During resin-bonding procedures, dentin surfaces are treated with acidic conditioners to remove the smear layer and decalcify the surface to expose the collagen fibrils of the underlying matrix. These decalcified surfaces are then either air-dried or treated with dehydrating solvents, procedures which may modify the physical properties of the dentin matrix. The purpose of this study was to evaluate the effects of dehydration on the stiffness of the decalcified dentin matrix. Small (8 x 1.7 x 0.9 mm) beams of dentin were prepared from mid-coronal dentin of extracted human molars. The ends were covered with varnish for protection, and the specimens were placed in 0.5 M EDTA for 5 days to decalcify. The stiffness was measured by both the cantilever technique and by conventional stress-strain testing. Specimens tested by the cantilever technique were sequentially exposed to water, acetone, alcohol, HEMA, and glutaraldehyde. Specimens tested by conventional stress-strain testing were exposed either to water, acetone, or HEMA, or were allowed to air-dry. The results indicate that the stiffness of decalcified human dentin matrix is very low (ca. 7 MPa), if the specimens are wet with water. As they are dehydrated, either chemically in water-miscible organic solvents or physically in air, the stiffness increases 20- to 38-fold at low strains or three- to six-fold at high strains. These increases in modulus were rapidly reversed by rehydration in water. Exposure to glutaraldehyde also produced an increase in stiffness that was not reversible when the specimens were placed back in water.

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The mechanism of formation of nodular material on the surface of a Pd-Ag-based alloy for porcelain during pre-porcelainization heat treatment was investigated using scanning electron microscopy, x-ray diffraction, quantitative metallography, and Auger electron spectroscopy. The nodules were found to form by a Nabarro-Herring creep mechanism driven by the internal oxidation of tin and indium. Implications of this process with regard to porcelain bonding and discoloration are discussed.

The bimaterial strip, particularly in the form of a split ring, has been used since 1930 (Schurecht and Pole) to determine the thermal expansion compatibility between porcelain and metal substrates. An analysis by Timoshenko (1925) relates the expansion difference to the radius of curvature and stress in the composite. Tuccillo and Nielsen (1972) used that theory to predict the stress from the curvature in such strips. The change in the gap in the split ring is a sensitive measure of the change in curvature, but there are end-effects (Tesk et al., 1981) that introduce a small error when curvature is calculated from gap width. DeHoff and Anusavice (1985) suggest using a half-ring to avoid those effects, and provide an analytical solution for the stresses that agrees well with a finite element analysis. All of this work has been based on the assumption that the materials in the composite are elastic. In many cases, the authors were well aware of the fact that dental porcelains are viscoe-lastic, but used the elastic analyses to obtain results in terms of readily measured properties, and to establish useful "rules of thumb". The purpose of this note is to emphasize that the viscoelastic character of the porcelain or glass layer invalidates the elastic analysis, and can lead to erroneous predictions of stress. In particular, it is not valid to use the elastic equations to predict the stress from the curvature of the composite. The stress (o'j) from thermal expansion mismatch in an elastic body has the form X=-KAEf (1) where K is a constant involving the elastic properties of the materials and a geometric factor, and-Efn = Ef2-Ef 1 (2) where Efn is the free strain in material n. The free strain is usually approximated by fTr Efn-fT on dT (3) where oXn is the linear thermal expansion coefficient of material n, Tr is room temperature, and Ts is called the setting temperature. It is assumed that the glass is fluid above Ts, so that it sustains no stress, and is elastic below TS. This approximation replaces the gradual increase of viscosity in the glass transition region with an instantaneous freezing process. It is assumed that the setting temperature is a property of the glass, so that the same value of Ts can be used in any composite of the same materials. However, it has been shown (Rekhson and Mazurin, 1977) that Ts depends on the geometry of the composite. Consider a thin layer of glass on a thick metal substrate. The more massive metal will force the glass to flow even when its viscosity is high, so the setting temperature will be low. On the other hand, if the glass is much thicker than the metal, it will stop flowing at a relatively high temperature, because the metal can exert little force on it, and the setting temperature will be high. If Ts is measured on a composite with a thick glass member, and the corresponding value of ACf is used to predict the stress in a seal with a thin glass member, the stress will be overestimated. In fact, even the sign of the stress may be incorrectly predicted (Scherer and Rekh...