The practical manual of dental casting : being the recorded experiences of many able and eminent men in the dental profession.

Date:: 1913

Credit: The practical manual of dental casting : being the recorded experiences of many able and eminent men in the dental profession. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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$shorter. Tliis loiulitioii \vc know would he impossible to oijtiiiii; and tiiu only chance for us is to force enough stretch into it to allow it to reach over the interior of such a cavi(;y. We are firmly of the opinion that the investment has a very great deal to do with shrinkage, or lack of the same. An investment material should expand suf- ficiently on heating to follow up the expansion of the brass flask which contains it. With such an investment the shrinkage can easily be taken care of, e.xcept in large bridge castings. During the autumn of '07 your essayist conducted a more or less exhaustive series of experiments in an effort to formulate a suitable investment compound. The result of the experiments was given at a regular meeting of the Pennsylvania Association of Dental Surgeons, during the same autumn, and also in a paper read at San Francisco in June, '08:—a record of which is in the Pacific Gazette, Vol. XVI, p. 586. The best results \vc obtained were gotten in a compound made up of cast plaster 1 part, and very finely powdered silcx 3 parts (by weight). In April and May, '09, we conducted a series of experiments to obtain data on shrinkage of gold castings. Castings made in Peck's investment compound sl.owed 83 - 100 of one per cent, shrinkage; S. S. White's showed IS - 100 of one per cent., and in silex and plaster as above given, we had only 16-100 of one per cent, shrinkage. These figures are for casts made in hot molds. With the same investments and technique, except that the flasks were allowed to cool to a tem- perature which allowed handling with the fingers, we had a loss of 1.64 per cent, by shrinkage in the plaster and silex investment, and proportionately more with the other investments mentioned. Quoting from your essayist. Pacific Gazelle, Vol. XVI, p. 589: A smoother casting is obtained if the investment has been allowed to remain some hours, or even days, before heat is applied. While this idea is all right for the purpose stated we find that if a flask is allowed to stand a long time before casting we have a marked increase in the shrinkage of the casting made in such a mold. We, therefore, find it advisable to choose the least evil, and cast within a few hours, at most, after investing. We, with others, believe that the time will come when the perfection of casting devices will make it possible for us to pre-determinc the amount of shrinkage. Quoting Dr. Baker, Denial Reviezv, Vol. XXIV, p. 130: I believe that the time will come when we can control the size of the cast we want by the temperature of our in- \-estment. When we get this work down to a certain point we will need a thermostat to tell the temperature of our investment when we cast in order to get the size we want. Quoting your essayist, Dental Digest, Vol. XV, p. 499: It yet remains for some genius to devise a form of casting apparatus with a pyrometer attachment to indicate the temperature of the flask while casting; this, in conjunction Avith an investment material of known physical properties could be a means of accurately determining the relative size of the casting. We believe that a misconception is more or less generally extant in regard to why any pressure should be needed to place and hold the molten gold in the mold in casting, the commonly accepted idea being that the pressure used is needed because of air and gases within the mold. Quoting Dr. Trigger, Dental Brief, ]u\y. '09 p 469- In order to force the molten gold into such a mold already described it requires a pressure exerted on the metal in the molten state to overcome the atmospheric pressure of air within the mold, that is, the force on the gold muse more than equalize the air within to allow it to flow readily to all parts. .According to experiments which we conducted we believe that air or gas resistance has so l.ttle$