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EFFECT OF METALS ON AMALGAMS. 247
Bismuth facilitates the working pr(Ji)erties uf anialgani,
hut increases tlie discoloration in the filhng, and lessens the
edge strength, hence is not employed.
Palladium was found to make the amalgam very dirty to
work, and caused leaky and hlack fillings.
Aluminum gives a soft easy working mass, hut very slow
to set, causes increased expansion and has a tendency to
further alter its shape.
Change of Volume.—The change of form in amalgam
fillings has long' been recognized as one of the chief disad-
vantages of this class of operation. Much tedious and
careful work has been done by dentists, metallurgists and
manufacturers to overcome this feature, and the more recent
researches of Dr. Black have gone far towards remedying it.
In testing amalgams for change of form, the usual method
is to firmly pack freshly mixed amalgam into a shallow glass
tube, leaving the top perfectly smooth and level with the
edges of the tube. If the amalgam contracts it will readily
slide out of the tube, while if there is any expansion it may
be determined by the slight projection of the metal above the
mouth of the tube.
It has been shown that alloys containing less than 50 per
cent, of silver first shrink and then expand, while 50 to 60
per cent, causes shrinkage only. Above 65 per cent, of sil-
ver gives more or less expansion. In Dr. Black's experi-
mental investigations, the contraction and expansion of sil-
ver-tin amalgams has been very carefully studied, both with
the microscope and a measuring instrument known as a mi-
crometer.
For the purpose of examining amalgams under the micro-
scope, the plastic metal is packed into a " Wedelstaedt test-
tube, made of hardened steel one-half inch deep and one
inch in diameter, with a cavity three-eighths of an inch in
diameter and one-fourth of an inch deep. The top of the