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for confining the material to tlie desired location. It not only prevents
the impression material from crowding into the interproximal space, but
holds it to proper conformation along the axial walls of the cavity.
These trays are constructed from a stock of copper or German silver
thimbles and may be made to conform to the tooth for each operation
with a loss of but very little time. The tray should be so fitted that the
length in the occluso-gingival direction will permit its edge to extend
about I mm. beyond the gingival margin of the cavity. The handle (Fig.

£

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Fig. 82.

Fig. S3.

83) which should be used with the tray permits it to be manipulated more
easily and prevents obstruction to view during the process of fitting.
When any of the above mentioned materials are to be used for taking
the impression, a small pellet of the material is made plastic and placed
in the impression tray. The material should be passed through the flame
to give its surface a glazed-like appearance. It is then pressed into the
cavity, which is moist, with the pressure directed on a line with the axis
of the tooth, and then chilled quickly and removed. Gutta percha does
not give so sharp an impression as modeling compound. It requires
more heat to make it plastic and more pressure to force it to the walls of
the cavity. Its use may be indicated, however, in those gingival cavities
extending beneath the gum margin, where a moderate amount of force is
required to press the gum from the cavity in order that an accurate im-
pression of the gingival wall may be obtained.