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l6 MECHANICAL DENTISTRY AND METALLURGY,

In partial cases, or where for any reason it is desired to
use a vacuum chamber, the simplest and one of the best
methods is to cut the form of the chaml)er in the impression
before using the varnish. The plaster in this wav will
be raised at a correspondiui^- point upon the cast, and will
have the same form and depth as the cavity cut in the
impression. Should it, however, be desirable to change
the general shape of the chamber, this raised portion can
readily be carved to the desired shape and size. With this
form of vacuum chamber there is no danger of displacement
or having lioles in the plate by the loosening and dropping
of pins used to attach the metal chamber forms to the cast.

PLASTER MODELS OR CASTS.
After securing- a perfect impression of the mouth, the
next important step in the construction of an artificial den-
ture is to obtain from the impression a correct representa-
tion of the parts in plaster. This counterpart or copy is
called a model or cast.
Manner of Obtaining Casts.—When preparing a cast
from a ^^ax or modeling- compound impression, it is not
necessary to coat impression wnth any separating material
excepting to dip them in soapy water just l)efore pouring
the plaster, water is more easily dispaced than air. Mix
plaster as for taking impressions, excepting that it should
be a httle thinner and that nothing is needed to hasten
the setting. In partial cases it is often desirable to
strengthen the plaster teeth to secure them against accident
in handling, especially such as are to be used in adjusting-
clasps, and those adjoining the space to be filled by the
artificial teeth. This support may be given by placing
short pieces of stiff w'ire or ordinary pins \'ertically in the
depressions made in the impression by the teeth, and in