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158 FILLING MATERIALS

their shape. Some, notably tin, possess more of this property
than others.
Shrinkage and Expansion.—Some metals, when heated
or amalgamated (mixed with mercury), shrink, while others

expand. The ideal amalgam would be one in which the con-
stituents are so combined as to possess neither of the above
qualities, and has not as yet been discovered.
Amalgams possess, , in addition to the above qualities,
certain advantages and disadvantages.
Advantages.

1. Crushing resistance.
2. Insolubility.
3. Convenience of manipulation.
4. Capability of polish.

Disadvantages.
1. Inharmonious color.
2. Tendency to molecular change.
(a) Spheroiding.
(b) Flow.
(c) Expansion.

(d) Shrinkage.
3. Lack of edge strength when built to thin edges.
4. High conductivity.

Metallurgical Properties of Silver, Tin, Copper and Zinc

The various metals in dental alloys possess certain prop-
erties, some of which they impart to the mass when combined
with mercury. Some desirable qualities of one metal over-
come certain undesirable ones of another in the combination,
if the metals are combined in the correct proportions.

Silver.
I. Unites with mercury fairly readily in definite atomic
proportions.

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