Bole is a clay which contains oxides and due to their typology they give various color shades.
The presence of the iron oxide is very typical, giving the red color but they may be also yellow or black. These clays are very similar to ochre but they differs from the latter, as they do not contain any carbonate. Therefore they have the typical oily feature and for the same reason a very different colloidal system.
It is a basic product for the "Guazzo", the water gilding technique. It is generally used as a base coat, above all the red bole as it affects substantially the color intensity of the gold leaf. It scatters the green light and absorbs the red wavelengths of the same: the red itself of the bole gives a renewed chromatic balance to the gold adding what is called more warm shade.
The use of bole in the various colors achieves particular chromatic effects when you scratch partially the leaf out from the gilded surface, and let the colored base show through.
Today there are a lot of synthetic products which recreate the characteristics of the natural bole.