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68 MECHANICAL DENTISTRY AND METALLURGY.

matrix, a, from the elevated portion, a, on the other half
of flask. Fig. 39, b, indicates a portion of a cut away,
illustrating the manner of forming vents ; in this cut it is
only carried half around in order show the condition before
and after preparing. Commence by cutting a deep groove
all around the piece close to the flask and gradually tapering
up to the tin-foil or the margin of the plate, marked c. By
this arrangement the material has free exit all around, yet

Fig. 39.

may not come out too rapidly. The plaster margins are not
likely to be broken away under pressure, as the vent runs
out almost at a right angle, thus leaving solid walls. An-
other advantage in this style of vent is that after the two
halves of the flask have been pressed home the surplus ma-
terial parts readily from the piece, leaving very little to
dress up.
Selecting and Preparing the Celluloid Blank.—After the
case is prepared as has been described, a suitable blank, one
as near as possible the size and form of mold, should be