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400 RESTORATION OF TEETH BY CEMENTED INLAYS.

MAKING MATRICES UPON MODELS FOR PORCELAIN INLAYS.
There are many wlio take impressions of the cavities intended for
porcchiin inlays, which impressions are run out in plaster or oxyphos-
})liate of zinc. On the models thus obtained the matrices are formed,
and the porcelain fillings finished according to shades selected when the
impression was taken. This has the advantage of saving much time for
the operator, as the work of construction may be done by an assistant
in the laboratory. As good results are claimed, the process should be
given a fair trial ; but on theoretical grounds the expansion of the
plaster, or the contraction of the oxyphosphate of zinc^ used for the
mould would tend to cause inaccuracy in adaptation of the edge.
Most inlay workers find it sufficiently difficult to get perfect adaptation
when the matrix is burnished to sharp enamel edges, and, except in
labial cavities, to get the exact contour. Also, the colors often require
such nice calculation that an assistant, not having seen the mouth,
could hardly mix them satisfactorily. While good results have been
obtained by giving the matrices for filling to an assistant especially
trained in the art of fusing and mixing the porcelains, thus proving
that such time-saving methods are practicable, the average dentist
should not attempt the art of inlays with the idea of saving time;
probably there will always be sufficient factors for failure if he works
directly from the tooth cavity and gives his entire personal skill to
the completion of the filling. With porcelain inlays the question of
artistic color eifect is paramount, and only through the most subtle
discrimination can the best results be obtained.
^ Wet oxyphospliate of zinc usually expands; dry oxyphosphate usually contracts.