Page 381 - My FlipBook

THE PORCELAIN INLAY. 379

freely away from the back, as in Fig. 374, «, 6, c, d. When the
fillings are between bicnspids the palatal or lingual walls should be cut
unsparingly whenever it is necessary. Where the filling is to stand the
force of mastication the edges of the cavity should be at least at right
angles to the grinding surface, as in Figs. 370 and 375. The prepara-
tion of the cavity being completed, if high-fusing porcelain is to be
used the matrix must be made with rolled platinum one one-thousandth
of an inch in thickness. If found desirable, thinner platinum may be
used for small cavities ; but the firm burnishing required usually re-
duces the foil of one one-thousandth of an inch to one three-thousandth
Fig. 361.

A, Cavity in anterior approximal surftice of first molar.
or one four-thousandth of an inch on the margins, where excessive
thickness of the metal is objectionable. Foil thinner than one one-
thousandth of an inch seems to lack sufficient body to stretch properly
Avithout tearing. This platinum, if annealed in a Bunsen burner or
blowpipe, will be harsh and unfit for use, but when annealed in a muffle
or electric furnace it becomes soft and tough. The platinum is placed im-
movably over the cavity and spun into all parts with a ball burnisher
(Figs. 353 and 354), great care being taken to define the edges sharply
and smoothlv. The edges that wrinkle mav be flattened and smoothed
with the broad surfaces of the spatula shown in Fig. 356. Should the
bott(Hu tear, it is of little moment as long as the edges are smooth and
intact. Sometimes it is useful to form the matrix by driving the metal