Water Gilding with Bole: Applying and Burnishing Gold Leaf

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.