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Plaster (Gypsum)

Plaster is made from gypsum, a soft white mineral that is also a major ingredient in wallboard (or sheetrock), in concrete (for highways and buildings), and in some paints and fertilizers. To make plaster, gypsum stone is ground into small particles, then heated until the moisture is driven off and only a very dry powder remains.

When dry plaster particles are mixed with water, a chemical reaction takes place which causes the plaster to harden.  If hardened plaster is placed in the kiln and heated, it starts to lose its strength at around 1300F/700C.  This decomposition continues until the plaster reaches around 1550F/840C, at which point it can fall apart in the kiln.  For this reason, plaster is often mixed with ground silica or a similar refractory product to make molds which can better withstand the heat of the kiln without falling apart.

There are many different types of plasters, including pottery plaster, hydrocal, hydroperm, and plaster of Paris.  Many people use the different types interchangeably, but each type has slightly different characteristics and properties.

Coming soon:  more about plaster and its uses in the kiln, including proper storage and mixing techniques.


Copyright 2005 Brad Walker.  All rights reserved.

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Copyright 2005-2006 by M. Bradley Walker.  All rights reserved.

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