Bole
A fine, soft clay, usually red, laid under gold leaf to give it warmth and a surface for polishing.
Also: Armenian bole, gilder's clay
Bole is a smooth clay rich in iron, most often a brick red, though yellow, grey and black kinds are used too. Gilders mix it with glue and brush it over gesso before laying gold leaf. It provides a slightly cushioned bed, so the gold can be rubbed to a mirror shine with a polished stone. Because gold leaf is thin enough to be slightly see-through, the red beneath warms its colour. On old frames and icons, the red bole often shows where the gold has worn away.