Plaster

Plaster is a cement-like material that primarily contains gypsum (CaSO4 2H²O) or lime. It may also contain aggregate or fibers (horse hair or fiberglass) to stabilize and strengthen the compound. "Gypsum was introduced in the United States in 1785 by Benjamin Franklin.  He had encountered the material known as 'Plaster of Paris' in France, where it was used as a wall finish, casting material and soil nutrient." Drywall is essentially pre-manufactured boards of compressed plaster with a paper skin.

When plaster is applied to a (wood, rock or metal) lath substrate on walls and ceilings, it is applied in two or three coats.  The first or scratch coat is a sand, cement and gypsum or lime mix.  The second coat of  the same material is then applied after the first coat has dried.  The final coat is a white coat that contains white plaster and a high lime content that forms a smooth, hard surface.

Plaster-Drywall Hybrid – 1/2-inch gypsum blue-board with a hard skim coat of plaster provides a gypsum board base and the rock hard finish of real plaster. This material is used in some custom construction when a buyer does not want drywall. At approximately $1.75 per SF, its cost is roughly 50% more than finished drywall and approximately 40% less than three-coat plaster.