Plastering has been around since olden times. The Egyptians plastered their houses and tombs with a lime-gypsum material similar to modern plaster. The Greeks and Romans also recognized the sanitary value of lime and coated walls with it to prevent the spread of disease.
In the Middle Ages, however, the level of craftsmanship dropped. Instead of lime and gypsum, a mixture of clay and dung with horsehair as a binder was used to plaster walls. But by the end of the 13th century traditional methods and materials resurfaced, and in the 15th century the craft reached new heights as artisans used plaster not simply as a wall covering, but also for ornamental molding and sculpture.In this century wallboard, rigid boards of gypsum plaster bound on both sides with paper, substituted for traditional plastering. In almost all houses built after 1945 the interior walls are of wallboard, which is also known as drywall and Sheetrock.
Older homes, however, have walls of plaster applied in the traditional three-coat system. First, thin strips of lath were nailed to the framework. The first, or scratch, coat of plaster, usually a mixture of lime, gypsum, sand and, sometimes, horsehair, was then troweled on to the lath. This "keyed" to the lath. It penetrated the slots between the lath and hardened to form a bond. But before it was completely dry the plasterer scratched it with a sharp stick or metal scratcher to help the second coat bond. That coat, applied after the scratch coat had dried, was made of gypsum and sand.
After the brown coat had dried, a third and final coat was applied. That was a lime-gypsum mixture that produced a smooth, hard surface.