Page 260 - My FlipBook

248 MECHANICAL DENTISTRY AND METALLURGY.

tube is ground flat and the margin of the cavity brought to
a perfect edge. The tubes are placed on the stage of the
microscope and so arranged that every part of the margin
of the cavity is brought under the lense as the stage is ro-
tated. A groove is cut in the inner wall of the tube at the
bottom, to hold the filling at that point, so that in case of
shrinkag'e the surface of the filling sinks down into the
tube."*
Edge-strength.—As the term indicates, it denotes the re-
sistance the edge of an amalgam filling offers to the force of
mastication. One of the requirements of a dental amalgam
is that it should have more than sufficient edge strength to
retain its edge integrity under all ordinary circumstances.
" The stress in the ordinary uses of the teeth has been shown
to be from sixty to eighty pounds upon the area of the
molars of medium size. This, if evenly distributed, would
give from seven and a half to ten pounds on a filling occu-
pying one-fourth the area of one of these teeth."f
Annealing and Aging of Amalgam Alloys.—With the
lapse of time, after an amalgam alloy has been cut, the par-
ticles of metal become more or less oxidized. This oxidation
has an influence upon the working properties of the alloy,
in some cases it is facilitated while with others it seems to
retard it. This change is technically called " aging," and
it is generally believed to have a beneficial effect. Dr.
Black's experiments demonstrate that oxidation or " ag-
ing " retards setting, though in varying degrees with dif-
ferent formulae; also that this change could be brought
about artificially. The explanation of the changes which
occur in a silver-tin alloy by annealing is that the alloy
becomes hardened in cutting, and that the heating or anneal-
ing causes the molecules to return to their normal relative
positions.
* Smith's Metallurgy.
t Dr. Black in Dental Cosmos.