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PORCELAIN INLAYS 177
cutting edge wall. Whether this is partly due to
a warping of the porcelain on cooling, drawing up
the matrix, or whether it is solely due to the heat
warping the matrix, the writer cannot say, but the
fact calls attention to the value of having a good
overlap of the matrix at these parts, and so cover-
ing up the overlap with the investment that it is
held firmly in place. Some dentists endeavour
to prevent warping by breaking up old artificial
teeth into small pieces, smearing these pieces
with thinly mixed body, and placing them in
the matrix. Thinly mixed body is then placed in
the matrix, so that all the interstices between the
pieces of broken-up teeth are filled up. The
porcelain on fusion unites everything together,
and the contour is then built up in the ordinary
way. This method, for a knowledge of which the
writer is indebted to Mr. Charters Birch, seems to
be of value principally in non-invested matrices of
a certain shape, for a well-invested matrix will be
found to rarely change its shape.
Dr. Jenkins recommends an investment of pow-
dered asbestos and water, and this will be found
the most satisfactory material to use, provided it
is the right kind. The investment must be
dried with heat, as, unlike mixtures of plaster of
Paris and marble dust, &c., it does not set in any
M