painting executed on a rigid support—ordinarily wood or metal—as distinct from painting done on canvas. Before canvas came into general use at the end of the 16th century, the panel was the support most often used for easel painting. A variety of woods has been used, including beech, cedar, chestnut, fir, larch, linden, white poplar, mahogany, olive, dark walnut, and teak. Wooden panels were usually boiled or steamed to remove gum and resin and thereby prevent splitting and then were coated with size (a glutinous material) to fill pores and with gesso (a mixture of glue and whiting), on which the painting was executed. Metals used for panel paintings include silver, tin, lead, and zinc. During the Middle Ages, especially in Russia, paintings were executed on panels over which leather had been stretched.