What is Gypsum Plaster?

What is Plaster?

Plaster is a building material that is used to coat walls, ceilings, and other surfaces. It is made from a variety of materials, including clay, lime, gypsum, and cement. Plaster is a versatile material that can be used for many purposes, including:

  • Interior wall finishing
  • Exterior wall finishing
  • Molding and casting
  • Fireproofing

The History of Gypsum Plaster

The history of gypsum plaster dates back to ancient Egypt, where it was used to build the pyramids. The Egyptians also used gypsum plaster to create sculptures and decorative objects. In ancient Greece and Rome, gypsum plaster was used to build temples and other public buildings.

Gypsum plaster continued to be used throughout the Middle Ages and Renaissance. It was used to build castles, churches, and other structures. In the 19th century, gypsum plaster became more widely available due to the Industrial Revolution. This led to an increase in the use of gypsum plaster for residential and commercial construction.

How is Gypsum Plaster Made?

Gypsum plaster is made from gypsum powder. When gypsum powder is mixed with water, it forms gypsum plaster, otherwise known as plaster of paris. As it dries, it creates a hard, smooth, durable material, ideal for interior smooth wall finishes.

Gypsum powder is made from a soft white rock known by mineralogists as hydrous calcium sulfate. To create gypsum powder, hydrous calcium sulfate is heated to temperatures over 350° F (176.6° C) to remove most of the water. It is then pulverized into a fine powder.

How is Gypsum Plaster Applied to Walls?

Gypsum plaster is applied to walls in two or three coats. The first coat is called the scratch coat. The scratch coat is a thick coat of plaster that provides a solid base. The second coat is called the brown coat. The brown coat is thinner than the scratch coat and is used to even out major high and low spots. The third coat is called the finish coat. The finish coat is a very thin layer of plaster that gives the wall a smooth, finished look and feel.

Gypsum plaster must be applied over a substrate such as wooden strips called lath or metal mesh. A solid substrate gives plaster something to attach to and increases the strength of the finished product. When the scratch coat is applied it pushes through the slits of lath or metal mesh to create what are called keys. Plaster keys anchor the scratch coat to the substrate.

Gypsum plaster can be applied using a variety of tools and equipment. Trowels are used to apply plaster to the substrate and spread it evenly. When applying the scratch coat, plaster screeds are often used to smooth out large sections at a time. Screeds help ensure that the scratch coat has a consistent thickness. Once the scratch coat has been applied, plaster floats are used to smooth out small inconsistencies in the surface and create a finished look.

Pure gypsum plaster is not generally used for the scratch coat. Rather, perlite, calcium, and/or quartz are added to gypsum plaster to add strength and reduce cost.

Crushed limestone, or lime, is often added to the finish coat. Lime helps harden gypsum plaster and makes for 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.


Plaster is a versatile building material that can be used for a variety of purposes. It is very durable and can be used to create exceptionally smooth wall finishes. Gypsum plaster holds up better than drywall in humid wet environments. It is the most common type of plaster used in the United States. It is made from gypsum, which is a soft white mineral. Gypsum plaster is applied to walls in two or three coats. The first coat is called the scratch coat. The second coat is called the brown coat. The third coat is called the finish coat. It is applied using a variety of tools including trowels, hawks, floats, and screeds.

What is Drywall?