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COHESIVE GOLD IN THE MAKING OF A FILLING 125
of solidity is not possible of attainment unless the gold is eon-
fined and the wedging principle is takeji advantage of.
Cohesion of Gold. The surfaces of pure gold when absolutely
clean readily cohere. This cohesion is brought about by the fric-
tion of the surfaces of the gold M^hen in absolute adaptation. The
degree of cohesion is in proportion to the friction. The friction
is in proportion to the load, the extent of the surfaces in opposi-
tion and the speed of the travel of the surfaces one upon the other.
Hence, the greater the load, the smaller the surface, and the more
rapid the movement of one surface upon the other the greater
the cohesion. Polished surfaces of gold must be brought into co-
adaptation in order to get cohesion. The smaller the surfaces and
the thinner the sheets, the less load and speed will be required.
The Serrated Plugger Points are used in condensing cohesive
gold for the folloAving reasons: That these polished surfaces may
be kept small and uniform; that great pressure (load) may be eas-
ily exerted on the polished planes previously left in the surface
of the gold by the wedge-shaped serrations. The mallet is applied
to give the additional factor in friction (speed) as the fresh gold
is moved over these small polished surfaces. The above conditions
are obtained with the least exertion on the part of the operator
and annoyance to the patient by the serrated plugger point, which
is made of a collection of pyramids which act as so many Avedges
and exert great lateral force (load) upon the polished sides of
their previous impression. That gold coheres to polished surfaces
can be easily demonstrated by taking any cohesive gold filling
and burnishing its surface to a glossy finish. Pellets of gold from
the annealer will readily cohere and the filling may be continued
to full contour by applying a steel burnisher with heavy pressure
drawn over the surface of the fresh gold. This process proves that
burnished gold coheres, but it is slow and laborious and objection-
able to the patient, hence the serrated plugger point which ac-
complishes the same result, the friction of polished surfaces of
gold under pressure, causing their Avelding.
Bridging is the term applied to that faulty manipulation which
results in air spaces within the body of the filling, caused by the
gold failing to reach the bottom of the indentations of the serrated
plugger point.
The Cause may be insufficient pressure being given the plugger
point, the gold thereby stopping short of the bottom of the serra-
tions, or it may be caused by too much light malleting, going over
the gold surface repeatedly thereby bending down the crests of