What is Plaster?
Plaster can be made from different types of material. Plaster made of clay, lime, gypsum, or cement was commonly used in different parts of the world in building construction throughout history. Gypsum plaster is the type of plaster most people think of when referring to interior wall finishing in the United States. Although some clay plaster is found in early American construction, from the 1800’s on until today, gypsum is by far the most common.
How is Gypsum Plaster Made?
Gypsum plaster is created by heating gypsum (a soft white mineral rock) to remove most or all of the water and crushing it into a fine powder. When gypsum power is re-mixed with water it turns into liquid plaster slurry known as plaster of Paris. Plaster of Paris can be molded into a variety of things. As it dries, it creates a hard, smooth, durable material that is ideal for interior smooth wall finishes.
How is Gypsum Plaster Applied to Walls?
Since liquid slurry cannot span between 16 inch wide wall studs, it needs a base upon which to be spread. The first step in plastering a wall involves installing a substrate. There are different types of substrate that can be used, including gypsum boards or masonry. A very common method for plastering on wood stud framed walls is to install a substrate of wooden strips or metal mesh across the studs. This substrate of wooden strips or meshing is called lath. Lath provides support for the first coat of plaster as it dries.
Pure gypsum plaster is not generally used for the first layer over lath. Rather, perlite, calcium, and/or quartz are added to gypsum powder to create a base-coat material. The addition of sand adds strength to the gypsum plaster and reduces cost. Base-coat plaster looks and acts similar to Portland cement. The method of application is similar to stucco. Base-coat plaster is sometimes called a brown coat partly because it is not white like pure gypsum. Two base-coats of plaster, one called a scratch coat and the other called the brown coat, are needed when plastering over wood lath or metal mesh. The scratch coat pushes through the slits between strips of wood to create what are called “keys” that anchor the base-coat and top coat layers of plaster to the substrate.
After the base-coat of plaster is dry, one or two finish coats of gypsum plaster is spread over top of the brown coat to form to a perfectly smooth surface. Gypsum plaster mixed with lime is used for the final coat of plaster. Lime helps harden the gypsum which leads to a more durable surface. Pre-mixed top coat plaster uses Dolomite rather than Limestone as the alkaline calcifying agent. Only a thin layer of top coat plaster is needed. The finish coat of plaster fills in any pits and creates a very smooth wall.
The term plaster can refer to a variety of materials. Modern interior finishing systems in the United States rely primarily on Gypsum based plaster. Even among Gypsum plaster products, there is great variety in the amount and type of additives depending on the desired durability or substrate used. Don’t underestimate the complexity of choosing modern plaster materials and techniques. It takes a true expert to design a Plaster system that fits specific needs.