To make lime plaster, a limestone of almost pure calcium carbonate has to be chosen. This is fired in a limekiln at a temperature of about 1,000°C. The burnt stone taken out of the limekiln is quick lime (calcium oxide), a very caustic material that is difficult to keep, so it is almost immediately turned into lime putty (calcium hydroxide) by adding water, a process known as ‘slaking’ which generates a great deal of heat and steam.
Putty lime will harden slowly when exposed to air as the lime reacts with carbon dioxide to form calcium carbonate once again – a process known as ‘carbonation’. Fresh lime putty is therefore protected from hardening by being stored in waterproof containers in a damp state, permanently covered by a thin film of water.
Lime can be used by a mason to bed stones or modelled by a sculptor once the necessary aggregates have been added. (In plasters, aggregates such as sand are added in the proportions of up to around three-to-one for all but the finishing coat, principally to reduce shrinkage.) A modeller using lime plaster, or ‘stucco’ as it is often known, has time to change his mind some time after he has used it, for lime plaster will set over a five to ten day period. During this period it must be protected from drying out too quickly or it will crack. Once set, stucco will last for centuries.
Gypsum plaster behaves very unlike lime plaster. It is made simply by heating gypsum rock or alabaster – both of which are mineral forms of hydrated calcium sulphate – and grinding the result to a fine flour-like powder. At a relatively low temperature some of the water which makes up the crystalline mineral structure is driven off, forming calcium sulphate hemihydrate, which is then ground to a fine powder.
Gypsum plaster will set rapidly – within 15 minutes once it has been ‘knocked up’ with water – forming interlocking crystals of gypsum. This is not a material for modelling with, more a material for casting with, as it sets so quickly. So we have two completely different materials, for different purposes. A slow-setting lime plaster and a fast-setting gypsum plaster.
One of the earliest and most renowned sources of relatively pure gypsum rock was Montmartre, Paris, from which the material takes perhaps its most common name, plaster of Paris.
Plasterers, particularly since the late 18th century, have generally used gypsum plaster both to imitate earlier lime plasterwork and to create their own contemporary plasterwork of varying quality.