What is Plaster?

Photo by Philip MacKenzie
Photo by Philip MacKenzie

A putty-like material, plaster is applied to walls and ceilings to provide a smooth, uniform, airtight surface. Exterior plasterwork is known as stucco or rendering.

Plaster is made by mixing suitable quantities of quicklime (obtained from lime by calcination), water and white sand. The paste is applied in three successive coats to ensure uniform hardening throughout the thickness of the plaster. The surface to be plastered must have suitable grip. Plaster can be applied directly onto a masonry wall but not onto a solid wall. Wooden or metal laths are used to provide grip if necessary. Each coat is roughened to provide grip for the next, while the finishing coat, usually made up of slaked lime, sand, plaster of Paris and water, is troweled carefully to give a smooth surface.

Plaster of Paris is a quick setting substance obtained by heating finely crushed gypsum. When dehydrated, gypsum produces a powder which, when mixed with water, sets rock hard. It may be chemically treated to reduce its fast setting quality. Plaster of Paris is used in the manufacture of gypsum lath, plasterboard (a substitute for plastered walls) and casts and molds. The Egyptians and Greeks used plastered walls as surfaces for painted decorations (frescoes), while the Romans carried the medium further and created relief ornamentation in plaster. The Romans took their techniques to Britain where in medieval times plastering was used in buildings as a protective measure against fire. Roman relief work was copied by artists of the Italian Renaissance and Henry VIII introduced their craft to England. The influence of Italian plasterwork can still be seen today.

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