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308 OPERATIVE DENTISTRY
ia the lock tweezers at that portion where the metal has been left
to extend the farthest from the cavo-surface angle.
The matrix should now be passed through the alcohol flame when
the camphor or Avax remaining will be burned off leaving no ash.
Wood as an Impression. In simple small cavities it is well to
shape a piece of soft pine (as cork pine) to proximately fit the cav-
ity. This should be then introduced against the deepest portion
of the cavity and given a few blows from the mallet which will
cause the wood to conform to the floor of the cavity. This should
then be used as an impression and the matrix forming proceeded
with, as described when modeling compound has been used. The
use of the stick with modeling compound on the end is of advan-
tage in large deep cavities where the pulp chamber is to be filled
with porcelain in place of metal pin. By this means it is possible
to place a matrix well to the bottom of any cavity without tearing,
provided the walls are regular and have the proper draAv devoid of
under cuts.
Taking the Spring- Out of a Matrix. If a matrix seems to retain
''spring" and does not seem to lay well on all surfaces, as fre-
quently met with in complex cavity outlines, this may be removed
by the following method : When cavity is thoroughly packed with
wet cotton, stretch a piece of rubber dam over the matrix, cotton
and all, and thoroughly burnish the entire outline. If ** spring"
still persists, remove the matrix and anneal, and then repeat the
method when it will be found that the fault has been removed.
Selection of Porcelain. The selection of that portion of the in-
\ay which replaces dentine and that which replaces enamel should
be attended to before the process of building begins. The part
replacing dentine should be of foundation body coarsely ground
and of a yellow color in all vital cases. In devital cases this shade
may be darkened by the addition of the brown shade, and in vital
teeth for young patients, particularly if the cavity is shallow, or
on a distal surface, the addition of white powder is of advantage
to lighten the shade of yellow.
The enamel shades may be decided upon after a careful study of
the shades and hues found in each case. Delicate shading is se-
cured by building one layer upon another, thus getting the benefits
of reflected light. The deep and pronounced shades and colors are
best obtained by building in sections. Teeth that are much of one
color and not pronounced in lines of shades will be best represented
by the layer method, while teeth that are decidedly yellow at the