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- originally a mixture of clay
or lime, sand and water used either internally or externally, with various other
ingredients, usually locally available such as animal hair, dung, straw etc to help bind and prevent cracking. Plaster
of Paris was the best quality plaster, which involved mixing water with burnt gypsum which is sulphate of lime.
Montmartre was the early source, hence the name. It sets rapidly and is only suitable for internal work. Long, strong
hair free of grease was almost always used in internal plasters. Goats hair was often favourite horse hair was also
used, but was too stiff to be contained properly within the mix, and anyway, was required for furniture. One of
the best current sources for goats hair is China, British goats are undercover in winter as a result of which
their hair is not suitable. Old animal hair is a possible source of anthrax, but the risk of contamination from
handling hair reinforced plaster is extremely low, and while precautions such as the use of gloves and masks
are advised, threat of anthrax is not a reason to dispose of old plaster.
Plaster is usually laid in a number of coats, and local practices and terms vary. Three coats is most common, where
the first is applied to the base surface to provide a key, the second is a levelling coat, the final a finishing coat.
Two coat work is often referred to as "render and set", three coat work as "render float and set". Where plaster is
applied direct to the wall surface rather than built out on lath,
it is referred to as being on "the hard". Traditionally
the base coats would be
lime putty, sand and hair, while the finishing coat would be one part putty to one part
fine silica sand. Any scratching of the surface between coats to provide a key, was always diagonal to avoid
piercing the space between the laths.