Page 495 - My FlipBook

PHYSICAL PKOPERTIES OF FILLING MATERIALS. 231

occlusal surface of a molar that will receive severe wear. Pit-
ting of the surface of fillings is in a large degree the result of
the introduction of bits of gold that have not been sufficiently-
annealed. To prevent the possibility of this, it is well to first
anneal one part and then lay down the piece and catch it at
another point and anneal again.
While the gold should be heated to redness, it should not be
heated to the melting point. This ruins the plasticity of the foil
and makes it impossible to properly condense it. To avoid these
difficulties, the gold may l)e annealed upon a tray over the flame
of the lamp. A number of devices for this purpose can be had,
most of which are convenient and effective. The tray may be
made of metal, of porcelain or of mica; it does not seem to make
much difference which is used. In annealing in this way, the
gold may conveniently be exposed to the heat for a considerable
time, and therefore a full red heat is not necessary to the com-
plete development of the welding property. When done in this
way, there is no danger of hardening the gold by melting por-
tions of it. However, the heat must closely approach the point
of redness to be effective. Another advantage is that all parts
of the gold will become equally annealed.
The disadvantages of this method are the presence of such
an apparatus upon the bracket, which is needed for operating
instruments, and that the gold spread upon the annealing tray
is liable to movement by every motion, or in the effort to lift
pieces from it; and they roll together and stick to each other
in such a way as to cause much annoyance and delay in handling
them. Much of this annoyance can be avoided by having a tray
that is so roughened as to prevent the rolling or sliding of the
gold. The electric annealer is the most perfect appliance yet
devised for this purpose. Its use, however, requires the electric
attachments and arrangements for the control of the current.
In offices in which electricity is available, the electric annealer
should be employed because of its greater effectiveness, its
greater convenience and its greater neatness. The apparatus
is much less bulky, is not so high, and the gold is not so liable
to be disturbed by shaking ; the heat is more evenly distributed
and is under more perfect control. It is, therefore, in every
respect, more desirable.