Red clay pigment applied as an under-layer prior to the application of gold leaf in panel painting. It has the effect of making the gold appear more “golden” by adding a reddish tint.

Allegretto di Nuzio. Crucifixion (det.). Tempera on wood, gold ground. 1345-74. Metropolitan Museum of Art, New York. The gold leaf has worn away in places, revealing the bole beneath.