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212 MFXIIANICAL DENTISTRY AND METALLURGY.

between the latter should be diminished, care being taken
that the approximation be insufficient to clog or impede the
free action of the mills. The gold, which, in time, becomes
hard and brittle, and liable to crack in the mills, should be
frequently and well annealed l)y bringing it to a full red
heat ; this restores the pliancy of the metal and facilitates
the operation in the press.
When the ingot has been extended in one direction as far
as may be desired, it should always be reannealed before
turning it in the mills; a neglect of this precaution will
seriously interfere with the working of the gold by twisting
or doubling the plate upon itself; and in some instances,
provided the gold has not been well annealed throughout
the operation, or is in any considerable degree unmalleable,
the plate will be torn across and rendered unfit for use.
A thin or retreating edge may be given to the plate at
any desired point or points by passing such portions part
way between the rollers and withdrawing; repeating this,
with the rollers brought a little nearer to each other every
time the plate is introduced between them, and decreasing
the distance the plate passes each time, until it is reduced to
as thin an edge as may be desired.
Standard Gage Plate.—The degree of reduced thickness
obtained by rolling is determined by what is called a gage
plate (Fig. 178). This instrument is usually circular or
oblong in form, and is marked at intervals on its edge by
cross-cut grooves, or fissures, which successively diminish in
size and are indexed by numbers ranging from 5 to 36.
The sizes of the groove diminish with the ascending num-
bers. During the operation of rolling, the plates should be
tested, from time to time, by the gage, to determine when
it has undergone sufficient attenuation.
Thickness of Gold Plate Required as a Base for Artificial
Dentures.— In prescribing the thickness of plate proper for