Sculpture techniques: surface decoration
The rough surface of European limestone and sandstone sculptures was usually concealed by paint, gilding, or inlaid glass or semi-precious stones. Marble sculpture was rarely decorated because a plain white surface like those of classical Greek and Roman marble sculptures was considered the most desirable finish. In fact these classical sculptures would probably have originally been painted, but by the time they were rediscovered in post-classical times, the paint had worn away.
Wood sculpture was also painted, gilded, or decorated with incised and punched patterns and occasionally inset glass, though at the end of the 15th century, sculptors in southern Germany produced unpainted limewood sculptures.
The appearance of bronze sculptures could also vary, according to the particular alloy used and the surface treatment. Chemical and organic lacquers can produce a variety of colours including black, green and reddish-brown. Bronzes could also be gilded, sometimes using the fire-gilding method. In this process a paste of milled gold and mercury is spread over the sculpture which is then heated, fusing the gold to the surface and driving off the mercury. Fire-gilding is now outlawed in most countries due to the toxicity of mercury vapour. Gold lacquer can also be used to give a gilded appearance, though it is less permanent than fire-gilding.
In Renaissance Italy, terracotta (fired clay) sculptures were usually coloured. In the 15th century, Luca della Robbia, a sculptor from Florence (1399/1400-1482), developed a method of tin-glazing terracotta sculpture that had previously only been used on pottery. Della Robbia's method used pigment suspended in a tin-oxide glaze rather than painted onto slip. The resulting enamelled terracottas were particularly durable and had an attractive, bright and reflective, surface. By the late 18th century however, unglazed terracotta had become popular with many European artists such as the French sculptor Clodion (1738-1814).