Page 134 - My FlipBook

ii8 THE TREATMENT OF TEETH
In many cases, even if no break occurs, the
curling up of the amalgam edges produces a trench
or cavity for the collection of food particles, with
equally disastrous results. This curling up of the
edges takes time, and although there is a great deal
of difference in the behaviour of different alloys in
this respect, three years is generally sufficient to
produce these defects. It has been noticed that
this curling up of the edges is particularly manifest
on occlusal surfaces, and more particularly on the
occlusal surfaces of approximo - occlusal cavities.
There is no doubt that many amalgam fillings seem
to "grow out" of the cavity on buccal surfaces
where they are subjected to no force of mastication,
but that the force of mastication on an amalgam
filling increases this even if, in many cases, it does
not produce it, will be apparent to any one who has
used his powers of observation for a number of
years.
Dr. Black explains this as being due to the flow of
the metal (amalgam) under force or pressure. It has
also been pointed out by Dr. Black that an amalgam
filling made of filings, or shavings, varies consider-
ably both in its mixing or working properties, and
in its behaviour under tests for shrinkage and flow.
The explanation is, that filing or cutting up an
ingot hardens or tempers the particles into which