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20 S MECTTANICAL DENTISTRY AND METALLURGY.
Ingot-molds.—A'arions sul)stanccs are used for the con-
struction of ingot molds, Ijut those in most common use are
made of iron, and, for gold, are generally about two inches
square and from ^ to ,'• of an inch thick (Fig. 174). They
should be slightly concave on their inner surfaces, to com-
pensate for the greater shrinkage of the gold in the center
than at the margins of the ingot.
Soapstone is sometimes employed for the same purpose,
Fig. 174.

and is preferred by some. Molds made from this substance
should also be warmed and oiled before pouring the metals.
Molds are also made from charcoal, which is highly recom-
mended for the purpose, though it requires to be frequently
renewed.
Charcoal Ingot-molds may be very easily and quickly
made. Select a close-grained compact piece of charcoal of
suitable size, cut through it w^ith a saw% and then rub the
divided surfaces together until perfect coaptation is secured.
The required size and shape of the mold is then cut out in
one section of the block; the two pieces are then secured in
place by binding with wire, or with the use of clamps.
Asbestos Molds.—Comparatively inexpensive, and at the
^
same time more convenient and (lural)le contrivances de-
signed for the same puri)ose. combining both crucible and