What's Really in Your Oil Paint

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

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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.

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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.

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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.

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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.

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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.

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