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GUTTA-PERCHA. 327

of zinc oxid ; in medium-heat the ratio is 1 to 6 or 7 ; and in the high-
heat specimens the gutta-percha is ahnost saturated with zinc oxid.
Physical Properties.—Gutta-percha is an almost perfect non-con-
ductor both of heat and electricity. It is less hard and rigid than any
other filling material. It contracts in hardening, i. e. cooling. Softened
masses of it are coherent when dry, but not when wet. Its color may
be made to resemble that of the teeth. To vital tissues it is the most
bland, unirritating filling material known.
After it has served as a filling for a greater or less period it is found
to have increased in hardness and difficulty of softening, and its surface,
and perhaps its substance, has become porous in variable degree. The
increased hardness is observed in such situations as those in which
putrefactive decomposition occurs ; that is, in places where there is an
evolution of hydrogen sulfid ; the gutta-percha apparently undergoes a
species of vulcanization. It becomes somewhat porous in those situa-
tions where the formation of a solvent is active (lactic acid), which
abstracts the soluble zinc oxid from the mass. The pink variety con-
taining the insoluble mercury sulfid does not become porous, but wears
with a comparatively smooth surface w^hen subjected to attrition.
Examining in detail these several physical properties it will be noted
that gutta-percha has but one property in common with gold—its insol-
ubility. Its rational employment is therefore in such situations and
conditions as those in which the use of gold is contraindicated.
Indications for its Employment.—First, in its several forms it is
employed as a temporary filling material for both the temporary and
permanent teeth. Owing to its non-conductivity it is employed near
the pulp ; its insolubility recommends its use at the cervical margins of
cavities, particularly in the buccal cavities of molars which do not
extend to the masticating surface, where the non-resistance of the
material would cause its rapid wasting.
This is the most common of the situations in which gutta-percha
is
applied : very deep cavities upon the buccal surfaces of molars, extend-
ing beneath the gum, and having ragged enamel margins, the orifice
of the cavity being much smaller than its body. Owing to its non-
irritating quality, the condition of the gum in contact with a gutta-
percha filling remains normal.
It is used in approximal cavities of the anterior teeth which have a
similar form to those just described ; also in labial cavities, particularly
when these teeth are in any degree loose. For example : in a cavity
opening alone upon the distal wall of a canine tooth the carious process
has almost invaded the pulp, the enamel walls unsupported by dentin
still retain their form and have a good texture.
Pink base plate is invaluable for the temporary filling of spaces after