BoleA kind of clay, usually red due to the presence of iron oxide, used as an underlayer prior to the application of gold leaf.
The bole was placed between the white ground and the gold leaf. It has the effect of making the gold appear more 'golden' by adding a reddish tint. Without it the thin gold leaf can appear to have a greenish tinge.
The National Gallery, Trafalgar Square, London WC2N 5DN