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BASIC ZINC CEMENTS. 341

rests upon the lingual surface of the tooth it is burnished into contact
with the edges of the cavity, forming walls to the latter (320, B). The
cement is introduced as in the preceding case, and Mhen the cavity is
full the free end of the strip is drawn upon, compressing and round-
ing the filling. Should the cement be an adhesive specimen or mixed
thinner than described, the surface of the flexible mat- Fig. 321.
rix is to be faintly oiled by means of olive oil.
At the completion of the operation the cement should
be exactly flush with the margins except at the labial
aspect, and the surface of the cement should have such
smoothness that polishing is not necessary. Cement
fillings are polished dry with the finest of cuttlefish
disks.
The process of filling the body of any cavity is the
same, except when the enamel walls are thin and frail.
In the latter case, where space permits, it is preferable
to line the walls with the oxyehlorid of zinc, over which
the phosphate is placed. Before inserting a veneer fill-
ing of gold or amalgam, each cavity margin must be
scraped free from cement.
When orthodontia appliances such as rings or caps,
or prosthetic appliances, crowns and bridges, are to be
set it is preferable to use a cement prepared for that
purpose, although it is the general practice to use the
cement to which the operator is accustomed, mixing it
thinner than for filling purposes. Wherever possible,
it is advisable to operate under rubber dam, even while
setting orthodontia appliances.
The tooth is cleansed with chloroform—as, for ex-
ample, when a ring or cap is set^—to remove fatty mat-
ters, and a layer of shellac varnish applied, which is
then dried by the air blast (chip blower). Cement
paste is formed, of such consistence tliat it \\\\\ floAV
readily and yet not be watery ; the inside of the band
or cap is filled with cement by means of an appropriate
spatula (Fig. 321) ; a layer of cement is placed on the
tooth where it is to be embraced by the band, which is
then pressed into position and is to remain without
disturbance until it is hard. The application of bands
or ligatures should be deferred until the following day.
As soon as the cement is hard the surplus is cut away
and the dam removed. Pointed spatula.