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314 PLASTIC FILLING MATERIALS.

Quaternary Amalgams.—The metal additional to the basal alloy
is added for the purpose of modifying the color or increasing the edge
strength of the amalgam. The addition of copper 5 per cent, to an
alloy containing over 60 per cent, silver increases the crushing stress
and lessens both flow and contraction. The alloy is white Avhen fresh,
but in the presence of sulfur compounds discolors.
The addition of gold (5 per cent.), as clinical records testify, aids in
maintaining the color of the filling. It lessens shrinkage slightly (com-
pare this and following statements with table No. II.), and appears to
have little or no influence upon flow and crushing stress. The addition
of platinum causes dark fillings and notably increases the flow ; the
setting is slowed.
The addition of zinc increases rigidity ; the amalgams expand for
long periods after apparent hardening ; the crushing stress is moderately
high—a direct contradiction of statements of several previous ob-
servers.'
Additions of bismuth, cadmium, lead and aluminum were made to
the basal alloy, but all of them exhibited properties which exclude
them from introduction into dental amalgam.
Dr. Black ^ states that " alloys containing 5 per cent, of aluminum
have their setting attended by the evolution of much heat ; an enormous
expansion of the mass occurs ; the instruments used in packing are oxi-
dized, and a distinct crackling of gas-disengagement is heard." " The
formation of aluminum amalgam is characterized by an exhibition of
the affinity of aluminum for oxygen. Aluminum oxid is doubtless
formed, which increases the volume of the amalgam mass."
Washing of Amalgams.—Alloys which have been cut for some
time, and mercury the purity of which is questionable, are found to be
coated with oxids of the metals—in the case of mercury, with the oxids
of contaminating metals. The advisability of w^ashing the amalgam
mass in some solvent which will remove the oxids is a mooted question.
It has been stated that the washing of an amalgam mass increases its
shrinkage (Flagg). On the other hand it has been observed that
washed amalgams retain their color better. It is difficult to see how
the washing could affect the integrity of the set mass unless oxidizing
substances were left in it ; and this is clearly contraindicated by the
maintenance of color in washed amalgam. The writer prefers wash-
ing the plastic mass in chloroform prior to expressing the surplus of
mercury.

^ It is to be recalled in this connection that Dr. Black's measurements are made with
instruments of unequalled accuracy, those of previous observers with comparatively crude
instruments.
^Private communication.