What is Pottery Plaster?
The history of plaster
The story goes that a shepherd built a fire for his night's vigil, on an outcropping of soft white rock. It burned there throughout the night, and in the morning the shepherd was quick to note that the rock on which he had built the fire, had crumbled under its heat. When the shepherd emptied his water jug into the embers of the fire to extinguish it; a portion spilled into the white powder which turned from a liquid into a mysterious stone-like slab.
The first known use of plaster dates back over 9000 years and was used in the building of the first pyramids of Egypt. Then known as alabaster, it was used to decorate and seal the tombs of the Great Pyramids.
So plaster as we know it today was found to be made from the mineral gypsum.
The process of making a plaster mould was discovered about the 18th century when plaster moulds of different types were known and used for the forming of plastic clay.
Gypsum- the mineral
Making plaster -the technical process
The mineral gypsum is found in large deposits throughout the world. It is a mineral which chemically, is calcium sulphate plus about 20% water. Gypsum rock is mined in the midlands of the UK but deposits are also found in Nova Scotia, in the western part of the USA and many other areas of the world.
Although usually white, it may be found in shades of pink, yellow, brown or even black. To change gypsum rock into plaster, the rock must undergo a heat treatment, called calcination. This is done by placing the crushed rock in a heated vessel (calciner) at C for a period of time. Approximately two thirds of the water content of the rock is removed during this process to produce pottery plaster (beta plaster). But if the gypsum is made into a slurry first and then put into large autoclave (pressure cooker) heated to around 220C then the harder alpha plaster is formed.
Pottery plaster is off white in colour and chemically is the hemihydrate of calcium sulphate. The powder when mixed with water is used to produce working moulds for the production of pottery items. During this remixing with water a chemical reaction takes place called ‘recrystallization’ which creates a strong rock like material. This rocklike material is highly porous allowing the water to be sucked from the slip during the casting process.
Find out more about using plaster in my article how to make pottery plaster moulds.
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