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168 OPERATIVE DENTISTRY
means of dryness must be used. When one of the surrounding
walls is missing as in proxinio-occlusal cavities in bicuspids and
molars (Class Two) the matrix must be in place. The first pellet
of tin introduced should completely cover the base of cavity and
be thoroughly condensed by good steady hand pressure, with
points at least one square millimeter in size employing the rock-
ing motion. The points should have deep serrations and be so stepped
as to include the entire surface.
This hand pressure should be folloAved with the mallet force
using a plugger point of medium serrations and the surface en-
tirely gone over. A new pellet may now be applied and the plan
just given repeated. If the filling is to be entirely of tin the cavity
should be filled to excess and by a process of burnishing, con-
densed and rubbed to the size desired. This last method gives a
surface of the greatest density possible.
Tin and Gold. When the fillhig is to be completed Avith cohesive
gold little dependence should be put upon the gold adhering to the
tin as the union is only slight. With a round-pointed instrument
new convenience angles should be made in the substance of the
tin near the line angles. The remainder of the cavity should be
retentive independent of the space occupied by the tin.
Tin and Amalgam. No special care is needed when the filling is
to be completed Avith amalgam. Amalgamation takes place in that
portion of the tin next to the amalgam proper and the union is
([uite strong, even more than tin to tin. The amalgam should, if
possible, be more thoroughly mixed and the process of kneading
prolonged that all amalgamation possible be secured before con-
tacting Avith the tin as the tin Avill take up some of the mercury
from the amalgam for which it has a great affinity. This is liable
to injure the amalgam as to strength unless the mixing has been
thorough. The use of tin and amalgam is not advised where the
surface of the tin is to be exposed by forming any portion of the
contour as the presence of the mercury absorbed causes the tin to
rapidly disintegrate. Gold should be used for topping in such
cases.
Tin in Bifurcated and Punctured Roots, When through decay
or by accident the cavity extends to the exposure of the peridental
membrane the use of tin has no substitute. The opening should
be rendered as clean as possible, sterilized and dried. The open-
ing should be covered with a mat of pure tin made from folded
sheets, being lightly burnished to place and covered with amal-
gam and the cavity finished with the desired material.