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
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
Copyright 2005 Brad Walker.
All rights reserved.