with the glaze layer intact. The glaze is effective in reducing crack propagaion within the outer surface because the surface flaws may bebridged and the surface will be under a state of compressive stressive stress. However, the results from one study indicate that porcelains with highly polished surfaces (1-umabrasive paste) have comparable strength to that of specimens that were polished and glazed (Fairhurst et al, 1992). This observation is of clinical importance because after the porcelain prosthesis is cemented kin the mouth, it is common practice for the dentist to adjust the occlusion by grinding the surface of the porcelain with a diamond bur, unfortunately, this procedure weakens the porcelain markedly if the glaze is removed and the surface is left in a rough condition. If the porcelain surface is rough, a natural glaze treatment is recommended since the fracture resistance of the surface is greater than that of unglazed porcelains. Porcelains for metalceramic and ceramic prostheses, porcelain veneers, or denture teeth may be characterized with stains and glazes to provide a more lifelike appearance. The fusing temperatures of glazes are reduced by the addition of glass modifiers that lower the chemical durability of glazes somewhat. Stains are simply glazes and are subject to the same chemical durability problem However, most of the currently available glazes have adequate durability if they are as thick as 50 um or more. One method for ensuring that the applied characterizing stains will be permanent is to use them internally. Internal staining and characterization can produce a lifelike result, particularly when simulated enamel craze lines and other features are built into the porcelain rather than merely applied to the surface. The disadvantage of internal staining and characterization is unsuitable. It is logical to assume that fine polishing of a roughened surface followed by glazing produces smoother surface than polishing alone, sandblasting followed by glazing, or diamond grinding followed by glazing. A highly polished and glazed surface is smoother than the surfaces of glazed specimens that have been sandblasted or roughened with a diamond followed by glazing. Cooling of Metal-Ceramic Prostheses The proper cooling of a porcelain prosthesis from its firing temperature to room temperature is the subject of considerable controversy. The catastrophic fracture of glass that has been subjected to sudden changes in temperature is a familiare experience. The cooling of dental porcelain is a complex matter, particularly when the porcelain is fused to a metallic substrate. Multiple firings of a metal-ceramic restoration can cause the coefficient of thermal contraction of the porcelain to increase and can actually make it more likely to crack because of tensile stress development. The chief limitation to the use of an all-porcelain crown in fixed prosthodontics is its lack of tensile strength. A method for minimizing this disadvantage is to fuse the porcelain directly to a metal coping that fits the prepared tooth. Such a metal-ceramic prosthesis is shown schematically in figure 21-1. The metal on the facial side is approximately 0.3 to 0.5mm thick. It is veneered with opaque porcelain approximately 0.3mm in thickness. The body porcelain is about 1mm thick. If a stronger material is used as an inner core of a ceramic crown, cracks can develop only when the stronger material is deformed or broken, assuming that the veneering porcelain is firmly bonded to the stronger substrate. With proper design and physical properties of the porcelainand metal, the porcelain is rein-forced so that brittle fracture can be abided or at least minimized when these crowns are restricted to anterior teeth. Although most metal-ceramic prostheses involve
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