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PORCELAIN AND GOLD INLAYS. 353

fill all of its parts perfectly. The process is divided into three
distinct parts: (1) an impression made by filling the prepared
cavity with a wax specially prepared; (2) forming the mold
in which to cast the gold; (3) the process of casting. Each of
these has its special devices and at the present time these are in
the stage of development. The form they will finally take can not
now be stated, but the general ideas of tlie requirement for prac-
tical work in each may now be made out with a fair degree of
clearness.
The cavity is first prepared, as has been described in the
articles on cavity preparation. (1.) The prepared cavity is then
moistened with clean water to prevent the wax from sticking to
its walls. Then the cavity is solidly filled with the prepared
wax, and a finished filling made in the form of the desired inlay.
A metallic pin prepared for the purpose, called a sprue by
molders, is attached to this filling by warming and inserting
its end into the wax and maintaining it in position imtil cold.
With this, the wax filling is cautiously removed from the cavity.
Prom this wax filling, and its sprue, the mold is made for casting
the inlay.
(2.) The wax filling held by its sprue is covered carefully,
to see that no air bubbles are included, with a mixture of fine
plaster and silex, and buried in a mass of the same mixture
placed in a metallic ring, which forms a part of the casting out-
fit. The base of this ring is convex in its central portion, so
that the investing material necessarily takes the form of a small
crucible with the end of the sprue protruding into its center.
When the investment has become hard, the base and the sprue are
removed and the ring is turned over, with the crucible formed in
the investment, up; it is then dried by heat and the heat is
increased until the wax model has melted and become dissipated
in the investment. In order that this may be complete, the wax
must have been perfectly cleaned by filtration while melted.
(3.) The apparatus for casting consists of the metallic ring
with its contained mold, a blow-pipe for melting the gold and
bringing it to a very high degree of heat, and an arrangement
for suddenly covering the metallic ring and at the same instant
turning on the pressure of condensed air, nitrous oxide gas, or
steam. The hole left connecting the mold with the little cruci-
ble by the removal of the sprue is too small for even the very
highly heated gold to run through by its own weight, but it is
forced through it readily by the increased pressure and fills the
mold completely and very solidly in its minutest angles, making

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