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AMALGAMS. 253

amalgamation. It is a notable fact that while freshly comminuted alloy
will more readily amalgamate, it requires more mercury than aged
alloy.
More recent investigations by Dr. Black tend to the conclusion that
the difference in capacity for mercury observed in freshly cut alloy and
that which has been cut for some time is due to the difference in molec-
ular arrangement of the alloy, brought about by the comminuting pro-
cess, which has the effect of hardening the grains and condensing their
texture in the same manner that hammering the ingot would harden the
entire mass. By the application of sufficient heat the particles of alloy
mav be " aged " artificially, and this aging is presumed to be simply an
annealing process. The capacity of the aged alloy for mercury is
markedly different from that of the freshly cut alloy, as are also the
Avorking qualities of the resulting amalgam mass, the aged alloy form-
ing a slower setting and much smoother working amalgam than that
made from freshly cut alloy. For the further details of this subject
see Chapter XIII., on Plastic Fillings.
It is held by many that the proportion of mercury should be in some
excess, to give decided plasticity to the mass for the purpose of securing
complete amalgamation of the particles of the alloy. When the amal-
gamation is complete the redimdance is forced out through chamois
skin, or the mass is kneaded in a napkin or piece of China silk, which
forces through the meshes most of the excess. It is claimed that this
method of conducting the amalgamation effects an approximately cor-
rect atomic relation of the metals with each other; it being held that
the freer proportion of mercury during the mixing process tends to this
result, as the redundant metal is carried out with the excess of mercury
as it is expressed. On the other hand, it is maintained that the propor-
tions of any given alloy and mercury which will produce an amalgam
fulfilling every requirement should be established by experimentation,
and thereafter be weighed out in those proportions.
The Distinguishing Features of a Good Ama/gam.—An amalgam (1)
Should be non-shrinking ; (2) Should have edge strength ; (3) Should
maintain lightness of color under the varying oral conditions ; (4) Should
tend to assume a spheroidal surface. A further qualification is that the
surfiices of the material may not undergo electrolysis.
Indisposition to shrinkage is secured by a close conformity of the
alloy with the proportions above given.
Edge strength is a term which has not as yet had a clear defini-
tion in respect to the causes which determine the deficiency of this
quality. The maintenance of unchangeability of the surface is directly
related to this important desideratum, as roughening and erosion of the
margins is the result of molecular w^aste, which causes a ragged and