Page 83 - My FlipBook

CELLULOID AS A DENTAL BASE. 7 1
ber, for the purpose of carrying off the camphor which is
set free in the process.
With the celhiloid blank adjusted to its proper position
in the flask, the latter is placed in the clamp and the top
screwed down until it tightly presses the flask. It is then
placed in the oven or tank, and heat applied.
If gas is used, the form of burner shown underneath the
heater in Fig. 40, which gives a pure blue flame without
smoke, may be used. If gas cannot be commanded, how-
ever, any of the alcohol or kerosene lamps commonly em-
ployed in vulcanizing may be substituted.
Having applied the heat, it is of the first importance that
unremitting attention should be given to the process of
molding until it is completed. If pressure is applied be-
fore the celluloid is rendered somewhat plastic, or too
great force is exerted during the earlier stages of the proc-
ess, and without sufiicient intervals of rest, there is danger
of crushing or fracturing the model and of impairing the
articulation by displacement of the teeth. On the other
hand, the nature of the celluloid is such that if it is ex-
posed to a temperature of 270, without being under pres-
sure, the camphor evaporates, and the material, besides
being rendered hard and intractable, is puffed up, exactly
as a loaf of bread is raised by yeast, and filled with air
cells, and thus rendered porous.
Celluloid begins to soften at about 225, and will then
yield slightly to pressure, but this should be applied very
gently at first, with no more force than can be readily
exerted with the thumb and finger. As the heat increases,
and the celluloid becomes more and more plastic and yield-
ing, the pressure should be correspondingly increased, but
always interruptedly, giving the material time, between
each turn of the screw or nuts, to escape from under the
pressure. No considerable amount of pressure will be re-