A clay material that is used as the preparatory layer for the application of very thin layers of metal leaf, most commonly gold or silver. Bole is prepared by mixing colored clay with a water-based adhesive (such as rabbit-skin glue). Although traditional boles are usually dark red in color, many variations can be found, ranging from white to light yellow to deep violet. Because metal leaf is very thin, the color of the bole has a substantial effect on the appearance of the gilding. In pictures that have suffered deom cracks or abrasion in gilt areas, the bole can often be seen in areas where the metal leaf is missing.
In water gilding, the bole mixture is applied to the area to be gilded and allowed to dry. A smooth piece of agate may be used to burnish the surface of the colored clay. The bole is then re-wetted in order to activate the adhesive, and the metal leaf is applied. The bole layer may also be referred to as the poliment (a term used to describe a preparation layer for gilding).
Source: Looking at Paintings: A Guide to Technical Terms / eds. Tiarna Doherty and Anne T Woollett (Los Angeles: J. Paul Getty Museum, 2009), p. 7.