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PLASTER MODELS OR CASTS. 19

that they can be readily separated, either l)y taking- the
model in the liand and tapping- the handle of the cup, or
by slipping' a wedge-shaped instrument between the impres-
sion and cast at its posterior border.
We usually find more or less under cuts, however, and
often the ridge is thin and prominent, or we may have a
number of teeth remaining. Jn all these cases, and they
represent a large majority, the impression can only be
removed with safety, that is, so as not to deface the model,
by being carefully broken away piece by piece. The rim

Fig. 6.

of plaster on the outer side of the ridge is first removed
this is best accomplished by removing the cup from the
impression, and then cut a groove around the latter at a
point directly over the ridge, as deeply as possible without
marring the face of the cast, then begin at one corner and
break away the rim piece by piece, after which the palatine
portion can be readily removed. Fig. 6 indicates where
grooves should be cut so that the impression may easily be
removed from the cast. When it is desired to prepare a
model for permanent preservation, it should be thoroughly
dried and then immersed for a short time in a solution of