Apply the Boles
After all the coats of gesso have been applied, several coats of pigmented clays, called boles, are applied to the surface. The layers of clay provide additional cushioning for the gold leaf, and the pigment adds luster to the finished piece. Boles come in different colors, including black, yellow and red. The luster provided by a black bole base is popular in the United States, whereas yellow topped by red is more common in Europe. Rabbit-skin glue is added to the dry clay pigment to create a solution that is brushed on the piece in layers. The piece receives five or six coats of bole, and each must be allowed to dry overnight.