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A method for viscosity measurement is described and representative data are given for opaque, gingival, and incisal porcelains. The significance of viscosity in annealing and in stress development due to thermal expansion mismatch is discussed. Thermal history of the porcelain is shown to be important only over a relatively small temperature interval in affecting residual stresses.
UNLABELLED: Although the metal crown is aesthetically improved when porcelain is fused to it, a large number of different factors often cause the porcelain to break. Here we report a method to prevent this from happening. As a hardening treatment for the porcelain surface, the Na-ions in the porcelain, which have a short radius, were replaced by K-ions having a long radius to strength the surface against compressive forces. RESULTS: 1. The bending test showed that the strength of the porcelain had increased from 20 to 38%. 2. The hardness test indicated that the strength had increased from 23 to 24%. 3. The breaking test showed that although the strength of the porcelain had not increased to any great extent, the strength of the veneer shell had increase by more than 30%. 4. The EPMA measurement showed that ion replacement had penetrated to as much as 50 to 80 microns. 5. Clinical tests indicate that even after 19 months, the surface of the porcelain remains the same and that the condition is quite stable.
A machine designed to simulate the physical parameters of masticatory function was used to investigate the amount of wear produced on perspex plates opposing discs of porcelain which were glazed, unglazed or finished to varying stages of a polishing sequence recommended with a proprietary finishing kit. The perspex specimens were abraded, under water at 37 degrees C for a total of 800000 contacts using a contact time of 0.2 s, a sliding distance of 15 mm and a constant load of 0.19 N/mm2. Assessments of the wear of the perspex were based upon depth measurements of the wear track recorded on surfometric tracings. Further measurements of the cross-sectional area of the wear track were made using an image analysing computer. The investigation confirmed that the best finish and least abrasive surface was produced by glazing of porcelain. The finish produced by intermediate components of the proprietary finishing kit did not reduce the abrasiveness of the porcelain surface. It was necessary to complete the polishing sequence with diamond paste to achieve a surface which approached the wear characteristics of glazed porcelain. It is recommended that any adjusted porcelain restoration should be re-glazed or subjected to a finishing sequence which is followed through to a final stage of polishing with a diamond paste.
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The bond strength of porcelain to the four gold alloys and the three nonprecious alloys was tested with a simplified pull test method, having wide test surfaces. The tests showed that all gold alloys tested were suitable for fusing porcelain. The bond strength in nonprecious alloys was somewhat lower and the location of the fracture lines was more variable. It seems that nonprecious alloys are more sensitive to laboratory procedures. The bond strength and the location of the fracture line had a good correlation in all metal alloys.
Many ceramists advocate polishing, rather than glazing, to control the surface luster of metal ceramic restorations. There is concern that compared to a natural glaze, the polished surface might have inferior properties. This study compared apparent fracture toughness (Kc) and stainability of a commercially available metal ceramic system with either a glazed or polished surface finish. The indentation technique was used for Kc determinations and colorimetry measurements of specimens immersed in a coffee solution for staining. Higher fracture toughness values were found for the polished specimens. No differences were detected in the staining characteristics.
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This study of the fluorescence of natural enamel and of dental ceramics shows the fluorescence of ceramics not containing rare earths decreases when the color saturation increases; the fluorescence of samples of the same shade guide are not homogenous; some guides show a strong green fluorescence; and two shade guides of the same origin can present completely different fluorescence. The cementing medium can affect the fluorescence of a ceramic prosthesis.
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