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It is not necessary to invest a matrix which is constructed of a ma-
terial i/iooo or 1/2000 of an inch in thickness. It should be held with a-
pair of lock tweezers during the process of applying and carving the por-
celain (Fig. 100). The tweezers should grip the matrix at the point
wliere it will offer the greatest resistance. It has been found that this
point is at or near the linguo-gingival angle of a matrix for a single ap-
pro.ximal or an inciso-approximal cavity. Care should be exercised, how-
ever, to hold the matrix in such manner as to prevent its distortion while
applying and carving the porcelain (Figs. loi and 102). Although the
matrix has a limited degree of rigidity, it can not be handled roughly
without changing its form.

-m

Fig. 100.

In case thin, pure gold is used for a matrix it is necessary to invest it
(Fig. 64), to prevent its distortion during the process of applying the
porcelain and to prevent its warping during the fusing. The investing
cup should be made of some metal that does not readily oxidize, and that
does not fuse at ordinary temperatures. The cup should be filled with
powdered asbestos, which has been made plastic with water or alcohol.
Apply a small quantity of the paste around the matrix and force it gently
into the cup. Care must be exercised in investing the matrix that its side
next to the asbestos may be supported at all points by the paste. This
may be easily accomplished by lightly tapping the cup on the table. The
operator should be careful, however, to prevent the asbestos from coming
in contact with the margins of the matrix. After properly investing the
matrix the investment is to be dried slowly until all steam has escaped,
or until all alcohol has burned away. If the matrix is filled with wax
before removing it from the cavity, and this is left in the matrix until the
investing is finished, none of the paste could flow through any holes in the
matri.x. But in case there is no wax in it during the process of invest-

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