PLASTER (IN ART). - Chemically, gypsum is hydrated calcium sulfate, poorly soluble in water. This solubility increases, however, as the temperature rises. Its use in the figurative arts is of great importance and of various kinds. First of all, it is used by sculptors both for preparing sketches to be transferred into marble and metal or into terracotta, and directly in the final execution of the work, since it is more solid than wax and virgin clay, even if less stable than stone and metal. In the Middle Ages, beginning in the 13th century, one finds tabernacles, pulpits, ciboria, and chancel screens in gypsum, often of exquisite workmanship, and many Italian regions, especially Latium and the Abruzzi, are enriched with works executed in gypsum.
Medieval plasterers carved gypsum in the same way as stone, and often the decorative structures had a stone core to compensate for the poor solidity of gypsum.
It is more difficult to find gypsum used in sculpture in the round; into this category, however, fall the casts, which are made for didactic and popularizing purposes, especially for sculptures of antiquity. Important collections of casts exist: among them notable are the Museums of Gypsum Casts of Rome, at the University, and of Paris, with the casts of the tomb monuments of the kings of France.
To make a cast of a sculpture, gypsum is used twice: first to build on the original the master mold or shell that will contain the reproduction, and then to pour into the former the paste that will model the copy. The first must be of a more solid quality and of easier "set" and is called molders' gypsum; the second, on the other hand, is more fluid and crystalline and is called alabaster gypsum.
In painting, gypsum was used to prepare the ground on panels and, later, the priming on canvases. Amalgamated in glue, it was spread on the panel (v. the treatise on painting by Cennino Cennini) with the brush, so as to form a smooth and uniform bed for the color. On canvases too the priming of gypsum and glue is applied by the same procedure and serves similarly to level the irregularities of the weave, as well as to create the foundation for a solid impasto of colors.
In the restoration of paintings, gypsum has a very important function. Diluted hot in glue, it serves to fill the gaps in the original parts, which are then integrated in colors in neutral zones or toned to those surrounding them, according to various procedures.