometimes friends, sometimes foes, depending on the formulation, the pigment, and the time scale. Lead soaps formed in the early years after a painting is made can act as anchor points in the polymer network of the dried oil, contributing to the durability of the paint film. Many of the great paintings of the European tradition — works that have lasted four and five centuries — contain lead soaps. They are part of why those paintings have lasted.

Zinc soaps are different. The mechanism is documented, and there is no comparable beneficial role.

Why zinc white is different

Zinc oxide — zinc white — was introduced as an artists’ pigment in the 1830s and adopted widely by the 1850s. It is non-toxic, very white, and stable to light, all of which made it attractive as a replacement for the lead whites that had served oil painting for centuries.

The trouble showed up slowly, because the mechanism of failure operates on a timescale of years and decades rather than months. Zinc oxide reacts with free fatty acids in the drying oil to form zinc soaps. In the laboratory work by Rogala and colleagues at the Smithsonian, this reaction has a specific and consequential effect: zinc oxide forms a packed crystalline structure that traps the unsaturated fatty acid chains in the oil and prevents them from fully crosslinking with the polymer network. The film that results is unusually stiff and unusually brittle. It does not flex when the canvas behind it flexes. It cracks. It delaminates from layers above and below. Over time, in some paintings, it sheds in plates.

This is the mechanism behind the elongation findings from the Smithsonian’s 28-year study: 0.3 percent for zinc white and 4.5 percent for lead white. The same testing showed that even when zinc oxide was diluted in tints to 3.6 percent of total weight, the embrittlement persisted. The failure mode does not require zinc to be the dominant pigment. It only requires zinc to be present.

There is a second mechanism, independently documented. Zinc oxide is photochemically active: under near-ultraviolet light, it catalyzes the formation of hydrogen peroxide. This contributes to chalking and embrittlement of the paint film by a separate chemical pathway from the soap mechanism. The two effects compound.

A 2010 study by Rogala and colleagues in the Journal of the American Institute for Conservation examined Abstract Expressionist paintings in the collection of the Hirshhorn Museum and found that condition problems could be traced to zinc oxide underlayers. Subsequent work by Mecklenburg and others has identified similar problems in named paintings by major twentieth-century artists. Williamsburg/Golden Artist Colors, after replicating the Smithsonian findings in their own laboratory, removed zinc oxide from nearly the entire Williamsburg oil paint line in February 2018 — keeping it only in two whites that explicitly name the pigment. Gamblin reduced zinc oxide in their formulations to under fifteen percent of mixtures around the year 2000, after consulting directly with Mecklenburg.

This is the load-bearing point. Zinc oxide in oil paint is a documented mechanical failure mode. It operates at additive levels — concentrations under two percent, low enough that ASTM D 4302 does not require zinc to appear on the label of a tube whose primary pigment is something else. A painter who buys “Titanium White” expecting titanium dioxide and linseed oil may be buying a paint that contains zinc oxide as well, with no way to know from reading the tube.

Other documented consequences

Zinc is the sharpest case, but it is not the only one. The conservation literature has documented several other failure modes tied to common additives:

Castor wax and slow-drying oils have been linked to long-term softness, stickiness, and dirt absorption in twentieth-century paintings. The Helwig study of Rita Letendre’s 1960s paintings traced specific oil exudates and fatty-acid efflorescence to high abundances of aluminum stearate and castor wax in the tube paints used.

Beeswax, by basic chemistry, is non-drying — it does not oxidize and polymerize as drying oils do, but solidifies physically as it cools. Research on the molecular structure of oil paint films by Bonaduce and colleagues has demonstrated that low crosslinking density itself leads to water sensitivity and reduced film integrity. Wax additives that do not participate in crosslinking reduce the achievable density of the polymer network.

Magnesium carbonate filler has been linked to water-sensitive paint surfaces — surfaces that bloom or whiten when they are exposed to water during cleaning. The mechanism, documented by Silvester and colleagues in Studies in Conservation, involves reacting with atmospheric sulfur dioxide to form magnesium sulfate efflorescence.

Aluminum stearate, beyond its role in promoting metal soap formation that causes protrusions, is also a Lewis acid: the aluminum ion can destabilize the chemical bonds that hold the oil’s polymer network together, accelerating hydrolysis in humid conditions.

None of these mechanisms is hypothetical. Each is supported by named studies published in peer-reviewed journals and conducted on real paint samples by independent research groups across Europe and North America.

How to Evaluate a Paint

If you are a serious painter, this is the question that matters: what should you actually do about any of this?

There is no single answer, because painters have different priorities. A scene painter producing work that will be photographed once and replaced has different needs than a portraitist painting a commission that the client expects to outlive them. A plein air painter who values quick handling has different needs than a studio painter who works in slow layers. The right paint for a given person depends on what they are making and how long it needs to last.

But the standard for evaluating a paint can be stated cleanly.

Read what the manufacturer publishes. If a paint maker tells you only what’s on the label, you know about pigments and oil — nothing more. If they publish technical articles, ingredient disclosures, formulation notes, the names of any additives used, and the reasons for their inclusion, you know what they are putting in the tube. The willingness to publish is itself information. A manufacturer who is confident that their formulation choices are justified will tend to explain them. A manufacturer who lists only what the standard requires has chosen to tell you the minimum.

To investigate any paint: search the manufacturer’s website for the color name plus “formulation,” “ingredients,” or “technical notes.” If those searches return nothing beyond the label, that is your answer. If they return published technical articles with specific disclosures — including acknowledgment of any additives and the reasons for their use — you have something to evaluate.

Look for the absences as well as the presences. Aluminum stearate is the most common additive in commercial oil paint and is rarely disclosed because it is rarely required to be. Beeswax, castor wax, and the various driers are similarly absent from most catalogs. Absence from a label tells you nothing about whether an additive is present. Presence in a manufacturer’s published technical notes — with the reason stated and the amount specified — tells you the manufacturer trusts you to evaluate the choice.

Treat zinc oxide with caution, especially in whites and tints. This is the sharpest single recommendation in the conservation literature, and it carries the most consequence for actual paintings. If you use zinc-containing whites, use them thinly and on top of more flexible underlayers, not as the structural foundation of a painting. Many manufacturers have already reformulated their whites to reduce or eliminate zinc; some still include it as an additive without naming it. If a “Titanium White” doesn’t explicitly disclose its zinc content, the safest assumption is that some is present.

The historical alternative — lead white — is the paint that produced 4.5 percent elongation in the Smithsonian’s 28-year study. It remains the most mechanically durable white in the oil painter’s palette, and its long-term behavior in the paint film is better understood than that of any modern substitute. Rublev Colours Stack Process Lead White and Lead White are made from pigment and alkali-refined linseed oil, without zinc oxide, stearates, waxes, driers, or fillers.

Be skeptical of newness for its own sake. Modern formulations are often optimized for shelf life, consistent handling, and uniform appearance across batches. These are real virtues. They are not the same as longevity. Some of the additives that produce excellent shelf life and handling are the same additives that produce mechanical failure thirty years later. The paints that have lasted longest in the historical record were not the most convenient to manufacture or the most consistent in the tube. They were ground from pigment and oil.

Know what questions to ask before you buy. The four criteria above give you a framework, but they require you to do some work. Here, concretely, is what that work looks like:

• Look up the paint on the manufacturer’s website. Does the product page list only what the label says — pigment and oil — or does it say more?
• Search for the manufacturer’s name alongside terms like “formulation,” “additives,” “stearate,” or “zinc oxide.” What comes back?
• Check whether the manufacturer has published any technical articles on their website. If the only content is marketing copy, that is a signal.
• If you use a white, check explicitly whether zinc oxide is disclosed as absent or simply unmentioned. The two are not the same.

A manufacturer who has nothing to hide will have made it easy to find out what is in the tube. If finding out is difficult, that is information too.

What We Do

Rublev Colours Artist Oils are made without aluminum stearate, zinc stearate, beeswax, castor wax, driers, or carbonate fillers. Most colors contain only pigment and alkali-refined linseed oil. A small number of colors contain heat-bodied linseed oil, as disclosed on the affected product pages. We do not use zinc oxide as a pigment or as an additive in any color.

There is one exception, and we state it plainly: Minium (Red Lead) contains a small amount of aluminum stearate (less than two percent). Red lead dries faster in oil than any other pigment — fast enough to harden in the tube within months without intervention. When we first made Minium without additives, the shelf life was under a year. Adding aluminum stearate at less than 2% extended shelf life beyond a year and made the paint viable for sale. We also make only small batches for the same reason. The reasons are stated in full on the Minium product page, because we think a painter using this color deserves to know exactly what is in it and why.

This is what disclosure looks like in practice. It is not a claim that aluminum stearate is harmless — the conservation literature makes it clear that it poses real risks in long-term paint films. It is a claim that a painter who knows what is in the tube, and why, can make an informed decision about how and where to use it.

For every other color in the Rublev Colours line, the formulation is pigment and oil. Where the oil type varies — alkali-refined linseed or heat-bodied linseed — it is disclosed on the product page. Any use of extended pigments is disclosed. The ingredient information is there to read, not because ASTM D 4302 requires it, but because we think a serious painter is owed a straight answer.

Browse the full Rublev Colours Artist Oils line.

References

Banti, D., La Nasa, J., Lluveras Tenorio, A., Modugno, F., van den Berg, K. J., Lee, J., Ormsby, B., Burnstock, A., Bonaduce, I. (2018). “A molecular study of modern oil paintings: investigating the role of dicarboxylic acids in the water sensitivity of modern oil paints.” RSC Advances 8, 6001–6012.

Bonaduce, I., et al. (2021). “The stability of paintings and the molecular structure of the oil paint polymeric network.” Scientific Reports.

Burnstock, A., van den Berg, K. J. (2014). “Twentieth Century Oil Paint. The Interface Between Science and Conservation and the Challenges for Modern Oil Paint Research.” In: Issues in Contemporary Oil Paint, Springer, pp. 1–19.

Casadio, F., Keune, K., Noble, P., van Loon, A., Hendriks, E., Centeno, S. A., Osmond, G., eds. (2019). Metal Soaps in Art: Conservation and Research. Springer International Publishing.

Cotte, M., Checroun, E., De Nolf, W., Taniguchi, Y., De Viguerie, L., Burghammer, M., Walter, P., Rivard, C., Salomé, M., Janssens, K., Susini, J. (2017). “Lead soaps in paintings: friends or foes?” Studies in Conservation 62, 2–23.

Erhardt, D., Tumosa, C. S., Mecklenburg, M. F. (2005). “Long-term chemical and physical processes in oil paint films.” Studies in Conservation 50, 143–150.

Gamblin Artists Colors. “Zinc Oxide in Artist Oil Colors.” gamblincolors.com.

Helwig, K., Poulin, J., Corbeil, M.-C., Moffatt, E., Duguay, D. (2014). “Notes on Metal Soap Extenders in Modern Oil Paints: History, Use, Degradation, and Analysis.” In: Issues in Contemporary Oil Paint, Springer, pp. 167–184.

Hermans, J. J., Keune, K., van Loon, A., Iedema, P. D. (2019). “Metal Soaps in Oil Paintings: Structure, Dynamics, and Reactivity.” In: Metal Soaps in Art (Casadio et al. eds.), Springer, pp. 47–67.

Izzo, F. C., van den Berg, K. J., van Keulen, H., Ferriani, B., Zendri, E. (2014). “Modern Oil Paints – Formulations, Organic Additives and Degradation: Some Case Studies.” In: Issues in Contemporary Oil Paint, Springer, pp. 75–104.

Izzo, F. C., Balliana, E., Pinton, F., Zendri, E. (2014). “A preliminary study of the composition of commercial oil, acrylic and vinyl paints and their behaviour after accelerated ageing conditions.” Conservation Science in Cultural Heritage 14, 353–369.

Izzo, F. C., Kratter, M., Nevin, A., Zendri, E. (2021). “A Critical Review on the Analysis of Metal Soaps in Oil Paintings.” ChemistryOpen 10, 904–921. DOI: 10.1002/open202100166.

Keune, K., Boon, J. J. (2007). “Analytical Imaging Studies of Cross Sections of Paintings Affected by Lead Soap Aggregate Formation.” Studies in Conservation 52(3), 161–176.

Maines, C., Rogala, D., Lake, S., Mecklenburg, M. (2011). “Deterioration in Abstract Expressionist paintings: Analysis of zinc oxide paint layers in works from the collection of the Hirshhorn Museum.” MRS Symposium Proceedings 1319.

Maor, Y. (2008). “Delamination of Oil Paint from Acrylic Grounds.” Master’s thesis, Queen’s University, Canada.

Mecklenburg, M. F., Tumosa, C. S., Erhardt, D. (2005). “The Changing Mechanical Properties of Aging Oil Paints.” Materials Research Society Symposium Proceedings 852.

Mecklenburg, M. F. (Smithsonian Museum Conservation Institute). “The Chemical and Mechanical Effects of Pigments on Drying Oils” (28-year study, completed 2007).

O’Hanlon, G. (2007, with table revisions 2014; appendix revised November 2019). “Zinc White: Problems in Oil Paint.” Natural Pigments. naturalpigments.com.

Osmond, G., Boon, J. J., Puskar, L., Drennan, J. (2012). “Metal stearate distributions in modern artists’ oil paints: surface and cross-sectional investigation of reference paint films using conventional and synchrotron infrared microspectroscopy.” Applied Spectroscopy 66(10), 1136–1144.

Osmond, G. (2012). “Zinc white: a review of zinc oxide pigment properties and implications for stability in oil-based paintings.” AICCM Bulletin 33, 20–29.

Osmond, G. (2014). “Zinc White and the Influence of Paint Composition for Stability in Oil Based Media.” In: Issues in Contemporary Oil Paint, Springer.

Phenix, A. (2017). “The Might of White: Formulations of titanium dioxide-based oil paints as evidenced in archives of two artists’ colourmen, mid-20th century.” ICOM-CC 18th Triennial Conference, Copenhagen.

Rogala, D., Lake, S., Maines, C., Mecklenburg, M. (2010). “Condition problems related to zinc oxide underlayers: Examination of selected Abstract Expressionist paintings from the collection of the Hirshhorn Museum.” Journal of the American Institute for Conservation 49(2), 96–113.

Sands, S. (2018, updated 2023). “Zinc Oxide – Reviewing the Research.” Just Paint, Golden Artist Colors.

Sands, S. (2019). “On the Yellowing of Oils.” Just Paint, Golden Artist Colors.

Silvester, G., Burnstock, A., Megens, L., Learner, T., Chiari, G., van den Berg, K. J. (2014). “A cause of water-sensitivity in modern oil paint films: the formation of magnesium sulphate.” Studies in Conservation 59(1), 38–51.

Tempest, H., Burnstock, A., Saltmarsh, P., van den Berg, K. J. (2013). “The sensitivity of modern oil paints to aqueous solvents used for cleaning.” In: New Insights into the Cleaning of Paintings, Smithsonian Contributions to Museum Conservation 3, pp. 107–115.

Tumosa, C. S. (2001). “A Brief History of Aluminum Stearate as a Component of Paint.” WAAC Newsletter 23(3).

Tumosa, C. S., Mecklenburg, M. F. (2005). “The influence of lead ions on the drying of oils.” Reviews in Conservation 6, 39–47.

van Loon, A. (2008). Color Changes and Chemical Reactivity in Seventeenth-Century Oil Paintings. PhD diss., University of Amsterdam.

George O’Hanlon is the technical director of Natural Pigments and has written on zinc white in oil paint since 2007.

In oil painting, artists today have access to two broad categories of iron oxide colors: traditional earth pigments and modern synthetic iron oxides. Although both are based on iron oxide chemistry, they behave differently in paint. These differences arise not only from composition but also from particle structure (the size and shape of pigment particles), surface chemistry (how particle surfaces interact with oil), and the way each pigment organizes within the oil film.

This article describes broad tendencies, not universal rules. Both earth pigments and synthetic iron oxides vary by source, processing, and paint formulation. The aim is to clarify how these materials tend to behave, and why.

For a broader overview of these materials, see: Earth Pigments in Oil Paint: Natural vs Synthetic.

We focus on pigment volume concentration (PVC), refractive index, particle packing, and working properties—so artists can make decisions grounded in how paint films actually function.

What Are Earth Pigments?

Earth pigments are naturally occurring mineral materials composed primarily of iron oxides such as hematite (red) and goethite (yellow), along with accessory materials including:

• Clay minerals (aluminosilicates: plate-like particles of aluminum, silicon, and oxygen)

• Silica (quartz: hard, transparent mineral)

• Manganese oxides (can influence color and drying)

• Calcium carbonates (chalk-like materials)

They are extracted, washed, graded, and sometimes calcined (heated to change color and structure) to produce colors such as yellow ochre (CI Pigment Yellow 43, PY43), red ochre (CI Pigment Red 102, PR102), raw umber (CI Pigment Brown 7, PBr7), and burnt sienna (also typically PBr7, calcined).

Crucially, earth pigments are multi-phase systems—that is, they contain several different mineral substances mixed together (rather than a single uniform material)—with irregular particle geometry and variable surface chemistry.

Practical implication: even within a single color family (e.g., ochres), earth pigments can vary significantly in handling, color, and drying depending on their geological source and processing. This variability is especially evident in pigments such as green earths, where mineral composition strongly affects working properties.

What Are Synthetic Iron Oxides?

Synthetic iron oxides are manufactured through controlled chemical processes such as precipitation (forming solids from a solution) or thermal decomposition (breaking down compounds with heat). These methods yield pigments with:

• High chemical purity

• Narrow particle size distribution (most particles are similar in size)

• Controlled crystal morphology (consistent particle shape)

Examples include synthetic red iron oxide (PR101), synthetic yellow iron oxide (PY42), and synthetic black iron oxide (PBk11).

These are engineered materials, designed for consistency and strength rather than geological complexity.

Variability vs Uniformity (A Central Distinction)

A defining difference between these two categories is the range of behavior.

Earth pigments:

• Exhibit wide variation in particle size, composition, and surface chemistry

• Can differ noticeably even within the same nominal pigment (e.g., two yellow ochres)

• Offer a range of rheological behaviors—from short and stiff to smooth and flowing (as seen in applications such as flesh-tone painting with Verona Green Earth).

Synthetic iron oxides:

• Are comparatively uniform in composition and particle structure

• Deliver consistent behavior from batch to batch

• Show narrower variation in handling and optical response

For artists, this means:
Earth pigments provide a spectrum of working properties and subtle variation, while synthetic iron oxides provide predictability and repeatability.

Pigment Volume Concentration (PVC) and Paint Structure

One of the most important—and least discussed—differences lies in how these pigments behave at equivalent pigment volume concentrations.

Pigment Volume Concentration (PVC) is the proportion of pigment relative to the total volume of pigment plus binder (oil). It largely determines whether a paint film is dense and glossy or porous and matte.

Earth pigments typically:

• Often require higher oil content to wet their surfaces (fully coat each particle) on a weight basis

• Form more open pigment networks due to irregular particle shapes and accessory minerals

• Commonly fall within a CPVC range of about 45–55% by volume, with some siennas and umbers exceeding ~55%

Critical Pigment Volume Concentration (CPVC) is the point where there is just enough oil to fill the spaces between pigment particles. Above this point, the paint becomes more porous and matte.

Synthetic iron oxides:

• Require less oil due to smoother, more uniform surfaces

• Pack more efficiently

• Tend to produce less internal void space at a given PVC due to their uniformity

Practical consequence:
At comparable working consistencies, earth colors may incorporate more binder relative to pigment on a weight basis and often form more open internal structures. Synthetic oxides tend to form more uniform, compact films under similar conditions. These differences influence gloss, transparency, and surface character.

Particle Packing and Film Formation

Paint films are structured systems in which pigment particles are dispersed within a continuous oil matrix (the binder forming a continuous film).

Earth pigments:

• Broad size distribution (many different particle sizes)

• Irregular shapes

• Presence of clays and accessory minerals

Result:

• More variable packing structures

• Increased likelihood of microscopic voids (depending on processing and formulation)

• Greater variation in optical and surface effects

Synthetic iron oxides:

• Uniform particle size

• Controlled morphology

Result:

• More consistent packing

• More predictable optical behavior

These differences affect optical appearance and can influence surface character and mechanical behavior, although final film properties depend strongly on formulation and curing conditions. For practical applications of structure in thick paint layers, see: Creating Impastos in Oil Paintings.

Refractive Index and Optical Behavior

The refractive index (RI) of a material describes how strongly it bends light. Iron oxides have relatively high RI values (generally ~2.0–2.9 depending on phase), while drying oils are lower (about 1.48).

Optical behavior in paint depends on:

• Particle size relative to visible light

• Degree of dispersion (how well particles are separated)

• Presence of additional mineral phases

Earth pigments:

• Multiple mineral phases and varied particle sizes produce diffuse light scattering

• Often yield softer, less uniform color effects

Synthetic iron oxides:

• More uniform particles produce more consistent scattering

• Typically yield stronger, cleaner color response

Outcome (as a general tendency):
Synthetic oxides often appear stronger and more opaque, while earth pigments tend toward softer, more complex color.

Tinting Strength and Color Control

Tinting strength refers to how strongly a pigment influences a mixture when combined with another color.

Synthetic iron oxides:

• Generally, higher tinting strength

• Stronger influence in mixtures

Earth pigments:

• Lower tinting strength

• More gradual influence

• Greater tolerance for adjustment

This makes earth pigments particularly useful for controlled color modulation, while synthetic oxides are effective when stronger color impact is required.

Oil Absorption and Rheology

Oil absorption is the amount of oil required to wet and bind a given amount of pigment.

Rheology refers to how a material flows and responds to movement.

Earth pigments:

• Often higher oil absorption

• Wide range of rheological behavior due to variable mineral composition

• Can range from short and resistant to smooth and flowing—even within the same color family

Synthetic iron oxides:

• Typically, lower oil absorption

• More consistent rheology due to uniform composition

• More predictable handling

This variability is one of the defining practical differences: earth pigments offer a broader spectrum of handling qualities, while synthetic oxides offer consistency.

Drying Behavior and Surface Effects

Earth pigments may contain trace elements that can influence oil oxidation, but drying behavior depends on many factors, including oil type, pigment concentration, and environmental conditions.

In practice:

• Some earth pigments may modestly influence drying

• Surface appearance (matte vs gloss) is often related to pigment structure, PVC, and binder distribution, not a single cause

Synthetic iron oxides are generally more uniform and behave predictably in drying, but are not strong driers on their own.

Stability and Long-Term Performance

Both earth pigments and synthetic iron oxides are among the most stable pigments available:

• Highly resistant to light degradation

• Generally, chemically stable in oil painting systems

• Proven longevity in historical and modern works

Earth pigments benefit from long historical use, while synthetic oxides provide consistent modern manufacture.

Earth Pigments vs Synthetic Iron Oxides: When to Use Each

Earth pigments are well-suited for:

• Underpainting and block-in

• Flesh tones and subtle transitions

• Landscape palettes

• Situations where variation and control are desired

Synthetic iron oxides are well suited for:

• Strong color passages

• High-opacity applications

• Situations requiring consistency and repeatability

A Note on Formulation and Quality

Artists work with paints, not raw pigments. Final behavior depends heavily on formulation:

• Degree of dispersion

• Pigment concentration

• Presence of extenders

• Oil type and additives

Well-formulated paints preserve the intrinsic properties of the pigment more faithfully, while heavily modified formulations can mask or alter them. For a broader discussion of how oils and additives influence paint behavior, see: Ultimate Guide to Oil Painting Mediums.

Examples (to be developed)

Future additions will include side-by-side comparisons of:

• Natural yellow ochre (PY43) vs synthetic yellow iron oxide (PY42)

• Natural red ochre (PR102) vs synthetic red iron oxide (PR101)

• Raw umber vs synthetic equivalents

Conclusion

Although earth pigments and synthetic iron oxides share similar chemistry, their behavior in oil paint is governed by structure and variability.

Earth pigments offer a wide range of properties—even within a single color family—due to their multi-phase nature. Synthetic iron oxides offer consistency and predictability due to their engineered uniformity.

Understanding this distinction allows artists to use each material deliberately—choosing variation or consistency as needed.

Related Reading

• Earth Pigments in Oil Paint: Natural vs Synthetic

• Green Earth Pigments in Art—Uses, Properties and History

• Verona Green Earth for Vibrant Flesh Tones

• Ultimate Guide to Oil Painting Mediums

• Creating Impastos in Oil Paintings

In traditional water gilding, bole is the colored clay and glue layer that sits between gesso and gold leaf. It is not a cosmetic filler. It is an interface that controls how the gold adheres, how it burnishes, and how its color appears in light. In this article, we review the section from the booklet Gilding Notes: The Traditional English Method by Judith Wetherall.

Wetherall describes bole as fine “pipe” clay bound with animal glue, whose tiny plate-like particles can slide over one another under pressure during burnishing [Wetherall 2025, p. 27].

Gold leaf is extremely thin, and according to Jane Wetherall, light can pass through it and interact with the color beneath. Therefore, the bole tone, thickness, and burnish have a direct, visible effect on the finished surface [Wetherall 2025, pp. 27–28]. The belief that substrate color influences the gilding is widespread. However, a comprehensive study provides clarity on this issue, unraveling the misconceptions and exploring the intricacies of gilding techniques and materials in the medieval era. Read the article, The Influence of Gilding Base Color on Gilded Surfaces: Insights and Misconceptions.

Historically, bole also reduces the porosity of the gesso. It slows the absorption of size water, so the gilder has enough open time to lay the leaf cleanly [Wetherall 2025, p. 28]. Without that control, the water sinks too fast, the gold bridges, and the surface becomes patchy or unburnishable.

Traditional Bole Colors in Water Gilding

Historically, European ateliers used local clays, and their geology still shows in historic frames and altarpieces. Wetherall notes that the most prized bole historically came from Armenia. It produced a rich, deep red color and a particularly good burnish, which made it expensive and closely guarded in workshops [Wetherall 2025, p. 28].

By contrast, English clays needed help. English bole recipes added graphite (“black lead”) and mutton suet to pipe clay and earth pigments to improve burnish and modify sheen [Wetherall 2025, p. 28]. Yellow clay was often used, with red or black reserved for highlights. Yellow burnishes poorly, so it was left matte, while red and black clays on accessible peaks were taken to a high polish [Wetherall 2025, pp. 28–29].

Today, modern commercial boles are usually pastes made from refined clays and sold in colors named after historic uses (for example, “Georgian Orange”) [Wetherall 2025, p. 29]. However, these names are marketing shorthand, not strict archaeological labels; the same hue can appear in fifteenth‑century Italian work as well as Georgian English examples.

Table 1. Typical bole colors and historic uses (after Wetherall)

For conservation and historically accurate reconstructions, matching period-appropriate clay color and behavior is as important as matching gold.

Preparing Bole for Water Gilding

Historically, bole was sold as dry cones. The gilder soaked, ground, and sieved the clay before mixing it with glue size [Wetherall 2025, pp. 28–29]. Today, most artists will start from commercial paste bole. Nevertheless, the preparation principles remain the same.

Wetherall recommends using rabbit-skin or parchment glue, prepared slightly weaker than glue for gesso or chalk ground, because too-strong size makes a glassy, brittle film that does not take burnishing well [Wetherall 2025, p. 30]. Soak the glue granules for several hours or overnight, then dissolve them in a bain-marie or carefully in a microwave oven, and add them gradually to the clay in a white vessel so that the color can be judged accurately.

Unlike gesso, bole is not judged solely by viscosity. Instead, the gilder evaluates coverage and tone. A test stroke over scrap gesso should produce a translucent veil: the white ground should still “grin through” after a single coat [Wetherall 2025, p. 30]. If the layer is opaque or heavily streaked, the mixture is too clay‑rich or too thick.

Once the color and coverage are correct, the bole is strained through fine silk or mesh to remove grit and agglomerates. Wetherall suggests warming the strained bole gently so it remains fluid but not hot. Overheating risks skinning and bubbles [Wetherall 2025, p. 30].

Glair (settled beaten egg white) can be used to temper bole instead of glue, especially in Italian practice, but it is harder to control and more brittle. Therefore, for contemporary studio work, protein glue remains the more flexible and predictable binder.

Applying and Polishing Bole for Water Gilding

Before applying bole, the gesso must be fully dry, shaped, and smoothed. Wetherall advises rubbing flat areas with fine silicon carbide papers from about P120–P400 and “ragging” carved surfaces with damp silk, using minimal water [Wetherall 2025, p. 27]. The goal is a clean, continuous surface without ridges, knife marks, or powdery areas.

Next, apply the bole in thin, swift strokes with a soft brush. Historically, a squirrel mop was commonly used, although soft nylon or sable flats also work when used lightly [Wetherall 2025, p. 30]. The brush should hold only a modest charge of liquid. The gilder sweeps rapidly across the surface, avoiding reworking fresh areas until they dry matte.

Wetherall, therefore, recommends the following layering scheme for traditional English gilding work [Wetherall 2025, pp. 29–30]:

1. One to three layers of yellow bole over the entire gessoed surface, until all white is covered, but the layers remain thin.

2. One or two layers of red bole over high points that are intended for burnishing.

3. Optional touches of black bole on very selective highlights, such as the outer tips of petals, when a sharp, cold flash of burnish is desired.

Each layer is applied when the previous one is dry to the touch. Unlike gesso, bole can be thoroughly dry between coats; there is no penalty for bubbles. Total bole layers rarely exceed four or five [Wetherall 2025, p. 30].

After complete drying, the bole must be polished. Wetherall stresses that this stage is about levelling and nib removal, not pre‑burnishing [Wetherall 2025, p. 30]. Suitable tools include:

• A cut-down bristle house-paint brush with its ferrule taped;

• 1000‑grit wet‑and‑dry paper, used gently;

• Grade 0000 oil‑free steel wool.

The object is a satin, even surface, free of brushstroke ridges. Using an agate burnisher on bare bole dulls the clay, risks glazing the glue, and spoils the later burnish under gold.

Water Size in Traditional Water Gilding

In water gilding, gold adheres because the bole is momentarily rewetted with a dilute glue solution called size water. Wetherall’s typical English recipe uses approximately one teaspoon of glue size to half a pint of water plus about one teaspoon of methylated spirits [Wetherall 2025, p. 31]. The alcohol lowers surface tension, so the water wets the clay evenly instead of beading.

Because craft conditions vary, Wetherall advises adjusting the proportions by how the bole behaves, not by fixed numbers [Wetherall 2025, p. 31]. If the bole absorbs the water immediately and flashes dry, the size may contain too much alcohol or too little glue. If the water beads on the surface, more alcohol is required, or the bole needs further polishing.

Other traditional water sizes include gin and water or water with a trace of glair plus alcohol. Gin evaporates quickly and is suitable only for experienced gilders, since the working window is short [Wetherall 2025, p. 31]. Regardless of recipe, the size must keep the bole uniformly glossy just long enough for the leaf to land and pull down.

Laying Loose Gold Leaf in Water Gilding

Only loose leaf is suitable for water gilding [Wetherall 2025, p. 13]. Wetherall emphasises technique and sequence more than force. The gilder wets an area, then immediately lifts and lays the gold:

• Work from upper surfaces downward so that water size cannot run over the finished leaf.

• Avoid flooding; a shallow, coherent gloss is ideal.

• Hold the gilder’s tip parallel to the surface and move in decisively; the gold will jump to the moisture as the tip nears [Wetherall 2025, p. 31].

She warns against pulling the tip toward the gilder when lifting it, as that motion tears the leaf [Wetherall 2025, p. 31]. If air is trapped, the leaf is gently tamped with a dry sable writer held end‑on at ninety degrees. Using the tip for tamping contaminates its hairs with size and shortens its working life.

Leaf should always overlap slightly so that any seepage of size water occurs under a second sheet of gold, not along a bare bole join [Wetherall 2025, p. 31]. Faulting and patching should be done with pieces as large as practical. Tiny patches invite the accumulation of size, which stains and darkens the surface.

On mouldings and sharp sections, the leaf must be cut and placed so that it folds into hollows rather than bridging them. Wetherall notes that leaf willingly follows a U-shaped groove but tends to crack if forced into a sharp V [Wetherall 2025, pp. 25, 31].

Burnishing Gold Leaf in Water Gilding

Burnishing is the controlled compression of gold, bole, and gesso with an agate or similar polished stone. It creates the characteristic mirror-like surfaces of water gilding. However, the timing is critical. Wetherall describes the ideal state as one where the preparations are neither obviously wet nor bone dry, but retain just enough internal moisture to act as a cushion [Wetherall 2025, p. 32].

She suggests practical tests. The gilder lightly taps the burnisher on a discreet area; the surface should sound hollow and resonant, not soft or silent. In a temperate, moderately humid studio, this window might arrive in fifteen minutes or several hours, and in some cases only after a day or two [Wetherall 2025, p. 32].

Once ready, the burnisher is drawn in smooth, overlapping sweeps, increasing pressure only until a clear shine appears. Wetherall advises working in one dominant direction and finishing each zone completely before moving on [Wetherall 2025, pp. 32–33]. Circular burnishing leaves visible arcs, and overworking can bruise or delaminate the layers.

She also notes:

• Dirty or faceted stones will score the gold.

• Skewings stuck to the burnisher must be removed, usually with industrial methylated spirits.

• Only raised areas and surfaces naturally accessible to the stone should be burnished; leaving enough matte zones preserves modelling and prevents the object from reading as flat metal [Wetherall 2025, p. 33].

The Science Behind Water Gilding

Clay platelets and burnishability

Bole clays are composed of microscopic platelets. Under pressure, these plates can slide past each other and then align, reducing surface roughness and increasing specular reflection.

Studio translation: a well-formulated bole can be compressed into a smooth, continuous plane under the gold. That plane behaves almost like a polished stone, giving a deep, hard-looking shine without needing excessively thick leaf.

Protein glue networks

Rabbit-skin and parchment glues form networks of long, spring-like molecules that swell with water and contract on drying. When they bind clay and chalk, they create a rigid but slightly elastic matrix.

Studio translation: glue strength determines whether the ground behaves as a resilient cushion or a brittle eggshell. Therefore, keeping bole glue slightly weaker than gesso glue helps the burnished interface compress rather than crack.

Surface tension and wetting

Pure water tends to bead on oily or very smooth surfaces. Adding alcohol lowers surface tension, so water spreads as a continuous film across the bole.

Studio translation: a small amount of alcohol in size water gives you even wetting and a more predictable lay of the leaf. However, too much alcohol shortens the open time and can starve the bole of moisture.

Transmitted light and bole color

Standard gold leaf is thin enough that part of the incident light passes through it, interacts with the underlying bole, and then re-emerges. That interaction shifts the perceived hue and depth.

Studio translation: red bole warms and deepens the gold; yellow gives a neutral, “clean” metal; black sharpens contrast but can cool and gray the tone. Consequently, the choice of bole is a critical color decision, not just a historical detail.

Moisture, humidity, and burnishing windows

As the gesso–bole stack dries, water migrates outward and evaporates. Ambient humidity and temperature strongly affect how long the layers remain in the “cushioned” state when burnishing is possible.

In dry, heated studios, the burnishing window may be short and arrive quickly. In cool, humid weather, the same surface may need many hours before the agate gives a clean polish. Therefore, you must test, not rely on fixed times.

Critical Appraisal of the Water Gilding Source

Scientific merit

Wetherall’s notes are explicitly practical guidelines compiled over more than forty years of professional work rather than formal experiments [Wetherall 2025, Foreword]. She does not report sample sizes, instrumentation, or quantitative measurements of glue strength, moisture content, or leaf thickness in the bole and burnishing sections. Instead, she offers phenomenological criteria such as resonance when tapping with a burnisher, or the “grin” of gesso through bole [Wetherall 2025, pp. 30–32].

This experiential approach has clear limitations. There are no controlled comparisons of different clays under identical conditions, and no statistical analysis of failure rates. Certain historical recipes, such as the use of gin as size water, are described but not verified with ageing studies or mechanical testing [Wetherall 2025, p. 31]. Therefore, the notes should be read as a distilled record of best practice in one well-documented craft tradition, not as a universal, lab-validated protocol.

External validity

Despite these limits, the external validity for studio practice is high. Wetherall’s descriptions are rooted in repeated work on historic English frames, furniture, and architectural decoration, often under conservation constraints [Wetherall 2025, Foreword]. Her observations about bole colors, burnish behaviour, and glue strength align with the experience of many gilders working in similar climates and with comparable materials.

However, contemporary studios may introduce variables that are under-represented in the notes. These include modern MDF supports, synthetic glues, premixed acrylic sizes, climate-controlled interiors, and LED lighting. Wetherall briefly warns that PVA-based sizes stay indefinitely soft and are unsuitable for gilding, but she does not systematically test modern products [Wetherall 2025, p. 35]. Artists adopting her procedures must therefore adapt them cautiously when substituting materials.

Facts versus interpretation

Facts from the text include:

• Bole is a mixture of colored clay and protein glue, applied over gesso and under gold [Wetherall 2025, p. 27].

• Yellow bole is generally used overall; red and black are placed on highlights according to burnish qualities [Wetherall 2025, pp. 28–30].

• Wetherall’s size water recipe combines weak glue, water, and methylated spirits, adjusted by observation [Wetherall 2025, p. 31].

• Burnishing should be attempted only when the preparations give a hollow, resonant sound and resist gentle touch without feeling soft [Wetherall 2025, p. 32].

Interpretations in this review include:

• Treating bole as a deliberately engineered interface layer, comparable to a controlled ground in painting.

• Emphasising bole color selection as a primary color-design decision for contemporary work, not merely a historical convention.

• Arguing that glue strength calibration is central to mechanical stability and that this area would benefit from quantitative testing.

• Highlighting environmental control (humidity and temperature) as variables that could be formalised, rather than left entirely to intuition.

These interpretations build on Wetherall’s descriptions but extend them toward a more explicitly materials-science mindset.

Best-Practice Water Gilding Guidance for Contemporary Artists

The following protocol adapts Wetherall’s traditional English method for professional studios today.

Support and Gesso Preparation for Water Gilding

1. Choose a stable, appropriate support. Use seasoned softwoods, hardwoods such as lime or oak, or high-quality plywood or MDF for panels. Avoid oily species and woods treated with fire retardants, which can repel gesso [Wetherall 2025, p. 21].

2. Size bare wood with hot rabbit-skin glue at about 1:10 by volume, on both faces of panels. Allow thorough drying [Wetherall 2025, p. 22].

3. Apply at least eight coats of gesso on panels, more on areas intended for re-cutting. Lay coats in alternate directions and keep gesso near body temperature to minimize bubbles [Wetherall 2025, pp. 23–25].

4. Allow to dry overnight. Then smooth flat zones with fine silicon carbide paper, and lightly ragged carved zones with damp silk, using minimal water [Wetherall 2025, p. 27].

Health note: rabbit-skin glue dust and gesso powder can irritate the respiratory system. Use a well-fitted dust mask when sanding.

Mixing and Applying Bole for Water Gilding

5. Prepare rabbit-skin glue slightly weaker than that used for gesso. Soak, melt without boiling, and keep warm [Wetherall 2025, p. 30].

6. In a white vessel, gradually add warm glue to the paste bole while stirring gently with a soft brush. Aim for consistency similar to that of whole milk [Wetherall 2025, p. 30].

7. Test on scrap gesso. Adjust with more glue (if patchy and too absorbent) or more bole (if streaky and weak in color). Strain through fine silk or mesh.

8. Apply one to three thin coats of yellow bole over all gesso until no white remains. Allow each coat to dry matte [Wetherall 2025, p. 30].

9. Apply one or two coats of red bole on all highlights intended for burnishing. Add black bole only where a very cold, bright flash is desired, and then only on the highest points [Wetherall 2025, pp. 28–30].

10. Let the bole dry thoroughly, then polish gently with a cut-down bristle brush or very fine abrasive. Remove nibs without cutting through to gesso.

Size Water and Gold Application in Water Gilding

11. Prepare the gilding size by mixing weak glue size with clean water and a small amount of alcohol. Adjust until it wets the bole smoothly without severe beading or instantaneous sinking [Wetherall 2025, p. 31].

12. Work in manageable sections. Wet the bole to an even gloss using a gilder’s mop, avoiding runs and puddles.

13. Immediately pick up loose gold leaf with a squirrel-hair tip and lay it from the top downward. Overlap leaves slightly to conceal joins and to trap any seepage beneath metal [Wetherall 2025, p. 31].

14. Tamping is used only when necessary to release trapped air. Use a dry sable writer held end-on and avoid contaminating the tip with size [Wetherall 2025, p. 31].

15. For mouldings and deep profiles, cut the leaf and position it so it drops into the hollows rather than bridging them.

Ventilation note: methylated spirits and other alcohols are flammable and produce vapors. Work away from ignition sources and ensure adequate ventilation.

Burnishing Strategy in Water Gilding

16. Allow the gilded work to reach the elastic-cushion stage before burnishing. Test readiness by gently tapping an inconspicuous area and listening for a hollow ring, then by light touch with a fingertip [Wetherall 2025, p. 32].

17. Use a clean, highly polished agate with an appropriate shape (dog-tooth, pencil, or book-edge). Inspect under raking light for chips or facets.

18. Burnish in long, even strokes, gradually increasing pressure until a shine appears. Keep strokes largely aligned and avoid tight circles that leave visible arcs [Wetherall 2025, pp. 32–33].

19. Burnish only where the tool naturally reaches: ridges, beadings, outer curves. Leave shadowed hollows and recesses, matte to support visual modelling [Wetherall 2025, p. 33].

20. Stop immediately if resistance increases sharply or if the gold greys; this indicates that the bole is being disturbed or that layers are too dry.

Safety, Storage, and Display for Water Gilding

• Keep protein glues refrigerated between uses and discard at the first sign of strong odor or mold [Wetherall 2025, p. 20].

• Avoid rapid environmental swings. Large changes in humidity or temperature can stress the gesso–bole–gold stack and increase the risk of cracking over time.

• Store finished gilded works in padded supports to protect burnished areas from abrasion. For display, balance lighting so that specular highlights do not obliterate modelling.

Adapting Water Gilding to Modern Materials

Many contemporary artists prefer not to use animal glues or to work on supports that were rare historically. The principles in Wetherall’s book still translate, but some substitutions are riskier than others.

• Synthetic sizes: Wetherall reports that PVA emulsions and similar synthetic sizes form uneven films, dry irregularly, and remain permanently soft, leading to inferior, unstable gilding [Wetherall 2025, p. 35]. Therefore, these should be avoided for serious work, especially if the object is expected to have a long life.

• Gelatin: high-grade gelatins behave similarly to refined animal glues and can substitute for rabbit-skin glue in bole preparation when prepared carefully [Wetherall 2025, p. 20]. However, bloom strength and impurities vary, so tests are essential.

• Premixed boles: modern paste boles save grinding time but conceal clay character. Because quality can only be assessed after application and burnishing, artists should test each new batch on sample boards before committing to important work [Wetherall 2025, pp. 28–30].

• Alternative supports: MDF, high-quality plywood, or plaster can all be used if appropriately sized. The porosity of plaster demands extra glue sizing before gesso [Wetherall 2025, p. 21].

In every case, the key is to preserve the functional sequence: stable support, elastic gesso cushion, thin and well-bound bole, controlled wetting, and correctly timed compression.

Water Gilding with Bole

Overall, Wetherall’s sections on bole and burnished water gilding capture a mature English craft tradition in a concise, practical way. For contemporary artists, her book offers not only recipes but a way of thinking: treat each layer as an active component with mechanical and optical roles, test constantly, and allow timing and touch to guide decisions.

When applied thoughtfully, these methods still produce gilded surfaces that balance brilliance with depth. They respect historic practice while leaving room for measured innovation, provided that substitutions honour the underlying physics of clay, glue, water, and gold.

Bibliography

Wetherall, Judith. Gilding Notes: The Traditional English Method. Los Angeles: The Getty Conservation Institute, 2025.

This booklet gathers practical instructions for traditional English water gilding, including the preparation of gesso and bole, and the application and burnishing of loose gold leaf. It is directly relevant for artists and conservators seeking historically grounded yet studio-tested methods for bole formulation and gilding technique.

Artists have long valued the subtle beauty and durability of natural pigments. For centuries, colors derived directly from minerals, plants, and insects have given artwork a richness that synthetic dyes struggle to reproduce. Today, as sustainability and eco‑consciousness take centre stage, there is renewed interest in earth‑based hues; natural pigments are making a comeback across art and design. These colorants offer unique, muted tones and are typically produced with minimal environmental impact. This shift toward natural materials is reflected in contemporary art trends: many artists now prioritise eco‑friendly materials and ethical sourcing, creating works that use natural materials and sustainable practices.

In response to this movement, Rublev Colours introduces a collection of five Armenian earth oil paints made with pure mineral pigments and linseed oil. The colors—Yellow Travertine, Orange Travertine, Red Barite, Olive Umber, and Violet Lalvarit—are derived from geological formations in the Caucasus region of Armenia. Each paint highlights the inherent qualities of its source rock or earth, bringing artists closer to the origins of their materials. To accompany this article, Natural Pigments will share videos of the tubes being squeezed onto canvas and spread with a palette knife, along with drawdown images showing each color at full strength and tinted with titanium white.

What Armenian earth colors mean to artists

• Earth‑derived colors: All five paints are made from naturally occurring mineral pigments, connecting artists to a long history of using ochres, umbers, and other iron oxides.

• Sustainability: Natural pigments are produced with minimal environmental impact, making them a more sustainable choice.

• Authentic hues: These pigments provide rich, earthy tones and subtle variations, lending depth to the painting. Many have been used since prehistoric epochs and have stood the test of time.

• Local geology: The colors come from specific Armenian deposits—travertine springs, barite‑rich clays, iron‑manganese earths, and volcanic andesite—offering a geographical story behind each tube.

Yellow Travertine

Travertine is a form of limestone precipitated from mineral springs. In the Lori Province of Armenia, deposits of pale travertine contain small amounts of iron oxide that produce a soft yellow‑beige tint. The pigment is a natural, inorganic earth classified in the yellow iron‑oxide family (Colour Index PY43) and consists mainly of calcium carbonate with traces of silica and iron. When ground for oil paint, the pigment has a mean particle size of about 8 μm and an oil absorption of roughly 24 g per 100 g of pigment. This fine particle size produces a smooth film and semi‑opaque coverage.

Rublev Colours’ Yellow Travertine Oil Paint captures the delicate warmth of this stone. In mass tone, it appears a pale straw color; when tinted with titanium white, it yields gentle beige tints ideal for mixing skin tones, atmospheric landscapes, or luminous neutrals. The linseed oil binder gives a moderate drying rate and a slightly matte finish. As with all Rublev Colours paints, no additives, fillers, or extenders are used—just pigment and oil.

Origins

Travertine forms in mineral springs when dissolved calcium carbonate precipitates out of solution and forms porous stone—a form of calcite. The Armenian deposits used for this paint originate from spring waters in the Lori Province. These travertine beds have been quarried since antiquity for building and ornament; now they also provide a unique artist pigment.

Orange Travertine

Like its yellow sibling, Orange Travertine comes from the same travertine springs but contains higher levels of iron oxide, which give it a warm orange hue. This unusual earth pigment is composed not only of calcite but also of germanite. Germanite is a rare, copper-iron-germanium sulfide mineral with the chemical formula Cu₂₆Fe₄Ge₄S₃₂. The pigment is classified as a natural inorganic earth (Colour Index PY43) and has similar physical properties to the yellow variety: mean particle size around 8 μm, density around 2.7 g/cm³, and oil absorption about 24 g per 100 g pigment. The composition, which includes calcite (calcium carbonate) with increased iron content, yields a color range from burnt orange in mass tone to soft apricot when mixed with white.

Rublev Colours’ Orange Travertine Oil Paint displays a semi‑opaque, medium‑strength tinting power. It is perfect for mixing warm highlights, autumnal landscapes, and lively flesh tones. Because the pigment is derived from stable iron oxide, the paint offers excellent lightfastness and permanence.

Origins

Both the yellow and orange travertine pigments originate from Lori Province, Armenia, where mineral springs deposit layers of calcite enriched with iron. These naturally colored stones connect artists with the geological processes that created them.

Red Barite

Red Barite is unlike typical red ochre. The clayey earth from northern Armenia contains hematite (red iron oxide) mixed with quartz and barite (barium sulfate). The presence of barite, rather than calcite, distinguishes this pigment; barite’s high density adds body to the paint and contributes to its opacity. The pigment’s colorant is hematite (Colour Index PR102), giving a deep, earthy red. Analyses show a mean particle size of about 10 μm and a composition of approximately 41.67% barite (Ba), 28.16% iron, 16.14% sulfur, 8.79% silica, and 2.06% aluminum.

The resulting Red Barite Oil Paint is a powerful, semi‑opaque red with a slightly granular texture. In mass tone, it appears as a warm brick red; when extended with white, it produces pinkish‑terracotta tints. The paint’s high density gives it body without requiring much additional oil, and it dries at a moderate rate.

Origins

This pigment is collected from earth deposits near Akhtala in the Lori Province of Armenia. Local miners have long extracted barite for industrial use; the mineral’s vivid color also makes it an exceptional artist’s pigment.

Olive Umber

Umber pigments derive their color from iron and manganese oxides. Olive Umber contains α‑Fe₂O₃ (iron oxide) and MnO₂ (manganese oxide) and is listed under Colour Index PBr 7. The combination of iron and manganese gives this earth a greenish‑brown hue, deeper and cooler than standard raw umber. Physical data show a particle size distribution with 80–85 % of particles under 15 μm and 15–20 % between 16 and 29 μm, with a density ranging from about 3.3–4.3 g/cm³.

In oil, the pigment yields a semi‑opaque paint with a fast drying time, thanks to the catalytic effect of manganese. Rublev Colours’ Olive Umber Oil Paint has strong hiding power and a dark, olive‑brown mass tone. When mixed with titanium white, it produces muted khaki and sage tints ideal for naturalistic shadows, foliage, and subdued palettes.

Origins

The olive‑umber pigment used in this paint comes from Armenian earth deposits rich in iron and manganese oxides. The natural combination of minerals gives this color its distinctive greenish cast.

Violet Lalvarit

The most unusual of the new colors, Violet Lalvarit, is ground from andesite lava rock found near Mount Lalvar in Armenia. Lalvarit is not a formally recognised mineral name but rather a local term for a unique grey‑violet andesite containing iron and manganese oxide impurities. The pigment contains about 30.26% silica, 8.17% aluminium, 5.89% iron, 1.59% potassium, and 1.17% manganese. It has a mean particle size of around 8 μm, a Mohs hardness of 7, and a relatively high oil absorption of 32 g per 100 g pigment. These physical characteristics create a smooth, yet slightly absorbent, paint film.

Rublev Colours’ Violet Lalvarit Oil Paint delivers a semi‑transparent grey‑violet tone with subtle warm undertones. In mass tone, it resembles dusty plum; when tinted with white, it shifts toward cool lilac and mauve. The hardness of the andesite gives the paint a delicate sparkle and ensures excellent permanence.

Origins

This pigment is sourced from andesite lava flows in the Alaverdi ore region near Mount Lalvar, a mountainous area of the Lori Province. The rock’s violet cast arises from iron and manganese oxides present in the lava.

Earth colors for a grounded palette

Natural pigments connect artists to the earth and to centuries of color‑making tradition. As sustainable art practices gain momentum, materials that are produced with minimal environmental impact become increasingly important. The new Armenian earth oil paints from Rublev Colours offer a palette of authentic hues—yellow, orange, red, olive, and violet—each rooted in Armenian geology and free from synthetic additives. Whether you’re painting luminous skin tones, rugged landscapes, or subtle shadows, these colors provide unique qualities that synthetic pigments can’t replicate. With their rich histories and eco‑friendly credentials, they invite you to explore a deeper connection between your materials and the natural world.

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 Granulation is the visible mottling that appears when pigment settles unevenly during a wash. You see soft speckles, pools, and lighter islands. It changes how edges read, how glazes alter hues, and how a sky or stone texture feels. Because it depends on paper, dilution, and handling, it often surprises even experienced painters. Our method measures granulation in a way you can repeat, compare, and trust—so you can choose watercolor paints with far less guesswork.

What we mean by “granulation”

Granulation is the pattern of tone change caused by pigment grains gathering in some small areas and thinning in others as the wash dries. It differs from flocculation (grains clumping in the liquid before or after they are on the paper) and from staining (how strongly a pigment embeds into fibers). Here, we measure the visible result on the dry paint film—not the behavior while wet.

Why we built an objective scale

Artists need objective evaluations of paint behavior and consistency. Brushstrokes vary, paper lots differ, and lighting shifts perception. We use a method that produces a number first, then a word description. The number, the Granulation Index (GI), comes from a scan of a watercolor swatch prepared under carefully controlled conditions that reduce paper and tone effects. The index allows us to categorize the granulation (see below). The same method applies to every color and every paper we test, so results from different sessions can be compared directly.

Summary of the method

Figure 1 — Workflow overview. End-to-end process: from a standard swatch to a Granulation Index and granulation category.

Materials and setup

1. Watercolor paper under test—the baseline is taken on the same sheet and in the same scan as the color. Since we are not rating the granulation effect on any specific watercolor paper, we use Whatman No. 42 filter paper, since it is also used in lightfastness and staining tests. Note 1
2. Paint from the tube or a freshly re-wetted pan
3. Deionized or distilled water.
4. Round brush (size 12) for pre-wet only.
5. Pipettes (1 mL for the 0.10 mL bead; additional for pre-wet).
6. Pencil (orientation mark).
7. Flatbed scanner capable of 16-bit/channel at 1200 dpi with all automatic corrections off.
8. Optional: IT8 target and profile for color-managed scanning.

Preparing the watercolor swatch

Mount the watercolor paper on a clean board using low-tack tape around the perimeter. Mark the 12 o’clock position for orientation. Pre-wet the entire circle with deionized water to an even satin sheen (no puddles), and record the amount of water used (mL). Set the board to 15° with the 12 o’clock edge raised. Prepare the paint at 1:8 (mass ratio of paint to water); mix for 60 seconds, then rest for 2 minutes. Using a calibrated pipette, dispense 0.10 mL of the mixture along the top arc just inside the paper edge. Do not manipulate the flow; allow gravity to carry the bead down the wetted circle. Cover from dust and air-dry undisturbed, then condition 24 hours at room conditions before scanning. Here are the steps in order:

1. Mount the filter circle (we use 110 mm diameter filter paper) on a clean board with low-tack tape around the perimeter.

2. Mark the top for orientation.

3. Pre-wet the entire circle with deionized water to an even satin sheen (no puddles) and record the mL used.

4. Set the board to 15° with the 12 o’clock edge raised.

5. Prepare the paint at a 1:8 (paint-to-water, by mass) ratio. Mix the water and paint for 60 seconds, then allow to rest for 2 minutes.

6. Using a pipette, dispense 0.10 mL of the mixture along the top just inside the paper edge. Do not manipulate the flow—let gravity carry the paint down the wetted paper.

7. Cover from dust and air-dry undisturbed; then acclimate for 24 hours at room conditions before scanning.

Notes on consistency: Keep the brush angle, speed, and pressure the same for every color. Also, stir the stock wash before each new swatch. Replace the water if it becomes dirty. Meanwhile, note the room temperature and humidity, since drying speed can change granulation strength.

Scanning the swatch and the paper

Scan the entire dried swatch and the paper-only at 1200 dpi, 16-bit per channel (48-bit RGB), saving as TIFF with all automatic corrections turned off (sharpening, auto-levels, dust/grain removal). If your workflow is profiled, embed the scanner ICC; otherwise, scan linear unmanaged and record model, software, and settings so runs can be repeated.

Selecting the test area on a swatch

On filter paper, the wash doesn’t settle in the same place each time, so we follow the same visual rules rather than fixed coordinates. We choose one 1-inch square that represents the broad mid-tone field of the wash—where granulation is most visible and not distorted by edge effects.

Where we would place the square on this swatch (Alizarin Crimson):

• Good choice A: the left square, roughly one third in from the left edge and halfway down from the top arc—an even, pink mid-tone with clear grain but no white channels or bead marks.

• Good choice B: the right square, between the central white “river” and the right edge, again in the middle third vertically. We select the most typical representative of the overall mid-tone; they should read similarly.

Zones to avoid on this and any swatch:

• the top band where the bead first touched (darkest, compressed texture),

• the pale tail near the bottom (too thin to show the pattern),

• white channels, backruns, dust, creases, or tape marks,

• the outer paper edge of the circle.

Removing the paper’s influence:
On the same sheet and scan, place an identical 1-inch square on bare paper near the paint square (but away from edges/tape). That tells us how much mottling is due solely to the paper. We subtract that baseline (in quadrature) from the paint’s variation, so the number reflects pigment-driven granulation rather than paper texture.

What the result means:
From the paint tile, we measure how much the lightness varies and scale it by the square’s average lightness. After removing the paper’s values, you get the Granulation Index (GI).

• Higher GI ⇒ more visibly mottled, grainy wash.

• GI near zero ⇒ as even as the paper itself at that spot.

Quality check:
If two nearby mid-tone patches both look representative and you’re unsure, measure both. If their GI values differ by more than ~10% (relative), discard that swatch and remake it—this prevents “cherry-picking” and keeps the method honest.

The math we use

Figure 3 — From Texture to Granulation Index. Color and grayscale images comparing paint and blank paper tiles. After correcting the difference in the paper, Alizarin Crimson shows no granulation.

We measure tone variation in L* within the paint tile and remove the paper’s own texture. The result is a paper-corrected coefficient of variation, which we call the Granulation Index (GI).

Let SD_p be the standard deviation of L* for the paint tile and SD_b for the blank paper tile on the same sheet/scan. Let Mean be the mean L* of the paint tile.

We report GI either as a fraction (e.g., 0.125) or as a percent (12.5%)—but we use the same choice consistently throughout the article. If SD_p ≤ SD_b, GI = 0 (indistinguishable from paper).

Another illustrative example

Suppose the paint area has SD_p = 2.8, the blank paper has SD_b = 1.2, and the mean paint lightness is Mean_p = 70. Raw Change is 1.6. The Granulation Index is 100 × 1.6 / 70 = 2.29. Therefore, the rating is Moderate.

What if the paper texture is exaggerated

Some papers may show an exaggerated fiber texture. To focus on the broader mottling you see at a normal viewing distance, you may apply a slight soft blur before the math. Set the blur strength to two pixels at 1200 dots per inch. That setting reduces fiber‑scale noise yet keeps the larger pattern. Use the same blur on the paint and paper areas.

Turning numbers into categories

Figure 4 — Category gallery. Reference swatches from None to Extreme with their Granulation Index values.

We map the index to these labels using the boundaries as follows:

• None: < 3%

• Slight: 3–7%

• Moderate: 7–12%

• Strong: 12–18%

• Very Strong: 18–26%

• Extreme: > 26%

Because the index is continuous, colors near a boundary can be interpreted differently by different viewers. However, repeat measurements of the same paint under this method cluster within a narrow band.

What matters for contemporary artists

Plan texture instead of chasing surprises

You can select paints for soft, broken skies or for flat, graphic shapes with less trial and error. Additionally, the categories travel well across brands, since the same recipe is used for every color we test.

Paper choice is part of the look

Also, paper plays a role in the effect. Our starting level removal reduces the paper’s role but does not erase it. Therefore, if you change paper, re‑test a favorite color once to learn the new starting level.

Mixes and glazes

Also, low‑granulating colors can wake up when mixed with high‑granulating partners. Conversely, a strongly granulating color can calm down when glazed over a dense underlayer. Because the index is measured at midtone, it correlates well with tinting strength. However, very dark passages reduce change and may look smoother than the number suggests.

Control with water and timing

More water and longer drying times often strengthen granulation. Meanwhile, fast‑drying conditions reduce flow and soften the pattern. Use these levers with intent. The index still shows the paint’s bend, but your technique can nudge the outcome.

Digital capture and reproduction

Scans and prints of granulating washes can look harsher if the sharpening is left on. Therefore, turn off capture sharpen and apply only gentle output sharpen matched to print size. The scale of the pattern matters; too much sharpening creates edges that weren’t there.

The science behind the effect

Particle size and density

Larger or heavier grains settle faster as the water flows. They gather in small dips between fibers. Smaller or lighter grains ride fluid currents longer and even out. Therefore, pigments with wide size spreads often granulate more.

Surface chemistry and clustering

In water with a gum binder, grains carry surface charges. Salts and other additives can screen those charges. When screening is strong, grains can approach each other and form soft clusters. These clusters act like larger bodies and settle unevenly. Gentle clustering is not the same as flocculation, which is runaway clumping in the pan or palette.

Flow within the wash

As water dries, wicking flow drags grains toward drier regions. Edges dry first. Meanwhile, gradients form inside the wash. This internal “micro‑weather” helps build the mottled field we see as granulation.

Paper shape and sizing

Hot‑press cotton paper looks smooth, yet its fiber network has small valleys and plateaus. Sizing on and inside the paper guides how water flows and where the binder sits. Hygroscopicity is the extent to which a material absorbs water from the air and from liquids. Hygroscopic sizing swells when wet and opens short‑lived paths. So, pigments find preferred paths and resting places. Our starting level for paper captures much of this role.

Binder concentration and solid content

Critical pigment volume concentration is the point where there is just enough binder to hold the solids together. Below that point, the film looks smoother. Above it, tiny voids appear, and the texture becomes more apparent. Watercolor washes stay below this point, yet local shifts occur as water leaves. Crosslink density is the degree to which the dried binder network is crosslinked. In a tighter network, flow stops sooner, and the pattern freezes earlier. Therefore, pigment‑to‑water ratio and gum strength matter.

Refractive index and visibility

A pigment sitting in a shallow depression may appear darker because it scatters less light and absorbs more. The refractive index is a measure of how much a substance bends light. The contrast between pigment, binder, and air sets the strength of that effect. With greater contrast, the same physical texture produces a stronger tone signal.

Quality assurance and repeatability

Run two swatches per color and report the average index. If the second copies differ by more than 10%, discard both and remake the pair. Also, keep a log of scanner settings, paper lot, and room conditions. Re‑scan a known “control” swatch at the start of each session. If its index exceeds 10%, check the lighting or settings before you continue.

Using the scale in daily work

Choose paints rated None or Slight for smooth skies, skin, and graphic passages. Choose Moderate to Strong ratings for stone, earth, foliage, or lively atmospheric washes. When a mix must be even, glaze a low‑granulating color over a high‑granulating base rather than mixing them wet. Conversely, for a single-pass texture, mix them in the palette and paint once. Finally, remember that paper still speaks. Retest when you change sheets.

Troubleshooting

• Backruns and blooms: These are separate effects. Avoid them by steering clear of puddles and drafts.

• Staining pigments: Strongly staining colors can read smoother than expected. Consider a slower, wetter wash to reveal their texture.

• Metallic or pearly colors: Their sparkle is directional and not well described by this metric. Treat their ratings as a guide only.

• Very dark swatches: Aim for a gradient with a clear midtone. If the entire swatch is near black, the index will be unfairly low.

• Near the paper baseline: If a swatch’s SD_p is ≤ the paper baseline SD_b, the corrected term becomes zero and GI = 0. That means the visible mottling is indistinguishable from the paper’s own texture under this method. It’s expected for some smooth colors and papers.

How to reproduce this at home with minimal gear

You can perform a simplified version with a modern phone camera. Place the card under bright, even window light. Hold the phone parallel to the surface and disable automatic filters. Photograph the paint area and a blank paper area. Then use a basic editor to view the lightness histogram and judge the spread. Although the numbers will differ from a set scan, the relative ranking of paints will be similar. Therefore, you can still sort colors by granulation tendency.

Recording your results

What this method does not cover

It does not rate sparkle or metallic effects. It does not predict edge darkening or blooms. It focuses on the visible mottling in a stable midtone. Even so, this is the part of granulation that most often shapes the look of skies, rocks, and foliage. Therefore, it is the most useful single number to guide choice.

Notes

1. Whatman 42 is a quantitative, ashless filter paper with the finest particle retention (2.5 µm), making it the world standard for critical gravimetric analysis and sample preparation for instrumental analysis. It is made from high-purity cellulose and is suitable for applications that require filtering extremely fine precipitates.

Watercolor artists often describe a color as staining or non-staining, but what do these terms really mean? At Natural Pigments, we wanted to understand staining more precisely—beyond casual impressions—and to offer data that artists can trust. This article explains how we study staining scientifically while remaining meaningful to painters. It details how we prepare swatches, take measurements, and translate those numbers into the familiar artist labels. We also explore alternative approaches and explain why we chose this quantitative method over others.

Understanding “staining” in watercolor

For artists, staining refers to how strongly a pigment binds to or penetrates the paper fibers and how resistant it is to lifting once dry. High-staining colors are difficult or impossible to remove; low- or non-staining colors can be lifted easily, revealing the paper's white beneath. This usage is consistent across artist literature and manufacturers.

Bruce MacEvoy’s authoritative resource, Handprint, defines staining pigments as those whose particles or dyes embed deeply into paper sizing and fibers, resisting re-wetting and blotting. Winsor & Newton explains the same concept: “Staining colours are those which are difficult to remove once dry; non-staining colours lift easily with a damp brush or sponge.” Daniel Smith likewise classifies its colors from low to high staining, noting that highly staining pigments leave a permanent mark even after repeated lifting. Schmincke’s HORADAM Aquarell line describes staining as color particles penetrating the paper, so they cannot be entirely lifted once dry. All agree that staining means difficulty of lifting.

Historically, artists judged staining by brushing out a color, letting it dry, and then trying to lift a portion with clean water and a soft brush. The results depend on paper type, sizing, drying time, and technique, so qualitative tests vary. Our goal was to preserve that artistic meaning—how much color remains after lifting—while eliminating subjective bias.

Why we developed a numerical system

Visual observation alone cannot reliably compare pigments that differ in depth or hue. A pale green and a dark phthalo blue may appear to lift similarly, but differ significantly in how much color actually remains. To make comparisons fair, we measure relative residual color using a spectrophotometer, following the workflow described below. This approach is more objective yet still reflects the artist’s concept of staining: how much color remains after a standardized lifting process.

We call this value the Residual-Color Index (RCI). It expresses the color remaining after lifting as a percentage of the color in the original wash. The math is simple:

RCI (%) = ΔE(Lift, Paper) ÷ ΔE(Wash, Paper) × 100

Here, ΔE is the color difference in CIE Lab* space measured by the spectrophotometer. An RCI near 0% means the lifted area is close to white paper—non-staining. Near 100%, the lift is almost identical to the original wash—high-staining.

How we prepare our test samples

Our swatches are prepared on Whatman No. 42 filter paper, which provides a consistent, absorbent surface. Filter paper ensures uniform capillary action and minimizes surface texture variations. Each piece measures about 5 × 10 cm. The process follows our internal laboratory procedure for color property evaluation.

We first dilute a measured portion of watercolor paint with distilled water to a fixed paint-to-water ratio. Using a clean 25 mm flat brush, we draw a single, even stroke across the paper at a controlled speed. The wash dries for five minutes before the lifting step begins.

To simulate a controlled lift, we flood a narrow central band with clean water, lay a damp cellulose sponge across it, place a one-kilogram weight on top, and leave it for ten seconds. The sponge is removed and the strip allowed to air-dry. The result is a clear band of lighter color across the wash—identical in preparation for every pigment.

Figure 1 — Preparation of watercolor strips on filter paper. The center band is the area designated for lifting.

Once dry, we record CIE Lab* values for three spots: bare paper, unlifted wash, and lifted area. These readings form the data for calculating the RCI.

Measuring and interpreting results

We compute ΔE—the perceived color difference—between each region. ΔE(Wash, Paper) tells us how far the original wash color is from the paper. ΔE(Lift, Paper) tells us how far the lifted area remains from the paper. Dividing one by the other gives a percentage that represents how much color remains.

For example, if the wash is 80 ΔE units from the paper and the lift is 40 ΔE units away, then RCI = 40 ÷ 80 × 100 = 50%. The lifted area retains half the color difference of the original wash, indicating medium staining.

We assign labels based on the RCI scale:

• Non-staining: less than 30 %

• Low staining: 30–55 %

• Medium staining: 55–75 %

• High staining: above 75 %

These ranges translate numerical data into terms artists understand. A high-staining pigment resists lifting, showing only a slight change between the wash and lift. A low-staining pigment shows a significant change—the color nearly disappears.

Preparing paper for better lifting tests

Not all papers lift color the same way. Sizing and absorbency are crucial. For artists wanting to prevent staining and lifting, a well-sized surface helps keep water out. We recommend using a traditional natural-resin varnish, such as Rublev Colours Lac Water Varnish, to lightly seal the paper. This thin varnish is brushed on, creating a less absorbent surface that more closely resembles an external-sized watercolor sheet and produces more consistent lifting behavior. When applied evenly and allowed to dry thoroughly, it prevents excessive pigment penetration and makes lifting reproducible across sessions.

Why this numerical method matters

Our method turns a subjective judgment into a repeatable measurement. Because the Residual-Color Index normalizes for the initial strength of the wash, it allows comparison between very different pigments. Artists can see that a pale yellow and a deep blue are both “low-staining” for the same physical reason: both lose most of their color difference relative to paper after lifting.

Manufacturers and artists describe staining qualitatively, but those descriptions rest on the same foundation—resistance to lifting. The RCI expresses it as a ratio. For colors like phthalo blue or quinacridone magenta, the lifted band remains close to the wash, giving an RCI above 75% and a high-staining label. For pigments like cobalt yellow (aureolin) or cerulean blue, the lifted band moves much closer to white paper, yielding an RCI below 30% and a non-staining label.

Figure 2 — Examples of swatches showing Non-staining, Medium, and High-staining behavior after identical lifting cycles.

This system aligns with definitions used by Winsor & Newton, Daniel Smith, and Schmincke, but provides a quantitative scale rather than a visual guess. It’s also reproducible: any lab with the same procedure and instrument can replicate the numbers.

Alternate visual labeling—and why we don’t publish it

During early testing, we explored a “visual” rule that combined absolute residue (the ΔE between the lift and paper) with hue-specific adjustments and changes in lightness (ΔL). This approach produced labels that matched visual impressions for some colors—especially yellows and oranges that appear bright even when most pigment has been removed—but it added subjective judgment. Because different hues appear lighter or darker at the same chroma, this rule cannot guarantee cross-color consistency. We therefore keep it as an internal reference but rely on the RCI scale for published results.

Figure 3 — Numeric (RCI) vs. Visual (Hue-Aware) Labeling illustrates how two different methods for classifying watercolor staining can produce slightly different results.

On the left, the green bars represent pigments ranked purely by their Residual-Color Index (RCI%), the quantitative value derived from color-difference measurements. This scale reflects how much of the original color remains after lifting, independent of hue or brightness. Higher RCI % means more color remaining (high-staining), and lower RCI % means most color has lifted (non-staining).

On the right, the red bars show how the same pigments might be judged using a visual or hue-aware approach, which adjusts classifications based on how the color appears to the eye. For example, yellows or light earth colors may look more intense than their measured RCI suggests, so they might be placed higher (more “staining”) in a purely visual system.

The figure demonstrates that while the visual approach can seem intuitive, it introduces inconsistencies between hues. The numeric (RCI) method, though more technical, provides a fair, repeatable comparison across all colors—showing why Natural Pigments chose the RCI-based system for its published staining ratings.

Why we use this method

This approach bridges science and studio practice. It captures what artists mean by staining—resistance to lifting—without depending on human perception of brightness or color memory. The numbers are not abstractions; they describe what painters see when a wash resists the brush and sponge. At the same time, because they are based on repeatable measurements, they allow us to compare colors precisely and publish data that any artist can verify.

When we say a pigment is High-staining, we mean it in both the artist’s and the scientist’s sense: the lifted color remains close to its original wash, difficult to remove, and rich in chroma. When we call it Non-staining, it truly lifts back toward paper white. The Residual-Color Index puts numbers to those familiar experiences.

References

Daniel Smith. “Watercolor for Beginners – Watercolor Properties.” Daniel Smith Artists’ Materials. Accessed October 2025. https://www.danielsmith.com/tutorials/watercolor-for-beginners/

Daniel Smith. “Making Sense of Staining, Sedimentary and Transparent Pigments.” Daniel Smith Artists’ Materials. Accessed October 2025. https://www.danielsmith.com/tutorials/staining-sedimentary-transparent-pigments/

Daniel Smith. “Watercolor Pigment Characteristics – Staining.” Daniel Smith Artists’ Materials. Accessed October 2025. https://www.danielsmith.com/daniel-smith-watercolor-pigment-characteristics-chart/

Holbein Works Ltd. “Artists’ Watercolor.” Holbein Works Ltd. Accessed October 2025. https://www.holbein.co.jp/en/products/watercolor/artists-watercolor.html.

Holbein Works Ltd. “Artists’ Watercolour – Tubes & Sets (Specification listing includes ‘Staining’ values).” Jackson’s Art (product page). Accessed October 2025. https://www.jacksonsart.com/en-us/holbein-artists-watercolour-paint-15ml-neutral-tint (or raw sienna example).

MacEvoy, Bruce. “What the Ratings Mean: St- (Staining) & VR (Value Range).” Handprint. Accessed October 2025. https://www.handprint.com/HP/WCL/pigmt8.html

MacEvoy, Bruce. “Watercolor Palette: Staining.” Handprint. Accessed October 2025. https://www.handprint.com/HP/WCL/palette2.html#staining.

Maimeri. “MaimeriBlu Superior Watercolors – Staining Colors.” ArtMaterialEssentials.com. Accessed October 2025. https://artmaterialessentials.com/maimeri.html.

Natural Pigments Laboratory. Watercolor Properties Evaluation. Willits, CA: Natural Pigments, 2025.

Natural Pigments. “Rublev Colours Lac Water Varnish.” Accessed October 2025. https://www.naturalpigments.com/lac-water-varnish.html.

Schmincke. “HORADAM Aquarell.” Schmincke Website. Accessed October 2025. https://www.schmincke.de/en/products/horadam-aquarell.

Winsor & Newton, “Professional Watercolour – Composition & Permanence Tables,” includes a note: “In water colour, … colours with [staining properties] cannot be lifted completely with a damp sponge and are marked ‘St’.” https://www.winsornewton.com/pages/professional-watercolour

An educational page on their site (“How to lift watercolour”) states: “Non-staining watercolours are likely to settle on the paper after the water has evaporated… once dried, these colours can be lifted off to reveal the white paper beneath them.” https://www.winsornewton.com/blogs/guides/how-to-lift-watercolour

5Barite (also spelled baryte) and its synthetic form, blanc fixe (precipitated barium sulfate), are found in historic and modern oil paints. Artists add it to adjust body, transparency, tinting strength, and sheen. Conservation scientists also use its presence to help date artworks and explain some aging behaviors. This article gathers verified research so you can decide when barite is the right tool in your studio.

Key takeaways for artists

• Barite is highly inert and compatible with drying oils and most pigments (CAMEO, 2022; Eastaugh et al., 2004).

• It lowers tinting strength and hiding power, giving smoother blends and softer chroma without significant oil additions (Eastaugh et al., 2004).

• It usually improves yellowing and oil-bleed control when used moderately in oil paint bodies; however, very high extender loads can give fragile, matte surfaces (Bonaduce et al., 2012; Mayer, 1991).

• Specific mixtures can age poorly under strong light (for example, chrome yellow with sulfate-bearing extenders, and some early cadmium yellow formulations) (Monico et al., 2011; Monico et al., 2013; Monico et al., 2020).

What is barite and blanc fixe?

Barite is the mineral barium sulfate (BaSO₄). Blanc fixe is the fine, precipitated synthetic analogue. Both are white, dense powders with low refractive index relative to oil, so they appear nearly transparent in linseed oil (CAMEO, 2022). Their oil absorption is low compared to chalk, so they add body without demanding much additional binder (Eastaugh et al., 2004).

Historical and modern roles

From the late eighteenth century onward, barite was used as a low-toxicity extender and as part of mixed whites (for example, Venice, Hamburg, and Dutch white), and as a support for organic lake pigments (Eastaugh et al., 2004). In modern practice, it remains a common extender, used with titanium white, or as a bodying filler in colors where handling or gloss needs adjustment (Mayer, 1991; CAMEO, 2022).

Conservation studies frequently report barite in nineteenth-century double grounds, including those used by Vincent van Gogh; the barite often contains natural strontium from celestite, which can fingerprint sources (Marino et al., 2006; Janssens et al., 2013; Van Gogh grounds study [verification required]).

Visual and handling effects in oil paint

Barite reduces tinting strength and hiding power, allowing more subtle mixtures and smoother gradations. Because it is hard and fine, small additions can help break pigment agglomerates during grinding and impart a short, buttery feel in paste (Eastaugh et al., 2004). It also tends to lower gloss and produce a more matte surface as loading increases (Mayer, 1991).

Practical upshot: If a color is too strong, too glossy, or too long and stringy, a modest barite addition usually solves it without flooding the paint with oil (Eastaugh et al., 2004; Mayer, 1991).

Stability and aging: the good news and the caveats

Inert by default. Barium sulfate is chemically stable and generally does not participate in oil reactions or form soaps. It does not blacken lead white as sulfur gases do, because it already is a sulfate (CAMEO, 2022; Eastaugh et al., 2004).

Oil yellowing and bleeding. Mock‑ups show that extender pigments can influence yellowing and oil migration. Aging studies of linseed oil reconstructions indicate that appropriate extender use can reduce oil exudation and limit yellowing; however, results vary with formulation, thickness, and light exposure (Bonaduce et al., 2012; Mallégol et al., 2001). Therefore, use moderate loads and favor good drying conditions.

Light‑sensitive mixtures. The most important caution concerns mixtures with certain yellow pigments:

• Chrome yellow (lead chromate). Under light, chrome yellow can reduce to darker chromium(III) species. Studies link faster darkening to paints where chrome yellow coexists with sulfate-rich phases (including barium sulfate) (Monico et al., 2011; Monico et al., 2013). Keep such combinations out of high‑UV light.

• Cadmium yellow (cadmium sulfide). Several teams have reported cadmium sulfate and related products in degraded cadmium yellows. Sulfate-bearing extenders such as blanc fixe may contribute to these pathways in some formulations (Monico et al., 2020; Comelli et al., 2019). Use caution with high loads of blanc fixe in cadmium yellows.

Analytical and dating value

Barite is clearly visible in elemental and imaging analyses (e.g., X‑ray fluorescence mapping and NanoSIMS). Its appearance in grounds and tube colors after the late eighteenth century helps authenticate and date works. Case studies on Van Gogh’s grounds identified barium sulfate with strontium impurities, consistent with natural barite (Marino et al., 2006; Janssens et al., 2013).

When (and when not) to use barite in the studio

Smart uses

• Taming high‑tint colors. Add a small percentage to phthalocyanine blues and greens, quinacridones, or other powerhouse colors to gain control without chalking the color (Eastaugh et al., 2004).

• Softening gloss. Mix into very glossy colors to create a more satin surface (Mayer, 1991).

• Strengthening paste without extra oil. Improve brush handling of oily or long paints while keeping the oil fraction reasonable (Eastaugh et al., 2004).

• As a lake substrate. Use blanc fixe as a substrate when making organic lakes; it is inert and does not shift hue (CAMEO, 2022).

Use caution

• Chrome yellow or cadmium yellow mixes. Limit blanc fixe loads and avoid harsh lighting, as research links sulfate‑bearing environments to specific degradation routes (Monico et al., 2011; Monico et al., 2013; Monico et al., 2020).

• Very high extender loads. Extremely matte, porous surfaces can collect grime and may be mechanically weaker; keep loads moderate and test (Mayer, 1991).

Working methods and simple tests

1. Bench mix first. Start at a 5–10% barite by weight of pigment in the color and test drawdowns at your normal film thickness. Increase gradually if needed (Mayer, 1991).

2. Check mass‑tone and tints. Compare a 1:1 white tint series with and without barite to confirm that the color still lands where you want it chromatically (Eastaugh et al., 2004).

3. Watch gloss and oil exudation. After a week and one month, inspect for surface bleed or hazing. Adjust load or oil type as needed (Bonaduce et al., 2012).

4. Light test sensitive mixtures. If using chrome yellow or cadmium yellow, expose a masked strip to a window for four weeks alongside a control. If darkening or chalking appears, lower the blanc fixe content and consider display limits (Monico et al., 2011; Monico et al., 2020).

Quick comparison: barite versus common extenders

Safety, storage, and disposal

Barium sulfate is considered non‑toxic due to its extreme insolubility. Nevertheless, avoid inhaling dust or ingesting powder. Use a particulate respirator when weighing fine powders, and work under local exhaust if possible. Clean spills with a HEPA vacuum. Do not wash significant quantities down the drain; collect dry waste and dispose of it in accordance with local regulations (NOAA CAMEO Chemicals, n.d.; PubChem, 2025).

Bibliography

Bonaduce, Ilaria, Maria Perla Colombini, Luciana Duce, et al. “New Insights into the Ageing of Linseed Oil Paint Binder.” Applied Physics A 111, no. 1 (2012): 119–127. https://doi.org/10.1007/s00339-012-7467-z.
Annotation: Analytical study of naturally and artificially aged linseed oil reconstructions. Useful for understanding oil migration, yellowing, and binder changes relevant to extender use.

CAMEO. “Barium Sulfate.” Museum of Fine Arts, Boston, last modified May 2, 2022. https://cameo.mfa.org/wiki/Barium_sulfate.
Annotation: Concise, vetted entry covering properties, synonyms (barite, blanc fixe), and typical applications. Good starting point for safety and identification.

Comelli, Daniela, et al. “Degradation of Cadmium Yellow Paint.” Analytical Chemistry 91, no. 10 (2019): 6465–6473. https://doi.org/10.1021/acs.analchem.8b04914.
Annotation: Case‑based spectroscopy on cadmium yellow degradation. Relevant where sulfate phases are present and for assessing extender risks with cadmium colors.

Eastaugh, Nicholas, Valentine Walsh, Tracey Chaplin, and Ruth Siddall. Pigment Compendium: A Dictionary and Optical Microscopy of Historical Pigments. London: Archetype Publications, 2004.
Annotation: Standard reference on historical pigments and extenders, including barite and blanc fixe. Details on names, optical traits, and historical uses.

Janssens, Koen, et al. “The Use of Synchrotron Radiation for the Characterization of Artists’ Pigments and Paintings.” Journal of Analytical Atomic Spectrometry 28, no. 1 (2013): 1–19. https://pubmed.ncbi.nlm.nih.gov/23772661/.
Annotation: Review of synchrotron‑based methods, including X‑ray fluorescence mapping that reveals barite distributions in grounds and paints.

Mallégol, Jacky, Jacques Lemaire, and Jean‑Luc Gardette. “Yellowing of Oil‑Based Paints.” Studies in Conservation 46, no. 2 (2001): 121–131. https://doi.org/10.1179/sic.2001.46.2.121.
Annotation: Classic study on oil yellowing mechanisms and variables. Helps artists contextualize extender choices regarding yellowing.

Marino, Elise, et al. “Imaging TOF‑SIMS and NanoSIMS Studies of Barite–Celestite in a Painting by Van Gogh.” e‑Preservation Science 3 (2006): 11–16. https://www.morana-rtd.com/e-preservationscience/2006/Marino-06-11-06.pdf.
Annotation: Demonstrates identification of barite with strontium in Van Gogh grounds. Shows how extenders inform dating and sourcing.

Mayer, Ralph. The Artist’s Handbook of Materials and Techniques. 5th ed. New York: Viking, 1991. https://www.penguinrandomhouse.com/books/322005/the-artists-handbook-of-materials-and-techniques-by-ralph-mayer/.
Annotation: Practical guidance on extenders, handling, and film properties in artists’ paints. Offers studio‑level context for barite use.

Monico, Letizia, et al. “Degradation Process of Lead Chromate in Paintings by Vincent van Gogh: Part I. Artificially Aged Model Samples.” Analytical Chemistry 83, no. 4 (2011): 1214–1223. https://pubmed.ncbi.nlm.nih.gov/21314201/.
Annotation: Shows how sulfate phases influence chrome yellow darkening under light. Important caution for using sulfate‑bearing extenders with lead chromate.

Monico, Letizia, et al. “Degradation Process of Lead Chromate in Paintings by Vincent van Gogh: Part IV. Artificial Aging of Model Samples of Co‑Precipitates of Lead Chromate and Lead Sulfate.” Analytical Chemistry 85, no. 2 (2013): 860–867. https://pubmed.ncbi.nlm.nih.gov/23051631/.
Annotation: Expands the mechanistic picture of chromate reduction in sulfate‑rich environments; underscores the role extenders can play in mixed systems.

Monico, Letizia, et al. “Probing the Chemistry of Cadmium Sulfide Paints in The Scream by In Situ Noninvasive Techniques.” Science Advances 6, no. 20 (2020): eaba4017. https://pmc.ncbi.nlm.nih.gov/articles/PMC7228759/.
Annotation: Documents cadmium yellow alteration products, including cadmium sulfate. Relevant when blanc fixe is present with cadmium sulfide paints.

NOAA CAMEO Chemicals. “Barium Sulfate.” Accessed October 27, 2025. https://cameochemicals.noaa.gov/report?key=CH25000.
Annotation: Safety and reactivity overview for barium sulfate. Useful for handling, storage, and disposal planning.

PubChem. “Barium Sulfate.” National Center for Biotechnology Information, accessed October 27, 2025. https://pubchem.ncbi.nlm.nih.gov/compound/Barium-Sulfate.
Annotation: Basic chemical data, manufacturing notes for blanc fixe, and identifiers; supports safety and sourcing decisions.

[Van Gogh grounds study: “Van Gogh’s Painting Grounds: An Examination of Barium Sulfate Extender Using Analytical Electron Microscopy.” Authors and year not fully verified; ResearchGate abstract accessed October 27, 2025. Link: https://www.researchgate.net/publication/227071458_Van_Gogh%27s_painting_grounds_An_examination_of_barium_sulphate_extender_using_analytical_electron_microscopy_-_SEMFIBTEMEDX.
Annotation: Frequently cited in technical literature; indicates strontium‑bearing barite in Van Gogh grounds. Include once a stable citation is secured.