Gypsum is a naturally occurring crystal of calcium sulphate. It is located in different areas of the world in various forms. In Britain, the most commonly available type is alabaster. Plaster is manufactured from gypsum by reducing it to powder and then heating it to remove the water of crystallisation content.

Gypsum plaster is not, however, a modern invention as some people might believe. It is recorded that it was used by the ancient Egyptians within some of the pyramids. In Britain, evidence indicates that shipments of Plaster of Paris were being imported from France during Henry VIII's reign.

In the 16th century, however, only small quantities of gypsum are now known to have been used in plasterwork. This compares with its use in the last 200 years, when it has predominantly been used for casting decorative elements and for gauging lime when running mouldings. Flat work, in contrast has mostly involved the use of plain lime plasters.

The use of lime and gypsum in different cases may have been partly to do with different local traditions, but may have as much to do with the availability of materials.

Later on, working methods and the materials used have become more standardised.

Gypsum was readily available throughout the 19th century and was used almost all the time to gauge lime mixes for running mouldings, as well as being used on its own, for cast work.

Although taking an interest in the composition of old plaster may appear something for the specialist historian, it is in practice an important part of ensuring that repairs on old buildings turn out as required. As a leading authority on period design, Aagaard Hanley Ltd can provide invaluable advice and guidance on interior decoration.

Next page...