Lime

• Many plasters are made from a base of lime. However, the proportion of lime to other ingredients in plaster is something that varies greatly depending on who originally made the plaster mix, which is something that people restoring old plaster buildings have to consider. The normal proportion found in most plaster, however, is one part lime to three parts sand.
  
  

Gypsum

• Although lime is still used in some modern plasters, many modern plasters are made with gypsum instead. Gypsum has been the most common ingredient in plaster in the U.S. since the beginning of the 20th century. Gypsum is a building material that is found on most walls because it is the key component of drywall, the most common covering used on walls.
  
  

Sand

• Sand is one of the key components of plaster. The sand used in plaster mixes is like regular sand that you find anywhere, but it has gone through a refining and cleaning process. Uniformly small particles free of debris and contaminants make for a good sand that will make a solid and smooth plaster mixture. The amount of sand added to plaster mix needs to be equal throughout a project to get a good looking and consistent plaster job.
  
  

Other Ingredients

• Plasters have a variety of other ingredients added to the mixture to create a product that is ideal for the intended purpose. Water is the one other ingredient that must be added to all mixtures. In lime plasters, sometimes a binding agent, such as hair, is added to the mix. Other additives like brick dust or various chemicals are added to create specialized types of plasters.