Editorial. Memoir of Prof. Chapin A. Harris. [Volume: 2, Issue: 5, December, 1860, pp. 274-288]

REVIEW OF DENTAL LITERATURE AND ART.

285

desired, care should be exercised that a good article is obtained, as much
of the white wax particularly. is mixed with fats that make it useless
for dental purposes. The yellow is more readily procured free from
adulterations, and is in many cases preferable.
" In introducing the impression cup, care should be taken that it is securely and directly pressed into position, as indirect pressure will destroy
any wax impression. The impression cup for the superior arch should be
so formed that the finger may press the wax into the spaces between the
teeth and against the palatine processes. The wax should also be pressed
firmly with the finger into the inequalities of the arch directly in the median line.
"I will now proceed to speak of plaster of Paris or sulphate of lime as
a material for taking impressions, which will, if properly manipulated,
produce better results than can be obtained with any other agent, though
all may, under some circumstances, be exceedingly useful. I will first
speak of obtaining an impression of the superior arch. Upon determining
to take a plaster impression, a cup should be selected that will nearly fit
the arch, and will extend around the tuberosities of the maxilla. As
remarked in the beginning, the mouth should be carefully examined, and
if the soft parts covering the palatine or alveolar processes are soft or
spongy, the plaster should be introduced thin, and with great care. The
soft parts covering the tuberosities will be found occasionally soft and
movable, requiring the greatest care to obtain a perfect result.
" The plaster of Paris and water, with a small portion of salt, (to hasten
its hardening,) should be mixed thoroughly in a glass or cup with a spatula,
which should be poured into the impression cup, and quickly transferred
to the mouth, care being taken that no particles fall upon the dress or face
of the patient. The proper introduction of the cup is one requiring nice
manipulation, and should be accomplished in the following manner:"The right side of the cup should be first introduced, pressing it well
toward the right cheek; the left cheek should next be distended by the
left index finger of the operator, which will allow the cup to pass readily
into the mouth. Having introduced it, the right index finger should be
placed in the centre of the cup, and a gradual upward pressure made, at
the same time that a slight forward and backward motion is given to the
cup.
" If before its introduction hardening of the plaster has commenced, or
there are bubbles of air between the plaster and the mucous membrane,
the last-named motions given to the cup will overcome these disadvantages. In obtaining plaster impressions, the head should be thrown
slightly forward, that portions of plaster may not flow over the soft palate,
and induce emesis. After the impression cup is in its right position in
the mouth, and previous to the hardening of the plaster, the finger should
be introduced between the cheek and the impression, that no muscular
folds may by their presence destroy the perfection of the impression.
" The length of time necessary for it to remain in the mouth must be
judged of by the quality of the material. Should the plaster have been
previously subjected to heat, by means of which a portion of its water of
crystallization has been driven off, it will harden very quickly. The ordinary plaster in use will usually set in four or five minutes.
i" In withdrawing the impression from the mouth after it has hardened,
the same care as in its introduction should be observed, that the soft parts
may not be injured, and the impression unnecessarily broken. If in its