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268 THE OPERATION OF FILLING CAVITIES.

Too much stress cannot be laid upon the question of its purity if the
best results are to be obtained from its use. While it is claimed by
manufacturers of dental gold foil that their ])roducts are absolutely free
from alloy, it is nevertheless true that but few sju'cimens of dental foil
show a fineness above 999. If this standard Avere always attained the
o})erator would have little cause for complaint. So small a percentage
of alloy as 1 in 1000 would not materially affect the working qualities
of the product, but when this is increased to 4 or 6 parts per 1000 it
manifests itself by harshness and intractability under the instrument.
Great care should be exercised in the preparation of the foil, since
so much depends upon its purity and cleanliness. For a detailed
description of the process of manufacture, from the ingot to the beaten
and annealed foil, the reader is referred to an article by a practical foil-
maker.^
In former times the dental surgeon w'as restricted to one form
of gold for filling. This was foil ranging in thickness from 4 to 10
grains to the leaf, but as the requirements of the operator broadened
the art of manufacture increased, and new preparations were offered,
until to-day the most fastidious can find such as will please his fancy :
foils ranging in weight from 4 to 120 grains to the leaf; cylinders of
various sizes and composed of non-cohesive and semi-cohesive foil ; cohe-
sive blocks prepared for use ; rolled gold, varying in thickness from No.
30 to 120, and crystal gold possessing great cohesive properties. These
are the more important forms in which gold is offered the oj^erator at
the present time.
Before entering upon a description of the classes of cases where each
of these seems best adapted, it may be well to describe somewdiat in
detail the peculiar qualities which each form of gold presents when
subjected to clinical use.
Soft or Non-cohesive Foil,—Prior to 1854, wdien Dr. Robert
Arthur discovered and promulgated the desirability of cohesive foil in
certain cases, the operator used gold which possessed very low cohesive
properties. Used as it then was, in the form of large rope, tape, or as
cylinders, the property of cohesion would have been a serious objection,
since there would be constant danger of the mass clogging and bridging
in the cavity, and the cause of many unfilled places along the cavity
walls.
The terms soft and hard, when used to designate the kind of gold, are
misleading, since all gold foil prepared from pure gold or gold that is
nearly pure possesses great softness under the instrument. The distin-
guishing characteristics between the two kinds of gold are the inability
to make a certain kind of foil cohesive when exposed to a reasonable
^ American System of Dentistry, vol. iii. p. 839.