Page 256 - My FlipBook

254 FILLING MATERIALS.
unclean appearance of the edges and an apparent sejiaration of tlic fill-
ing from the borders of the cavity. The causes Avliich produce this
condition are slowly progressive and are continuous.
This kind of erosion is most marked when contraction takes place,
from incorrect preparation or improper ratio of the metals entering
into the formula, or careless manipulation, when cai)illary defects are
liable to occur at the margins.
The most probable hypothesis to account for these observed
changes is that the presence of moisture having acid reaction, by
inducing electrolytic action between the metals, brings about the erosion
of the material immediately within the margins. In these cases tlie
exposed surfaces generally suffer little waste, for the reason that they
are subject to the continued movement of the oral fluids, but it is often
observed that entire fillings undergo a similar gradual loss and disappear.
This result is common where there is an excess of gold or mercury. In
some instances the above described action takes place to a limited degree
upon the whole surface in proximity with the dentin, when a residue
is found upon the filling as well as on the surface of the dentin.
The conclusion from these observed fiicts is that the securement of
edge strength depends upon an approximation to the chemical ratio of the
elements of the alloy. This would appear to be most nearly secured
when the material is subject neither to shrinkage nor expansion. Expan-
sion under some circumstances might produce marginal space and there-
fore lead to the same result ; for instance, if in apjiroximal or buccal
cavities the depth were greater at one division than another the expan-
sion of the thicker part of the filling would tend to raise the edge sur-
rounding the shallow part of the cavity, and would then subject the
edge of the filling to electrolytic changes.
A related condition sometimes appears when an amalgam filling,
quite hard at completion, after some years becomes comparatively soft,
which apparently is due to molecular changes.
The close conformity of the alloy to the proportions recently estab-
lished by Black, and foreshadowed by the much earlier experimentation
of Flagg, furnishes a result that is directly conducive to fixity of form
and edge strength when the margins of the cavity are overlapped.
This formula is approximately—silver, 68 ; tin, 26 ; copper, 5 ;
zinc, 1.
The maintenance of size, form, and strength dei3ends largely, if
not entirely, upon the influence of silver. When the proportion of
this element becomes less than 60 per cent, of the formula, the tendency
to shrinkage appears and holds a nearly direct relation with the diminu-
tion. When the ratio of silver advances above 70 per cent, the expan-
sion becomes marked, and at 80 per cent, is excessive.