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140 OPERATIVE DENTISTRY

poor edge strength; its spheroiding during setting, when not prop-
erly mixed from a perfect alloy. It is also liable to injury between
the time of introduction and complete setting through carelessness
of either dentist or patient.
The Extent of Expansion and Contraction of amalgam is not un-
der the control of manipulation by the operator, but is controlled
by the composition of the alloy both as to materials used and their
proportions; as well as the method of their preparation.
The Flow of Amalgam under pressure is the term applied to the
tendency of amalgams to flatten or move from under stress.
Most metals will yield or flatten under a given stress in proportion
to the load, up to a given point, and then cease unless the weight is
increased. However amalgam continues to yield as long as the pres-
sure is continued even though it is not increased.
This peculiarity in amalgam explains the phenomenon often ob-
served in the mouth. Amalgams differ as to the amount of force
necessary to produce flow, yet the peculiarity is exhibited by all
amalgams.
Edge Strength in a Filling is the term applied to the resistance
a filling shows to stress upon thin margins at that portion of a fill-
ing which covei's the marginal bevel.
Edge Strength in Amalgam. This depends first, upon the metals
entering into the alloy. The greater the proportion of silver enter-
ing into the amalgam up to seventy-five per cent, the greater the
edge strength. Above seventy-five per cent it becomes more brittle.
Second, the manner of packing. Third, the amount of actual union
between mercury and alloy. Fourth, bulk at margin.
The Maximum Strength will be obtained Avhen the alloy contains
just enough mercury so that the mass will take the impression of
the skin markings after prolonged kneading between the thumb and
forefinger. Any more or less weakens the edge strength.
The Length of Time the Alloy Stands has an effect upon edge
strength, as amalgams made from alloys lose their edge strength pro-
gressively with time, the more rapidly the higher the average tem-
perature.
However Aged Alloys Slioiv Less Variations in Expansion, Con-
traction and Bange, and artificial aging is resorted to for this rea-
son and is done by annealing. This annealing produces an amalgam
that shows more uniform and consistent properties.
Annealing of Amalgam is accomplished by subjecting the alloy
when freshly cut to either a dry or moist heat ranging from 110° F.