muller, in painting, an instrument used in conjunction with a slab to grind artists’ colours by hand. The modern muller and slab are made from glass or stainless steel, although from ancient Egyptian times until the 18th century porphyry was invariably used. After the introduction of the mechanical paint mill in the middle of the 19th century, the muller and slab became obsolete except when small quantities of pigment had to be ground.