Page 76 - My FlipBook

If oxyphosphate of zinc cement is used for an impression material
it should be mixed to a putty-like consistency, kneaded between the
thumb and finger, and then placed in the impression trav. The material
should be forced into the cavity, in which it should remain until the cement
has thoroughly hardened. The impression may fracture or change its
form if the cement is removed while it is in a semi-plastic state.
Models of cavities may be made in amalgam,
models of oxyphosphate of zincjnlay metal (a preparation pre-
pared by the S. S. White Company, which is similar
Gacities.
to Spence's metal) and low fusing alloys. Each of

Fig. 84.

these materials has some advantages, and if used when indicated, will
meet the requirements.
Models constructed of amalgam present a sharp outline and seem to
be as accurate as it is possible to obtain them unless they are constructed
under pressure. The advantage of accu'-acy is the only point in its favor.
It requires from four to six hours for the amalgam to thoroughly set,
and unless it is manipulated at the proper consistency it gives a granular
surface over which the matrix is to be swaged. There is always an ele-
ment of uncertainty regarding the accuracy of the amalgam model, and
the operator can not be relieved of the suspense until he has separated it
from the impression.
Oxyphosphate of zinc may be used for constructing models, but the
difficulty of securing accurate models unless constructed under pressure
causes the element of uncertainty to be an objection. There are, how-
ever, these advantages over amalgam : The model may be separated from
the impression in a comparativly short time, and if it is faulty another
one may be constructed without delay, and the operator is not required
to wait several hours before the matrix can be swaged.

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