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204 THE GOLD AND PORCELAIN INLAY

when they should then be deepened for retention. Surround-
ing walls should meet the basal wall or seat at an obtuse angle
to admit the ready withdrawal of the matrix. Retention
may be increased by pins, more generous step forms than for
gold, double steps, extension of porcelain into pulp chambers,

and similar measures. The cavo-surface angle should be a
right angle, or if a bevel angle exists, it should be deeply
buried to afford strength to the porcelain. The cavity should
be washed, first with chloroform to dissolve fatty matters,
then with alcohol; and thoroughly dried before setting the

inlay.

COMPOSITION AND GENERAL CHARACTERISTICS
OF PORCELAIN

The general composition of Porcelain for inlays is as fol-
lows: First, Basal ingredients, such as Silex, Kaolin, Feldspar;
Second, Fluxes, such as borax, sodium carbonate and calcium
phosphate; and third. Pigments, such as gold, platinum,
purple of Cassius, oxide of Cobalt, Titanium, Iron, Uranium
and Silver.

There are two general classes of porcelain used in inlay con-
struction, viz. ; high and low fusing.
High fusing porcelain requires 5 minutes or more to fuse at
a temperature exceeding the fusing point of gold (2,01 2°F.),
while low fusing porcelain requires less than 5 minutes.

A porcelain inlay usually requires several bakings in the
furnace. The first of these bakings is known as the biscuit
fuse. By the biscuit fuse is meant the act of heating the
porcelain sufficient to obtain shrinkage, but not enough to
glaze it. The first bake should never exceed a low biscuit
fuse, subsequent bakes being fused to a higher temperature,

but the final glaze being not completed until the last bake.