I know that the answer to the question "What was rubia tinctorum sometimes mixed with?" is in "The painters of the early Renaissance used two traditional lake pigments, made from mixing dye with either chalk or alum, kermes lake, made from kermes insects, and madder lake, made from the rubia tinctorum plant. With the arrival of cochineal, they had a third, carmine, which made a very fine crimson, though it had a tendency to change color if not used carefully. It was used by almost all the great painters of the 15th and 16th centuries, including Rembrandt, Vermeer, Rubens, Anthony van Dyck, Diego Velázquez and Tintoretto. Later it was used by Thomas Gainsborough, Seurat and J.M.W. Turner.". Can you tell me what it is?
Answer:
either chalk or alum