(3) Composition of the alloy. The ADA specification states that the
composition of the alloy must include a minimum of 65 percent silver, a maximum of 29
percent tin, a maximum of 6 to 13 percent copper, and a maximum of two percent zinc
by weight (see figure 1-1).
Figure 1-1. Approximate composition of an acceptable amalgam alloy.
(4) Correct proportion important. Immediately prior to use, the silver alloy is
mixed with pure and uncontaminated mercury. (Mercury, although an indispensable
ingredient, imparts undesirable properties to the amalgam if added in incorrect
proportions.) There are some alloys that are completely zinc free. They can therefore
be used more successfully in a moisture-contaminated environment.
(5) Properties of the finished product. Each element composing amalgam
imparts certain properties to the finished product. Table 1-1 summarizes these
properties. Silver imparts strength, durability, and color, gives the alloy desirable setting
expansion, decreases flow, and accelerates (decreases) the setting time. Tin makes
the amalgam easier to work, controls excessive setting expansion, and increases both
flow and setting time. Copper increases hardness, contributes to setting expansion,
reduces flow, and decreases setting time. Zinc increases workability and unites with
oxygen and other "impurities" to produce a clean amalgam.