Page 69 - My FlipBook

FILLING TEETH
53
to thoroughly blacken the mica, and another lot will
leave it quite clean. The safest thing to do is to
use rectified spirits of Avine. Absolute alcohol has
been recommended, but does not appear to be any
better in this connection than rectified spirit, and is
more expensive. It is difficult to anneal gold on a
sheet of mica ; the mica does not become red-hot
except at the edges, and the time necessary to hold
the mica over the flame can only be guessed at,
although some operators, from great experience,
seem instinctively to know when the desired effect
is produced. The writer made a few experiments
in trying to regulate this by timing Avith the
second-hand of a watch, but gave it up in despair.
The gold was always either under, or over, annealed.
There is no difficulty in using a thin sheet of
platinum, for as soon as the platinum glows with
the heat, the gold is. fully and satisfactorily annealed.
It has already been mentioned that skill and ex-
perience with the sheet of mica are particularly
valuable in the production of semi-cohesive gold.
All gold that is to be used cohesively, whether
foil or crystal gold, should be annealed before use.
It can be heated to a dull red without injury, and
in fact, if the greatest cohesion and hardness of the
filling is desired, it is necessary to heat it to this
point. The gold will, however, in many cases work