I think someone asked what this was a while back:
Bole: Clay mixed with rabbit skin glue for application to the
surface of an object being prepared for water-gilded. Usually the
clay is applied to a gesso surface. It is the clay surface that
receives the gold leaf. The clay and rabbit skin glue content dictate
the reflective quality of the gold finish. Clay is finer than gesso
and when burnished with a tool of hounds tooth, agate or hematite
stone its surface becomes more dense and thereby more reflective.