Page 29 - My FlipBook

PLASTKR MODELS OR CASTS. 17

order to support tliein in this upright position iinln the
plaster is ponred, the end of each pin shonld be shghtly
forced or imbedded in the impression material in the center
of the bottom of each cavity. The plaster shonld then be
mixed and ponred; in doing so it shonld not be poured
directly into the cavity formed by the teeth, but a little of
the plaster should be placed at a point just back of these
cavities, and then gently but hastily coaxed into them by
slightlv tapping the bottom of the impression cup against
the table. In this way the air and water are expelled and
the plasver more perfectly fills the cavities, which of course
will give more perfectly shaped teeth on the cast, that is
they will be minus the " air bubbles " so often seen in such
cases. Sufficient plaster should then l)e added to give to
the model a depth of about two inches.
To Separate Cast from the Impression.—The simplest
and most satisfactory method of separating the cast or
model from the impression is to immerse the impression
in warm water until the wax or modeling composition is
sufihciently soft to allow of its being readily removed. Or
dry heat may be applied to the impression until the same
result is accomplished.
To Obtain Cast from Plaster Impression.—The manipu-
lation required in securing a model from a plaster impres-
sion is much the same as when wax or modeling compound
has been used, though there are a few details which are
very essential. There are various methods of preparing
the impression, but in this, as in every subject treated, our
endeavor is to give the one generally accepted as the best
practice ; by that we mean the method which has proven
most sati factory, not only in our own hands, but to many
of the most progressive practitioners and teachers.
Separating Fluids.—Of the materials used for separating,
probably the best are shellac varnish, and soapy water. The
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