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Chapter TU.
Chf formation of matrices for Porcelain Tnlays.
Matrix Metals. Methods of Constructing. Technique of Burnishing. Impression
Methods. Models of Cavities; Amalgam; Cement. Devices and Methods
of Swaging. Refitting Matrices.

Before describing the methods of forming matrices, let us consider
the materials that are used for this purpose. Such a consideration shows
that gold, platinum and platinous gold are used for the construction of
matrices, and that each material has properties characteristic of it. In
formulating a table for comparison of these three metals, the numbers
used for such comparison refer to this particular table and not to
the general table used in the comparison of metals.
Fusing point 2016° F. Malleability, first rank;
Qeld. tenacity, third rank ; pliability, first rank ; ease of
annealing, first rank.
Fusing point above 3500° F. Malleability, third
Platinum. rank ; tenacity, first rank ; pliability, third rank, and
ease of annealing, third rank.
(One to three per cent) : Fusing point, ap-
Platinou$ Sold. proximately from 2030° to 2040 F. Malleability,
second rank ; tenacity, second rank ; pliability, second
rank ; and ease of annealing, equal to that of gold.
Gold and platinous gold can only be used to advantage with low
fusing, while platinum can be used with high or low fusing porcelains.
Matrices can be constructed of platinous gold as thin as 3/10000 of an
inch, while pure gold thinner than 4/10000 of an inch can not be manipu-
lated to good advantage. They may be constructed of platinum foil as
thin as 1/2000 of an inch, but the best results are generally obtained by
the use of foil i/iooo of an inch in thickness.
There are cases in which pure gold as thin as 4/10000 of an inch
may be used for the construction of a matrix, for small cavities of free
accessibility, but of this thickness, gold is so pliable that the danger of

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