. The principles and practice of dental surgery. overthe surface of the impression. After the plaster has sufficiently hardened, it may be trimmedand removed from the impression. This is done, in the case of 652 PLASTER MODELS. wax and gutta-percha, by softening in water, being careful tomake it so soft in partial cases, or when the ridge is thin, as notto break off any of the teeth or ridge. Plaster impressionsmay, in simple cases, be loosened by striking the back with theplaster-knife hamlle and removing entire. But in partial cases,and in undercut full cases, the cup must first be removed (
. The principles and practice of dental surgery. overthe surface of the impression. After the plaster has sufficiently hardened, it may be trimmedand removed from the impression. This is done, in the case of 652 PLASTER MODELS. wax and gutta-percha, by softening in water, being careful tomake it so soft in partial cases, or when the ridge is thin, as notto break off any of the teeth or ridge. Plaster impressionsmay, in simple cases, be loosened by striking the back with theplaster-knife hamlle and removing entire. But in partial cases,and in undercut full cases, the cup must first be removed (byhot water, if of gutta-percha), and the impression then carefullydetached in fragments. The same impression can sometimes beused a second or third time; but usually the shape of it is soaltered in the removal of the model, that a duplicate impressionis necessary if more than one model is wanted. The model maythen be shaped with a knife, until it presents an appearancesomething like that represented in Fig. 196, having a slight flare Fig. or taper, so as to admit of easy withdrawal when used in sand-moulding. This flare can be given by surrounding the impres-sion with a tapering tin ring, more regularly than it can betrimmed with a knife. The body of the model may be made in several ways: 1, byusing the tin rings, as just stated, which is the best for deepmodels used in moulding; 2, by surrounding the impression withsheet-wax, waxed cloth, sheet-lead or tin-foil; 3, by filling theimpression, then inverting it upon a mass of soft plaster builtupon the table to the required height. The last is, perhaps, themost convenient method, except for cheoplastic models and deepmodels used in sand-moulding. Models for vulcanite work need PLASTER MODELS. 653 no particular shaping, as they are subsequently to be set intoflasks. These should be made no deeper than is requisite forstrength and they must not be varnished, unless it be with dilutesoluble glass. Models from which the counte
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Keywords: ., bookcentury1800, bookdecade1860, bookpublisherphiladelphialindsa
