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distortion precludes its use in cavities where the matrix can not be easily
removed. Since the pliability of metals decreases, and their tenacity
increases in direct ratio to their thickness, the gauge of the metal used
should be determined largely by the conditions presented by the cavity
in hand.
Of the metals used for matrices, the author believes platinum to be
the best for the construction of matrices of large cavities. While gold
and platinous gold are more pliable than platinum and are apparently
more easily manipulated, the tendency of a matrix made of them to dis-
tortion, either in the process of withdrawal or during the process of

Fig. 64.

fusing the porcelain, seems to be somewhat of a disadvantage. Some
operators invest matrices, made of thin foil, to prevent the porcelain from
changing their shape (Fig. 64), but the time consumed in investing the
matrix and in cooling it after each baking will be greater in total than
the additional time consumed for the construction of a platinum matrix.
Platinous. gold has more elasticity than pure gold, but is more liable to
tear during the process of swaging or burnishing.
Gold and platinous gold are annealed by the
HniKallna '^^^^ °^ ^ S^^ '^^ alcohol flame. But extreme care
matrix nittaU. must be exercised to prevent these metals from par-
tially fusing during this process. The metals should
be passed through the flame several times and should never be held in
an intensely hot flame for any considerable length of time, because of
the comparatively low temperature at which they fuse. Platinum should
be annealed in the furnace or in an oxyhydrogen flame. The method of

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