Page 145 - My FlipBook

AMALGAM IN THE MAKING OF A FILLING 141
to 212° F. for some hours or days. The lower temperature for a
longer period produces the best results.
Effect of Annealing. The artificial aging increases the contrac-
tion, the flow, and the ability to withstand the crushing strain; the
amalgam requires less mercury, and sets slower.
The Alloy Showing the Least Expansion and Contraction Avhen
unannealed is composed of seventy-two parts silver and twenty-
eight parts tin and may be modified very slightly by adding a small
per cent of copper or other metals. When annealed the above for-
mula of silver tin alloy should be changed to seventy-six parts sil-
ver and twenty-four parts tin, to get a stable amalgam.
Cavity Preparation for Amalgam. Many of the failures in the
use of amalgam attributed to the property of the material used are
in fact due to laxity in cavity preparation, since many practitioners
believe that thoroughness is unnecessary in this particular. The
preparation of a cavity for the reception of amalgam is even more
exacting than for gold, as the operator is dealing with a filling ma-
terial possessed of a greater number of faults, each of which must
be given consideration, and the cavity should be prepared in such
a manner as to minimize these to the least degree. In compar-
ing amalgam with gold it might be said that amalgam requires
less access in awkward localities in the mouth, requires much
separation in proximal fillings, and that the outline form must re-
ceive more careful consideration as the margins must be farther
removed from positions of great liability to caries, as' well as stress.
Flat Seats for Fillings are even more imperative than Avith gold,
and the occlusal step must be broader bucco-lingually. The enamel
walls must be finished with as great care, w4th a cavo-surface angle
more acute, and a more deeply buried bevel angle. Cavities must
have more retentive form.
The Rubber Dam is very essential as it is imperative that amal-
gam be built against dry, freshly cut, walls and margins. It is as
impossible to make a good amalgam filling as it is a good gold fill-
ing against moist Avails. The residue from the saliA^a upon the Avails
Avill shoAV leakage more quickly Avith the amalgam filling than with
the gold. When operators come to the full realization of this fact
and manipulate all amalgam fillings Avith as great care as gold, Avith-
reference to dry conditions, the frequent failures of amalgam will
be materially lessened.
The Matrix. All cavities filled with amalgam must have contin-
uous surrounding walls. This will necessitate the adjustment of the