Page 30 - My FlipBook

^8 MECHANICAL DENTISTRY AND METALLURGY.

shellac is not a parting fluid, but is used to stain the plaster,
so that the line of demarkation between the impression and
cast will be more clearly indicated. After this has thor-
oughly dried the impression should be coated, by means of
a camel's hair brush, with the soapy solution, which gives
it a smooth, glossy surface, ensuring easy separation. Care
must be taken, however, to leave none of this solution un-
absorbed on the impression or in the imprints of the teeth,
or the face of the cast will not be as sharp and smooth as
is desirable. This is best removed by washing the surface
off thoroughly, when the impression will be ready to recei^'e
the plaster for the model.
The same measures for mixing and pouring cast and for
strengthening the plaster teeth should be pursued as directed
where wax or modeling compound has been used.
A very simple though effective method of staining the
plaster is to color the water used in mixing the plaster for
the impression with anilin red or rose pink. The latter
gives the plaster such a delicate pink color that it is quite
unobjectionable to the patient, and at the same time the
coloring is sufficient, so that the impression can be readily
distinguished from the cast in separating. When this is
used the shellac varnish, of course, is not necessary.
The objections to oil are that plaster will not flow
smoothly over an oiled surface, and that it has a tendency
to soften the surface of the cast.
Separation.—The manner of separation is more difficult
where plaster has been used for the impression material, and
requires more care. It is needless to say that one good
cast is better than several poor ones, so every care should
be taken in separating, and no effort should be made to save
the impression for further use, as it would usually be at
the expense of the cast should we do so. There are cases,
however, both upper and lower, where the mouth is so flat