Page 237 - My FlipBook

THE PORCELAIN INLAY 205

High fusing porcelains shrink 15 to 25 per cent, of their

volume, while low fusing bodies contract from 20 to per
35
cent, during the baking process.
The advantages claimed for the porcelain inlay are har-
monious color and non-conductivity, while its disadvantages

are friability, difficult manipulation, the presence of the
cement line and impossibility to preserve freshly cut cavity
walls during its construction.
Following is a list of the prominent porcelain bodies on
the market with their fusing points.

Whiteley's 2,300°F.
S. S. White Dental Mfg. Co 2,30o°F.
Consolidated Dental Mfg. Co 2,6oo°F.
Johnson & Lund 2,55o°F.
Ash & Son i,9oo°F.
I Jenkins i,55o°F.

The two latter may be considered low fusing bodies.
Obtaining the Matrix.—Platinum foil, o.ooi of an inch in
thickness for high fusing, and pure gold or platinized gold

for low fusing porcelain are used for this purpose. The
platinum foil may also be used for low fusing work.
For obtaining the matrix, first anneal the material in the
electric furnace, carrying gold or platinized gold to full red

heat (1,200 to i,300°F.), and platinum to a dull yellow color;
then follow one or a combination of the three following
METHODS, viz.:
I. Burnish Directly into the Cavity.—Cut the foil into
proper size and shape (Fig. 256), allowing sufficient excess to

extend one-fourth to one millimeter beyond the margins of
the cavity, except that in one location it may extend 2 or
3
mm. for convenience in handling. Force the matrix care-
fully into the cavity (beginning at the seat of the cavity first,

avoiding tearing and crumpling) by means of small pieces
of spunk or chamois discs and smooth burnishers. Now,