The main ingredient in most modern plaster mixes is crushed gypsum stone, according to Big Wally?s Plaster Magic. Gypsum stone dries faster than traditional lime-based plasters and usually doesn't need extra fiber binders added to the mix. Modern drywall and plaster of Paris are also gypsum-based.
Historically plaster was made of various materials, explains HowStuffWorks, and it has been used since ancient times to finish walls. Early plaster mixes were made with lime, sand, animal hair and water. The plaster walls found in many old American homes used a similar mixture up until the early 20th century. Other traditional plaster mixes also included cement, clay and mud. Builders would often make plaster out of whatever supplies they had at hand, states Big Wally?s Plaster Magic. In Louisiana some builders even mixed swamp mud, moss and deer hair to make plaster.
The extra ingredients in traditional plaster mixes, such as animal hair, were added to bind the lime-based plaster. However, gypsum-based plaster mixes do not need extra binders due to the particular type of crystalline particles found in gypsum stone. Most builders no longer use plaster to finish walls and instead use drywall, which is much faster to install and requires less skill to create a smooth surface.