Page 18 - My FlipBook

6 MECHANICAL DENTISTRY AND METALLURGY.

merce (sulphate of lime or plaster-of-Paris) in that it con-
tains two molecules of water of crystallization (CaS04,
2H2O). The water of crystallization is driven off at
392° F,, leaving a white, opaque mass, which is then known
as plaster-of-Paris. This, - however, readily recombines
with water, forming a hard mass. The setting process is
regarded as a chemical action, the water being taken up
in proportion of two molecules to one of the plaster. The.
result being chemically the same as it is in its native state,
CaSO^,2H20.
Professor Henry Leffmann says : " The facility with
which this combination occurs depends largely on the care
which has been taken in preparing the plaster. If it has
been too highly heated, the power of taking up water is
lost. The presence of impurities also, of course, interferes
with the setting qualities. It is well known that the many
saline substances, e. g., common salt, hasten the setting,
but the exact cause of this is not definitely known. In the
case of common salt a double composition occurs to a slight
extent, by which calcium chlorid and sodium sulphate are
formed. When a solution of the so-called liquid silex,
which is sodium silicate, is put on a plaster cast, a similar
reaction often occurs, forming sodium sulphate and calcium
silicate, and the sodium sulphate appears on the surface of
the cast in white, moss-like tufts."
Plaster-of-Paris has taken its name from its abounding
at Mont-Martre, near Paris, this being the most important
deposit known.
Modeling Composition.—Modeling composition or com-
pound is composed of gum dammar, stearin, and French
chalk, with carmin as a coloring-material, and a little per-
fume to render it more pleasant. There are several
varieties manufactured ; the degree of hardness of each is
due to the quantity of stearin and chalk incorporated with
the gum.