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FILLING MATERIALS. 101

By careful experiment it has been found that acid gases are
most likely to permanently obscure the welding property of
gold ; hence, if the gold be kept in an atmosphere containing a
liberal per cent of ammonia, ammonium salts will be formed on
the gold. These salts are readily volatile, hence, gold so kept
will always be readily cleaned by annealing and its welding prop-
erty restored.
"We may use gold, non-cohesive or cohesive, as we
choose, from the same book, or the same sheet, by sim-
ply keeping it in a dra-wer containing a small bottle in
which a bit of sponge, punk or cotton is placed and
occasionally saturated with spirits of ammonia. Used
without annealing this w^ill be perfectly non-cohesive,
or when annealed will be perfectly cohesive. This
should be taken advantage of in keeping gold in good
condition for use in either form.

Annealing Gold.
Gold is annealed to develop its welding property.
This annealing is not for the purpose of softening the metal, as
in annealing plate, but for the purpose of cleaning the surface of
the gold by volatilizing any gaseous film that may have' collected
upon it. This purpose is explained sufficiently in the preceding
article. The best means of accomplishing this will depend some-
what upon circumstances. For the general work of filling teeth
the annealing is very well done in the flame of the alcohol lamp
or the flame of a small bunsen burner. The element of time is
important in annealing, particularly if the annealing is done at
low temperatures, for the gases are not driven off" at once unless
a full red heat is obtained. Therefore, in annealing in the flame
the gold should always be brought to a glow, and so held for a
moment or two. It is also necessary to the best results that
every particle of the gold be brought to a glow. In taking
up the gold with the pliers, that portion of it that is caught
between the beaks, and that very close to the beaks, will not be
heated sufficiently to develop its welding property. This will
introduce into the filling spots of failure to weld and greatly
impair the strength of the work. For this reason great care
should be exercised in annealing to see that all parts of the gold
are brought fully to a glow. This is especially important when