Any one of several varieties of friable earthy clay, usually colored more or less strongly red by oxide of iron, and used to color and adulterate various substances. It was formerly used in medicine. It is composed essentially of hydrous silicates of alumina, or more rarely of magnesia. See Clay, and Terra alba.
A clay in liquid form that can be used over gesso before gilding. Bole can vary in colour from red, yellow and grey.
A fine colored clay applied to a work of art to support gold leaf. The extremely fine particle size allows the gold to be rubbed or burnished to create a bright metallic surface. Because the gold leaf is so thin it is somewhat transparent and its color is modified by the bole color. Red bole , the most common color, adds to it warmth.