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88 MECHANICAL DENTISTRY AND METALLURGY. :

54. In most cases, whether one or a greater number of
teeth are to be replaced, increased adherence and stabiHty of
the substitute will l)e better secured by permitting- the plate
to cover the larger portion of the roof of the mouth ; though,
in cases that present the best form of the vault, a diminished
surface may be given to the base with equally satisfactory
results.
Manner of Forming the Vacuum Chamber. —Where a
central cavity, or " Air Chamber " is to be stamped into the
base, the model should be prepared for the purpose before
molding. Fig. 54 shows its general form and position.
The model may be prepared in either of the following ways
I. The form of the chamber may be cut from the wax or
plaster impression, in wdiich case the plaster will be raised
at a corresponding point or points upon the model, and will
have exactly the same form and depth as the cavity in the
impression. 2. Cover the palatal face of the model with a
sheet of wax equal in thickness to the required depth of the
chamber, and cut out from this, at the desired point, the
form of the cavity; fill the latter with plaster, and when
hard remove the wax and trim the raised portion to the
proper form. 3. Cut a pattern of the required form and
depth of chamber from a sheet of wax or lead; place it in
the proper position in the arch, and press it down with the
fingers or burnisher until it conforms to the contour of the
palate ; it is then fixed in place either by confining it with a
small pin or tack driven through it into the plaster, or by
interposing softened wax or other adhesive material between
the pattern and model.
The metallic swages being secured, the plate is placed
between them and swaged until it conforms perfectly to the
face of the die. During the operation the plate should be
frequently annealed as has been previously described.
The Use of a Tracer.—Unless the plate used is purer and