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PORCELAIN INLAYS 189

matrix is stripped off, the thin film of porcelain
that covers the silex at the bottom of the inlay is
readily pierced and broken away with a steel point
or an excavator, and the silex is easily removed
with a damp brush. Mr. Rippon uses a platinum
or platinum-gold matrix without investment. He
places the matrix on a tray filled with powdered
silex, and consequently the fusing of his inlays pro-
duces the little lumps in the silex that are utilised
in this process. He mixes the inlay body with dis-
tilled water, in which orum-trasfacanth is dissolved.
This method is by no means difficult if all the
various details are grasped, and as its success
depends on this, the writer has endeavoured to
describe it as fully as possible. Whenever the case
will admit of it, Mr. Rippon cuts a depression in the
floor of the cavity in the tooth, corresponding to
the hole in the inlay, and also undercuts the
depression in the tooth. The writer finds that
although this is an extra precaution which is doubt-
less very valuable in certain cases, that the hole in
the inlay provides excellent retainage when no
corresponding undercut can be made in the tooth.
It is desirable for the film of porcelain at the base
of the inlay to be sufficiently broken away to pro-
vide a fairly broad or wide opening, although
whether the hole in the inlay is undercut or