Preparing Clay Bole
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 Clay bole, a smooth, silky clay comprised mainly of hydrous aluminum silicate, is used in traditional water gilding to add color underneath the somewhat translucent gold leaf and to provide a soft cushion to the hard gesso ground to help achieve a brilliant burnish in the gilding. Clay bole is available pre-mixed and in dry cones. The color range includes red, yellow, black, white, green, and blue and may be combined together for an infinite color palette.
Bole is first mixed with distilled water to create a fluid paste. The pre-mixed version consists solely of clay and water; dry cones are tumbled for a period of time with distilled water which is much more economical if a ceramic tumbler is available.
The bole is then mixed in specific proportions with either a 10% rabbit skin glue or a food grade gelatin solution as a protein binder, then heated warm in a double boiler or similar system where the bole is kept blood warm and no more than 120F. As with the preparation of any animal glues, care must be taken so the glue is not overheated or at too high a temperature which can cause the glue to lose its strength.
It is at this point where research may uncover a variety of approaches to the preparation of clay bole. For instance, some
recipes call for a slightly weaker protein binder solution, around 8%, although for general practice I have found 10% to work well.
Some gilders prefer to combine the RSG or gelatin and bole in proportions that are judged visually. I prefer to use metric weight in an attempt to maintain consistency with each batch of bole. The metric weight version that I currently use is
1:1.67 RSG (or gelatin) to bole. A sample batch therefore would equal 25 grams of 10% gelatin (or RSG) to 42 grams of bole.
Once the bole is prepared, a little more distilled water may be added to allow it to flow more readily. A simple guideline is to make the bole thin enough so three drops of bole will flow slowly from the tip of the brush.
The bole is then ready to be applied over the gesso ground. For best results a water color brush is recommended. Synthetic bristles are fine. Anywhere from 3-5 thin layers of bole are applied. Often I will lay three coats of bole, very gently dry sand the dried coats with 1,000 grit wet and dry sandpaper, degrease the bole with isopropyl alcohol and then apply two more thin layers of bole, being careful not to overbrush too much to minimize brushstrokes.
The final dry layer of bole is then polished with a stiff bristle brush or horse hair cloth to smooth the bole and to help gain a smooth, brilliant burnish to the gold leaf.
Look for the next issue during the second half of December: Gilding with Genuine Gold Leaf
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