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246 MECHANICAL DENTISTRY AND METALLURGY.

mercury is accompanied with expansion while the union of
tin and mercury causes contraction. The influence of silver
therefore upon an amalgam of tin and mercury is to lessen
shrinkage, and increases the hardness. Silver, however,
discolors very readily, owing to its affinity for sulphur. A
coating of silver sulphid forms upon the surface through
the action of sulphureted hydrogen in the mouth.
Tin as indicated causes contraction or shrinkage; it
facilitates amalgamation and helps to prevent discoloration.
Tin retards setting and decreases edge strength.
Gold in amalgam, that is in small quantities, makes it
work easier, that is makes the mass smoother and more
plastic; reduces shrinkage, increases edge strength and, it is
thought, assists in maintaining good color. If more than
per cent, of gold is used it makes the amalgam brittle.
7
Platinum, when added to a silver and tin amalgam in
any considerable amount, impair the properties of the alloy.
It causes dirtiness in working, retards setting, and increases
shrinkage. In small cjuantities it is said to impart strength
and density, and to resist discoloration.
Zinc in small amounts is occasionally employed in mak-
ing dental amalgam alloys. This is done to add whiteness
to the amalgam, and help it retain its color. So-called white
amalgams contain a large amount of zinc. Its presence,
however, causes rapid setting and a tendency to contract.
Copper diminishes shrinkage, hastens setting, is com-
patible with tooth structure and favors tolerance of metallic
fillings near the pulp. The great objection to copper is its
tendency to marked discoloration.
Cadmium f( )r a time was used in amalgam alloys, but has
been discarded as the tooth substance became stained, owing
to the formation of soluble salts of cadmium.
Antimony has also been experimented with. It was found
to lessen the strength of the amalgam and make it dirty to
work.