Page 44 - My FlipBook

2 8 THE TREATMENT OF TEETH
it before use, in order to drive off anything that
may happen to have collected on its surface. This
"welding property is made use of by the majority of
dentists in making gold fillings.
Non-cohesive gold is so prepared that there is no
possibility of one piece sticking to another. Any
welding is quite out of the question, and the filling
is made by so wedging and interlocking the pieces
of gold together in a cavity that it forms a tight,
dense plug. This non-cohesive property is probably
attained by subjecting the leaves of pure gold foil
to some vapour which deposits something on its
surface. If this non-cohesive gold is annealed, it
will sometimes be noticed that a vapour is given off,
and when brought to a dull red heat it becomes, as
a rule, thoroughly cohesive. There are a few makes
of non- cohesive gold which become so slightly co-
hesive on being fully annealed that this does not
prevent their being worked non-cohesively. These
foils, the character of which is only slightly changed
by annealing, are often spoken off as " true, non-
cohesive gold." They can be worked cohesively, but
it demands special care and manipulation. What
it is that gives them this particular property is a
trade secret. It has been stated that it is due to
the addition of a very small quantity of alloy to the
gold. These true, non-cohesive foils, however, seem