To produce the colors of classical art, Greek and Roman artists took three approaches: they used the natural color of their materials, manipulated the surfaces of these materials through various technical processes, and manufactured pigments and dyes to apply over the original surfaces of the artworks. Although the dramatic effects of ancient color are often lost to us today, traces of original surfaces and added pigments provide vivid evidence for the practice of polychromy.
Different materials required different techniques for adding or manipulating color. Naturally colorful material, such as stones, gemstones, and precious metals, was left relatively unchanged. Many materials, however, required artistic intervention to add, modify, or change color. For example, alloyed metals, such as bronze, could be manipulated by altering the proportions of the metals or chemically treating its surface, while glass could be colored by the addition of minerals to the relatively colorless base materials, silica (sand or quartz) and soda (lime). The addition of colored slips (liquid clay) to unfired terracotta, furthermore, allowed color to be bonded to clay surface when it was fired, rendering it more durable; similarly, dyes were absorbed into the fibers of textiles. Finally, pigments were added to the surfaces of buildings, walls, and statuary, creating visual effects that varied from lively polychromy to naturalistic likeness. Pigments were composed of both organic (typically plant) and inorganic (typically mineral) ingredients; the latter are usually better preserved today.