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The inlay metal is manipulated differently from amalgam, or the oxy-
phosphate cements. It fuses at a comparatively low temperature, and is
cast into the impression. It is largely composed of sulphur and graphite,
and if overheated it ignites, giving off the disagreeable odor of sulphur.
If the metal is permitted to burn, its composition is changed and its prop-
erties are affected. It gives a sharp casting, and models constructed of this
metal are fairly accurate. Low fusing allows, composed of tin, lead, bis-
muth and cadium, may be used for constructing models. They are par-
ticularly indicated in those cases where the matrix is to be burnished into
the cavity after it has been swaged. They cast with a sharp imprint, and
it requires but a few moments to construct a model with these alloys.
The technique of constructing models varies, and each material re-
quires a method of constructing peculiar to it. If models are to be con-
structed of amalgam or cement without pressure, the impression should
be invested with the impression of the cavity upward, in plaster of Paris
to reinforce the frail walls (Fig. 84). This also gives a base which tends
to hold the impression steady while the material for the model is packed
into it. A rubber ring one inch in diameter and one-half inch long, is
filled with plaster, mixed to a thick consistency, and then the impression
is forced into the plaster so that all except the imprint of the cavity and
the walls of the tooth are covered.
When amalgam is to be used for constructing
JImalaam models. a model, it should be mixed so that it has ample
plasticity after a portion of the mercury has been
expressed. It should be forced into the deep depressions of the im-
pression with small flat-faced pluggers, and the excess mercury should
be removed with cotton. The bulk of the impression should be filled by
condensing the amalgam with large flat-faced pluggers. This prevents
the "chopping" of the amalgam during the process of condensation and
forces the excess mercury to the surface, which should be removed with
pledgets of cotton or spunk.
If oxyphosphate of zinc is used for a model it
Oxyphosphatc should be spatulated to a putty-like consistency and
models. kneaded between the thumb and finger, until all air
bubbles are excluded. The cement should be formed
into an irregular conical shaped mass and forced into the impression with
the thumbs and large burnishers. If the model is so constructed from an
impression taken in cement, the impression should be coated with soap-
stone, and the cement used for the construction of the model should be
of different color from that of the impression.

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