Prosthetic dentistry / by James Harrison Prothero.

  • Prothero, James Harrison.
Date:
1916
Catalogue details

Licence: Public Domain Mark

Credit: Prosthetic dentistry / by James Harrison Prothero. Source: Wellcome Collection.

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    sistant to stress. Why or how it sets, or how much pressure it will withstand without becoming distorted, are points of vital importance, practically unknown to, or at least disre- garded by, prosthetists. this material. The greater portion is used in the arts, build- ing trades, and in glass-making industries. <A number of investigators, thoroughly competent, by training and with suitable apparatus, have made extended researches in the chemical and physical aspect of this material, and some of the results of their findings, together with some experimental work carried out by the writer, will here be presented. Professor M. Glasenapp, chief of the technical department of the University of Riga, Russia, an authority on building materials, has gone into this subject exhaustively. Although not relevant in all respects to the use of plaster in the pros- thetic field, the following extracts from his writings will shed much light on the chemical as well as the physical aspects of W. Michaels of Chicago. SETTING ‘‘Tf ordinary plaster of Paris, representing mainly the half hydrate CaSO, + 1/2H.0O is mixed with water and exam- ined under a microscope, a lively process of ee can be observed to set in after five_or_ six s. In the beginning, very thin needle- erystals form on the cover glass and shortly afterwards also in the liquid and on the particles of gypsum. After 15 or 20 minutes, single needle-crystals and groups of them can be seen in great numbers, and espe- cially the larger fragments of the half hydrate are covered with needle-crystals radiating from them, also the character- istic twin crystals appear abundantly. As fast as the crystals form, the original particles of the half hydrate disappear. iter an hour they are completely used up and transformed into crystals, whereby the larger fragments of them become centers of accumulation of crystals, while the smaller have been converted into isolated star-shaped groups. After the same gypsum was heated for several hours to a temperature of 170 deg. C. (whereby the amount of water still remained 6.2 per cent, corresponding with the half hydrate) crystal- lization began after 3 to 4 minutes, and was practically com- pleted after half an hour; only the largest particles required almost an hour to dissolve and re-crystallize.”’