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BASIC ZINC CEMENTS. 335

or even inflammation. The extent of the irritation is largely governed
by the fluidity of the cement paste, /. e. the amount of zinc chlorid present.
It sets in flfteen minutes sufficiently to permit the packing upon it
of an amalgam, and in half an hour a gold filling. After setting it is
whiter though less hard than the zinc phosphate; it shrinks, particularly
when used in large masses. It is a poor thermal conductor, and, like
all bodies containing zinc oxid, is soluble in lactic acid—the usual sol-
vent in the oral cavity. These several features are at present regarded
as limiting the application of oxychlorid to—first, a lining material for
carious cavities over which the insoluble filling proper is to be placed
;
second, as a root-filling material (its use in this connection is discussed
in Chapter XVII.). It is to be noted that the cement retains after
setting an antiseptic power for a greater or less period.
Use.—Zinc oxychlorid is usually employed as a lining material in
teeth having what is known as poor structure—those in which caries
proceeds to great depths without external evidence of the extent of
invasion. After these cavities have been partially excavated it is found
that further excavation and the removal of the deepest layers of the
leathery dentin which appear to have retained sensitivity would prob-
ably uncover the pulp ; it may be that the. pulp has given subjective
evidence of a mild attack of active hyperemia.
In such cases the deepest layer of the partially disorganized dentin
is permitted to remain and is subjected to the prolonged—fifteen minutes
or longer—contact of hydrogen peroxid in the 25 per cent, ethereal
solution (caustic pyrozone), 5 per cent, aqueous solution of formalin,
or preferal)ly a saturated solution of thymol in alcohol. The cavity
walls are well dried with bibulous paper and the warm-air blast.
Upon a mixing slab (see Fig. 315), a drop or two of the zinc chlorid
is placed, and beside it a quantity of the zinc oxid powder. The
powder is gradually incorporated with the fluid by means of a spatula
until a creamy paste is made. A number of balls of bibulous paper
are to be at hand. A portion of the paste is taken upon the end of
an instrument and placed in the cavity, where it is quickly pressed into
a layer against the cavity walls by means of the balls of bibulous paper.
The walls are to be covered to a uniform depth of about one-sixteenth
of an inch. The prompt application of the bibulous paper usually pre-
vents any irritation due to the contact of the oxychlorid with the dentin
overlying the pulp. Should the cavity be very deep it is advisable to
protect the pulp by interposing a film of ethereal varnish between the
oxychlorid and the dentin over the pulp.
At the completion of the lining operation, the margins of the cavities
are to be cleansed of the oxychlorid and the filling completed with the
material indicated.