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PORCELAIN INLAYS 175

painted over the interior of the matrix, and fused
so that it is lined with a thin glaze. This stiffens
the matrix, and it can then be placed back in the
cavity, and re-adapted. The thin glaze will break
or crack at certain parts during the re-adaptation,
but sufficient will remain intact to hold the matrix
more firmly in place during this final adaptation,
and a more perfect mould will result. If a gold
matrix is used, it should then be invested. Any
overlap that would interfere with the removal of
the matrix should be cut off with a lancet or
other suitable sharp instrument before it is re-
moved. It is usually necessary to invest a gold
matrix, not only to prevent warping of the metal
under heat, but also to permit of its being easily
handled without distortion. It is a question
whether the shrinkage of the porcelain has any-
thing to do with distorting the matrix. Two kinds
of shrinkage take place. One is the running
together of the powder during fusion, and the
consequent obliteration of the numbers of minute
spaces that existed between the particles of powder.
The other is the probable shrinkage of the fused
mass during cooling. It is doubtful if any appreci-
able shrinkage takes place in the fused mass when
used in such small bulk as inlays. When the
particles of powder run together during fusion, the