Plaster results from the calcination of gypsum (CaSO4, 2 H2O), which partially dehydrates to produce a hemi-hydrate (CaSO4 , ½ H2O).

The oldest traces of plaster renders are 9,000 years old, and were found in Anatolia and Syria.  We also know that 5,000 years ago, the Egyptians burnt gypsum in open-air fires, then crushed it into powder, and finally mixed this powder with water to make jointing material for the blocks of their monuments, such as the magnificent Cheops Pyramid for example. The ancient Egyptians used models of plaster taken directly from the human body.

The Greeks also used gypsum, in particular as window for their temples when it was of a transparent quality ("selenite gypsum"). The writer Theophraste (372-287 BC) described quite precisely the fabrication of plaster as it was done at that time in Syria and Phenicia.

The Romans cast in plaster many thousands of copies of Greek statues.

Plaster of Paris. Throughout the centuries, expertise was gained in many parts of the World with gypsum calcinations. In the 1700's, Paris was already the "capital of plaster" ("Plaster of Paris") since all the walls of wooden houses were covered with plaster, as a protection against fire. The King of France had enforced this rule after the big London fire literally destroyed this city in 1666. Large gypsum deposits near Paris have long been mined to manufacture… "Plaster of Paris".  

From Gypsum to Plaster of Paris. Gypsum is a sedimentary rock, which settled through the evaporation of sea water trapped in lagoons. According to the nature of its impurities, gypsum can show various colors, ranging from white to brown, yellow, gray and pink.

Gypsum selection and preparation (cleaning, classifying) are key factors to produce the best plasters. The chemical reaction is :

Several processes are available to calcinate gypsum into Plaster of Paris. We can distinguish two categories :

1st: Calcination under atmospheric pressure to produce Beta plaster ;

2nd: Calcination  under elevated pressure to produce Alpha plaster.

Controlling some critical calcination parameters is essential to master the growth of the plaster crystals. And the performance of the plaster depends a lot on its crystals' sizes and shapes.

Plaster of Paris is a calcium sulfate hemi-hydrate : (CaSO4, ½ H2O) derived from gypsum, a calcium sulfate dihydrate (CaSO4 , 2 H2O), by firing this mineral at relatively low temperature and then reducing it to powder. Calcination of the gypsum at higher temperatures produces different types of anhydrites (CaSO4), as shown on the table below

HISTORICAL USE OF GYPSUM PLASTER

Gypsum plaster is not a modern invention like Portland Cement, as some people might suggest. We know that it was used by the ancient Egyptians to plaster the pyramid at Cheops. In Britain, research being carried out by Claire Gapper, a PhD student at the Courtauld Institute, indicates that considerable quantities of Plaster of Paris were being imported from France during Henry VIII's reign for work on royal properties.

Our knowledge of the use of gypsum plaster prior to the 19th Century is limited. However Claire Gapper's research shows that it was being used in the 16th Century with lime in floors, walls and ceilings, but decorative plasterwork, which was previously assumed to contain gypsum, is proving to contain only minute traces; the sort of levels at which one would find it as an impurity in limestone. This contrasts with the use of gypsum over the last 200 years, when it was predominantly used for casting decorative elements and for gauging lime when running moldings, whilst most flat work has been executed using plain lime plasters. Although further investigation is required, it would appear that gypsum was being used in these early gypsum/lime plasters very differently from the way we expected and there is no evidence, at the moment, that it was also used for moldings or decorative work.