About gypsum
Pure gypsum is white, but impurities sometimes colour it grey, brown or pink. Its scientific name is calcium sulphate dehydrate, or CaSO42H2O, and for every molecule of gypsum, there are two molecules of water. If a piece of gypsum is ground to a powder and heated, it will lose about three quarters of its water. If this powder is then mixed with water, the paste or slurry will set rock hard.
Gypsum is a rock that’s found and used in every continent of the world.
The powder is called hemi-hydrate gypsum plaster, more commonly known as plaster of Paris. This is the material you may have used for modelling or for taking casts of paw prints, tyre treads or footprints in soft ground. It gets its name from the city of Paris which was built on land containing gypsum, which has been mined and quarried, particularly in the district of Montmartre.
As well as in France, gypsum is mined and quarried throughout Europe, India, Pakistan, parts of Africa, Australia, China, Japan and South America. There are gypsum mines across North America, and Utah possesses large deserts of powdery gypsum rock. In 1964, Jamaica issued a stamp picturing its gypsum industry.
Gypsum is now also obtained as a by-product of reducing the emission levels of coal-fired power stations. Flue-Gas Desulphurisation (FGD) systems are a great step forward in keeping the air you breathe clean and provide an alternative to mined gypsum.
Gypsum use through the ages
Gypsum is one of the oldest building materials in the world, and has been known in Britain for centuries.
6000-3700 BC
The oldest use of gypsum yet discovered was in Anatolia around 6000BC and later, in 3700 BC, it was used on the interiors of the great pyramids in Egypt. Artists painted magnificent frescoes on their smooth white surfaces.
710-705 BC
The ancient civilisations also used the type of gypsum we call alabaster. The great winged bulls of Assyria, which can be seen in the British Museum, are made of this stone. The Greeks coined the word albatross from the Egyptian town of Albastron, where small vessels or pots were made from gypsum.
1200 BC - AD 600
It was the Greeks also who gave gypsum it’s name using two words meaning ‘earth’ and ‘to cook’ (Gypsos’). They used a special form of transparent gypsum for windows, particularly for temples dedicated to the moon goddess Selene, and this, traditionally, is why they called it selenite or ‘moon-stone’.
43 - 410
410 - 1071
1201 -
The Romans knew of gypsum and used it during their occupation of Britain, as can be seen from bodies preserved in gypsum plaster at York. The Anglo-Saxons and Normans forgot about it however, and gypsum was not heard of again until plaster of Paris was brought from France in the thirteenth century.
1500 -
The earliest British houses were made of wattle and daub, where the walls and ceilings were made of reeds or wattles, and the gaps were filled with daub. At first mud of clay was used, but then builders found they could make a hard, white surface from lime plaster and the trade of plastering was born. The ‘Guild of Plaisterers’ was given a charter in 1501 and by that time plaster could be made of lime or gypsum. Gypsum plaster became more popular because it dried faster and didn’t crack. Today very little lime plaster is used.
1558 - 1603
1714 - 1830
The Plaisterers were also called Pargettors. Pargetting meant decorating the outside of a house with rough plaster, examples of which can still be seen on Elizabethan manor houses. The greatest use for plaster, however, was inside, and from the sixteenth century onwards plasterers developed marvellous skills in creating decorative plaster or stucco. Walls and ceilings were covered with flowers, leaves, fruit – and even musical instruments – all in plaster. Some of the finest stucco was created by Italian plasterers in the Georgian houses of Dublin.
The plaster for this decorative work had to be of the finest quality and one plasterer used eggs and cream as well as horsehair in his mixture!