Plaster of Paris is sulphate of lime--a combination of lime and
sulphuric acid; and exists abundantly in various degrees of purity;
it occurs plentifully in the locality of Paris, and is brought here
in masses of a greenish, pink, or brown color. To prepare the gypsum
it is first made red hot, to separate the water which it contains,
and then ground to powder of various degrees of fineness. Plaster of
Paris has the peculiar property of uniting with a certain quantity
of water and forming a solid compound with it; upon this property
all its usefulness depends. If a portion of Plaster of Paris be
mixed with sufficient water to form a liquid of the consistence of
cream, in a few minutes it will harden and become quite set, and
as it dries, it will harden still more, till it is of a hardness
almost equal to stone: this useful property causes it to be much used
for casting figures and ornaments; cheap plaster figures, &c., are
generally cast hollow, to save the plaster; this is done by pouring
into the mould a certain quantity of the plaster mixed with water,
and quickly turning the mould about so that it shall adhere and form
a layer on all the inside of the mould; when set hard, the mould is
taken to pieces and the figure finished by scraping off the marks
where the mould was joined. Plaster of Paris, combined with whiting,
forms what is called by plasterers “putty,” and is much used for
ceilings, and similar purposes.