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64 MECHANICAL DENTISTRY AND METALLURGY.
When metal is employed, a shell is more advantageous
than a solid cast and a plate, where there are any consider-
;
able undercuts, can be more readily detached from the
former. But before the cast is run, whether it be from
metal or plaster, the vacuum chamber, if used, should be
carved in the impression.
Fig. 36.

To Make a Hollow Metal Cast.—Prepare the impression
as instructed for running cast, or else secure mold in sand
from plaster cast, then fuse pure block-tin and pour in the
usual manner. The metal cools first at the surface; by
taking advantage of this fact and, after waiting a few
seconds, inverting the mold and quickly pouring out the
central fluid, a thin metal shell can be secured. When
obtained it can be filled with plaster, which will form, in
fact, a metal-faced plaster cast. When the case is finished
and the plaster removed, the edges of the shell may be