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THE PORCELAIN INLAY. 385

painted all over the under side of the porcelain filling, so that the under-
cuts may be well filled. These coatings should then be allowed to dry
to the consistence of a thick jelly, when the filling is put into the cavity
and ironed into place with a hot instrument. The cement sets hard
only through evaporation of the solvent, and if the filling is too soon
inserted it will act as a stopper, preventing the hardening of the cement.
Where great strain is brought to bear upon the filling, or where the
cavity is not deep enough to Avarrant the use of any but the most
adhesive cement, oxyphosphate of zinc should be used. The rubber
dam should be applied wherever practicable. The filling and cavity
should be carefully undercut and dried in alcohol. Creamy, slow-set-
ting oxyphosphate of zinc, corresponding in color as nearly as possible
to the tooth, should then be placed in the cavity, and the filling, picked
up by means of a little cement on the spatula, be pressed home. The
porcelain should be held in position for a minute or two until the oxy-
phosphate has lost its elasticity ; for, however perfectly the porcelain
filling may have been fitted, if it does not go accurately into place the
edges will be as imperfect as though an ill-adapted matrix had been
used. As before mentioned, a creamy, slow-setting cement is essential,
and up to the present time the Harvard cement seems best to satisfy
these requirements. When the filling is finally in position the setting
of the cement may be hastened by a blast of hot air or a hot instru-
ment applied to the porcelain. When the cement is wiped away and
the tooth cleaned, paraffin or varnish should be flowed over the filling,
in order that the cement may set for six hours before it is exposed to
the action of the saliva. On the following day the edges may be
ground with an Arkansas stone or polished with sandpaper. It is
better for finishing that the edges should be a little too low than too
high. If, however, the porcelain is too high it can be ground down
and still give good results ; but the original gloss is in most cases to be
preferred. Having described the general operation of putting in a
porcelain filling, a few cautions may not be out of place before describ-
ing the special operations.
A filling smaller than a pinhead should not be attempted, as good
edges can hardly be obtained. Simple circular cavities without com-
pound curves in the margin frequently require only one burnish,
especially if they be small, as the contraction of the fused porcelain is
often less than the probable personal error of the operator.
Labial cavities should be made deep if good color and adhesion are
desired. A bar of porcelain running into the tooth makes a much
stronger anchorage than a platinum pin, as the platinum may stretch
and it always tends to weaken the substance of the body.
In large contours excessive contraction may be avoided by adding one
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