Borghini, in describing the method of making a gold-size (the same as Cennini's), speaks of boiling the "buccie de' colori" in oil; this only means the skin or pellicle of the colour itself—in fact, he proceeds to say that they dissolve in boiling. Vasari, in describing the same process, uses the expression "colori seccaticci."

"Maggio 4 (1520) Per un cadin (catino) per depentori. Per scudellini per li depentori."—Mem. Trev., vol. i. p. 131. Pungileoni ("Memorie Istoriche di Antonio Allegri") quotes a note of expenses relating to two oil-pictures by Paolo Gianotti; among the items we find "colori, telari, et brocchette."—vol. ii. p. 75.

Salmon, in his "Polygraphice" (1701), gives the following direction:—"Oyl colors, if not presently used, will have a skin grow over them, to prevent which put them into a glass, and put the glass three or four inches under water," &c.

This varnish appears to have been known some centuries before Van Eyck's time, but he may have been the first to mix it with the colours.