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Page 100 text:
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The prognosis z Slnniana mental Qtullege 1 1904 mutual if its of each form and the ardent advocates of the one can see no good in the other. Both have their application, and the dentist should no more confine himself to one form of porcelain than he sho-uld to one form of gold or silver. All high fusing porcelains contain silica, kaolin, feld- spar, a flux and the oxides of certain metals. Silica is the oxide of silicon, it is almost infusible and inso-lub-le in all acids except hydrofiuoric. It gives the fused porcelain a translucent appearance and adds strength. Kaolin is the silicate of aluminum and is a very refractory clay. This is added to porcelain to impart stability of fo-rm, and it is this pro-perty which permits it to be molded and carved. Relatively large proportions of kaolin tend to give porce- lain an opaque appearance. Feldspar is a double silicate of aluminum and potassium. On account of its ready fusibility, it serves to agglutinate the more refractory in- gredients, and when diffused throughout the mass imparts a semi-translucent appearance. The flux is composed of some of the following: glass, the carbonates of sodium and potassium, calcined sodium borate, sodium and alumi- num Huorid fcryolitej. One or mo-re of the materials is added to the silica, or feldspar, or both, and are used to render the porcelain more fusible. The basal ingredients of high fusing porcelain may be used in a formula for low fusing, but the fusing point is regulated by the proportion of Hux added to the formula. The flux is incorporated with the basal ingredients and the mass heated until the Hux agglutinates the refractory ma- terials. The process of fusing and grinding is continued until all the ingredients are thoroughly mixed. The action of the flux on the refractory materials increases the fusi- bility and the porosity of the porcelain. It is impossible to always fuse the porcelain so that there is no surplus overlapping the platinum. All margins of the porcelain overhanging thecavity walls are ground away with line stones and cuttle fish disks. If proper care is exercised in constructing the inlay, only a small amount of the glaze on the exposed surface need be destroyed. The glaze should be removed from the seat and walls in order to- obtain a better adhesion with the cement. This is done
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Page 99 text:
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I - . 'QUE-19EDQIID5f5 1. ggnniana Dental QEDIIBQB 1-1904 annual inlays are Hat-face fissure and inlay burs, small Arkansas stones and excavators with short blades and their edgesso formed that they will make all angleseither right or obtuse and well defined. A q There are three' general methods o-f constructing matrices for cavities: . I. Swaging the foil over a negative or an impression of the cavities. A 2. Swaging into a positive o-f the cavity. 3. Burnishing directly into the cavity. In cavities involving only the labial or buccal or lingual surfaces or in simple proximal cavities, a matrix can be swaged over an impression and an' inlay constructed which accurately Hts into the cavity. The method of swaging into a' positive die of the cavity has some followers, but it hardly seems possible that a positive die can be obtained that will reproduce the margins of the cavity with that degree of accuracy that is required in inlay work. The method of burnishing the foil directly into the cavity has more advo- cates than either ofthe other methods, and many who have tried the swaging processhave returned to this method. A combination of 'theiswaging and burnishing methods can be used successfully. The technic of constructing a matrix is as follows: Take an impression of the cavity in dental lac or cement and then construct a'positive die of the cavity by mixing a mass of cement and imbedding the impression in it. Allo-w the cement to harden, then separate and invest the die' in the ring of a swaging device. Coat the die with soapstone, thennplace a piece of foil over it and swage with a water bag or unvulcanized rubber., Remo-ve the matrix and co-ver and burnish the metal to the margins. The heat required to fuse the po-rcelain thor- oughly, anneals the metal. This method is especially good in large cavities, and has the advantage of conforming the foil to the cavity by the use of the die so that the final bur- nishing is made quite easy. THE FUSING OF PORCELAINS Porcelain is divided into- high and low fusing bodies. There is a difference o-f opinion regarding the relative mer-
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