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Chapter un.
Ccchnlqut oT fusing Porcelain Turnaccs and Pyrometers.

Tlie purit)' of colcrs of porcelain is partially clepciKk-iU upon the
accurac}- of tlic fusiiiij. The variation of heat of the muffle will cause
dififerences in the shade of the porcelain. Under-fused porcelain is of a
duller hue and less translucent ; the blues appear lighter in color; the yel-
lows, browns and grays appear darker than properly fused porcelain.
Over-fused porcelain is much lighter in color and has more of a glass-like
appearance.
If porcelain remain in contact with the ma.ximum heat long enough,
or if it is brought to the maximum fusing heat by repeated firing, it tends
to form a glass-like mass. All the hues of a color from the normal to
a light may be made from the same porcelain by increasing the heat
above its maximum fusing point, but this is done at the sacrifice of
strength. This explains why each layer of enamel should be fired only
to a high biscuit, and all the enamels to the point of glazing only at the
final firing.
The method of firing porcelain at its maxinumi temjierature for a
short time should be condemned. It makes the porcelain more brittle
and causes the formation of minute bubbles throughout the mass. The
porcelain should not be ]:>laced in an intensely hot furnace, for it causes
a crust to be formed on the surface, which prevents the escape of gas.
This gas w'ill seek the point of least resistance, which will be along the
margin, and result in the formation of bubbles along the margin of the
inlay.
The three varieties of furnaces used in inlay
work are the gas, gasolin and the Gas
furnaces. electric.
furnaces have been used w-ith but a moderate degree
of success. In order to obtain a sufficient supply of air for proper com-
bustion the use of a bellows or compressed air becomes necessary. The
noise produced by the combustion is one of the objectionable features.

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