Biomedical subjects

J R Mackert

Publications and source records attributed to J R Mackert.

At least 19 recordsLinked to original sources

PubMed2001

Evidence of a critical leucite particle size for microcracking in dental porcelains.

The leucite particles in dental porcelains are often partially encircled by microcracks that are the result of the thermal expansion mismatch between leucite and the surrounding glass matrix. Although the magnitude of the stress at the particle-matrix interface is independent of the particle size (Selsing, 1961), Davidge and Green (1968) showed experimentally that there is a critical particle size below which microcracking is absent. The critical particle size is explained by a Griffith-type energy balance criterion: Below the critical size, the stress magnitude may be sufficient to cause cracking, but there is insufficient strain energy for the creation of the new surfaces of the microcrack. The purpose of the present study was to determine whether the mean leucite particle size of a dental porcelain influences the degree of microcracking in the porcelain. Microcrack density, leucite particle surface area per unit volume, and leucite mean volume-surface diameter, D3,2, were determined by quantitative stereology on 10 specimens each of 6 dental porcelains and Component No. 1 of the Weinstein et al. patent (US Patent 3,052,982, 1962). The fraction of leucite particles with microcracks around them, f(mc), was estimated for each porcelain from the microcrack density and the leucite surface area. Using the equations of Selsing (1961) and Davidge and Green (1968), we calculated the critical particle diameter, Dc, for leucite to be 4 microm. The porcelains were partitioned according to whether their mean leucite particle diameters, D3,2, fell above or below Dc, and their values of f(mc) were analyzed by a permutation test with random re-sampling. The porcelains with mean leucite particle diameters below Dc had a significantly lower fraction of cracked particles compared with the porcelains with mean leucite particle diameters above Dc (p < 0.05). This study provides evidence that microcracking in dental porcelain can be minimized by a reduction of the mean leucite particle diameter to less than 4 microm.

Aluminum Silicates↗

PubMed2000

Water-enhanced crystallization of leucite in dental porcelain.

OBJECTIVE: The purpose of this study was to determine whether long-term exposure of dental porcelain to saliva during temporary cementation of a porcelain-fused-to-metal (PFM) restoration could enhance leucite crystallization if the restoration is refired. Such water-enhanced leucite crystallization in dental porcelains could lead to porcelain-metal thermal incompatibility problems. METHODS: Six commercial dental body porcelains and the Component No. 1 (leucite-containing) frit of the Weinstein et al. [13] patent were studied. For each porcelain, 30 coupon specimens were randomly assigned to a treatment group. Ten specimens were placed in artificial saliva, 10 in distilled water, and 10 in a desiccator and were stored for six months. At the end of the six months, an additional 10 coupons of each porcelain were prepared to serve as a control. All 40 specimens of each porcelain were randomized and subjected to one additional firing. Leucite weight fraction was determined by quantitative X-ray powder diffraction analysis via an internal standard technique. RESULTS: Comparisons among the treatments via the least-squares-means test-adjusting for porcelain showed that the saliva group mean leucite weight fraction was significantly higher than that of the other groups. The change in porcelain thermal expansion that would be associated with a leucite change in this range would be between 0.2 x 10(-6) K-1 and 0.3 x 10(-6) K-1. SIGNIFICANCE: The results of this work constitute the first demonstration that moisture absorbed by a porcelain can act as a glass modifier and enhance the crystallization of the glass during subsequent firing. The effect was sufficiently large to generate thermal expansion changes that would exceed the maximum safe mismatch between porcelain and metal.

Absorption↗

PubMed2000

High-temperature X-ray diffraction measurement of sanidine thermal expansion.

Dental porcelains that are designed to be fused to PFM (porcelain-fused-to-metal) alloys are formulated by their manufacturers to be closely matched in thermal expansion to these alloys. The high thermal expansion of the mineral leucite has been exploited to regulate porcelain expansion. Leucite, however, has been observed to convert to the sanidine polymorph of feldspar during certain heat treatments within the normal firing range of dental porcelain. The effects of this conversion on porcelain thermal expansion and porcelain-metal thermal compatibility have been uncertain, due to the paucity of published data on the thermal expansion of sanidine. The purpose of this study was to measure the thermal expansion of sanidine by high-temperature X-ray diffraction over the temperature range in which thermal mismatch stresses can develop in porcelain-fused-to-metal restorations, i.e., from room temperature to 700 degrees C. The lattice parameters a, b, c, and beta were determined from the d-spacings and hkl values of multiple reflections by means of a least-squares iteration. The dependence of each lattice parameter on temperature was determined via analysis of variance, and the coefficient of thermal expansion, alpha, was obtained from this analysis. The lattice parameters of sanidine at room temperature were determined to be: a = 0.8524 +/- 0.0015 nm, b = 1.3020 +/- 0.0004 nm, c = 0.7165 +/- 0.0002 nm, and beta = 116.02 degrees +/- 0.01 degree (mean +/- 95% confidence interval). The linear thermal expansion coefficient, a, over the range from room temperature to 700 degrees C was determined to be 4.1 x 10(-6) K(-1) +/- 0.6 x 10(6) K(-1) (mean +/- 95% confidence interval). Because the coefficient of thermal expansion for sanidine is substantially lower than that of leucite (the effective linear thermal coefficient of thermal expansion of leucite over the range of 25 degrees to 700 degrees C is 28 x 10(-6) K(-1)), the conversion of leucite to sanidine during porcelain heat treatments would produce a detrimental lowering of the porcelain thermal expansion.

Aluminum Silicates↗

PubMed1999

Elimination, via high-rate laser dilatometry, of structural relaxation during thermal expansion measurement of dental porcelains.

OBJECTIVES: Thermal expansion measurement of glassy materials is complicated by thermal history effects. Excess volume--trapped in quenched dental porcelains after firing--collapses via structural relaxation on first slow heating during conventional dilatometry, making the thermal expansion coefficient (alpha) obtained on first heating unreliable. The purpose of this study was to determine whether porcelain thermal expansion measurement at high thermal rates could minimize the influence of thermal history. METHODS: Eight thermal expansion specimens each of six body porcelains and the Component No. 1 (leucite-containing) frit prepared according to the patent by Weinstein et al. (US Patent No. 3,052,982) were subjected to three heat-cool conventional dilatometry runs at 3 degrees C/min, while eight thermal expansion specimens of each porcelain were reserved as untreated controls. Eight hollow, cylindrical specimens of the same brands were subjected to three heat-cool laser dilatometer thermal expansion runs at 600 degrees C/min, while eight cylindrical specimens of each porcelain were reserved as untreated controls. Thermal expansion data (25-500 degrees C) of all specimens were subjected to repeated measures analysis of variance. RESULTS: The alpha obtained on first slow heating was significantly lower than values for succeeding slow heat and cool runs in all porcelains (P < 0.001). High-rate alpha obtained on first heating was not significantly different from values of succeeding heat and cool runs in all porcelains (P > 0.05). SIGNIFICANCE: Conventional dilatometer measurements demonstrated occurrence of structural relaxation, as evidenced by the significant difference in the first heating and subsequent runs. High-rate laser dilatometry eliminated structural relaxation, thereby providing a thermal expansion measurement that is free of interference from thermal history effects.

Analysis of Variance↗

PubMed1997

Mercury exposure from dental amalgam fillings: absorbed dose and the potential for adverse health effects.

This review examines the question of whether adverse health effects are attributable to amalgam-derived mercury. The issue of absorbed dose of mercury from amalgam is addressed first. The use of intra-oral Hg vapor measurements to estimate daily uptake must take into account the differences between the collection volume and flow rate of the measuring instrument and the inspiratory volume and flow rate of air through the mouth during inhalation of a single breath. Failure to account for these differences will result in substantial overestimation of the absorbed dose. Other factors that must be considered when making estimates of Hg uptake from amalgam include the accurate measurement of baseline (unstimulated) mercury release rates and the greater stimulation of Hg release afforded by chewing gum relative to ordinary food. The measured levels of amalgam-derived mercury in brain, blood, and urine are shown to be consistent with low absorbed doses (1-3 micrograms/day). Published relationships between the number of amalgam surfaces and urine levels are used to estimate the number of amalgam surfaces that would be required to produce the 30 micrograms/g creatinine urine mercury level stated by WHO to be associated with the most subtle, pre-clinical effects in the most sensitive individuals. From 450 to 530 amalgam surfaces would be required to produce the 30 micrograms/g creatinine urine mercury level for people without any excessive gum-chewing habits. The potential for adverse health effects and for improvement in health following amalgam removal is also addressed. Finally, the issue of whether any material can ever be completely exonerated of claims of producing adverse health effects is considered.

Absorption↗

PubMed1996

Castability, opaque masking, and porcelain bonding of 17 porcelain-fused-to-metal alloys.

Seventeen porcelain-fused-to-metal alloys, which represented a cross section of the various alloy types available, were evaluated for castability, opaque masking, and porcelain bond strength. The base metal alloys generally cast more completely than the noble alloys, with the presence of beryllium as an important factor for greater castability among the base metal alloys. Statistically significant differences were observed in the ability of an opaque porcelain to mask the different alloy substrates but no systematic effect of alloy type was observed. Porcelain bond testing revealed that nickel-chromium-beryllium alloys produced significantly better porcelain-metal bonds than nickel-chromium alloys without beryllium. In addition, it was found that palladium-copper alloys produced significantly better bonds with porcelain than palladium-cobalt alloys.

Analysis of Variance↗

PubMed1996

Potential interference of leucite crystallization during porcelain thermal expansion measurement.

OBJECTIVES: The elevated temperatures and slow heating and cooling rates associated with a thermal expansion run provide a thermal treatment comparable to those previously shown to produce leucite crystallization in dental PFM porcelains. The purpose of this study was to determine whether the exposure of dental porcelains to the time-temperature profile of a thermal expansion run could result in leucite crystallization, a potential interference with the accurate determination of the thermal expansion coefficient. METHODS: A total of 112 thermal expansion specimens were prepared, 16 each of six commercially available body porcelains, as well as the "Component No. 1" (leucite-containing) frit of the Weinstein et al. (1962) patent. In a randomized design, the leucite contents of specimens subjected to a 3 degrees C/min thermal expansion run were compared to those of untreated controls. The leucite content of each specimen was determined via quantitative X-ray powder diffraction using an Al2O3 internal standard. One-tailed t-tests were applied to determine whether the dilatometer thermal treatment produced a statistically significant increase in leucite content. RESULTS: The heat treatment afforded by a 3 degrees C/min dilatometer run was not sufficient to induce measurable leucite crystallization in any of the porcelains examined in this study. SIGNIFICANCE: The crystallization of leucite during a dilatometer heating run can be ruled out as a possible interference with the determination of the coefficient of thermal expansion of dental porcelain.

Aluminum Silicates↗

PubMed1996

Microcracks in dental porcelain and their behavior during multiple firing.

Dental porcelains rely on the high-thermal-expansion mineral leucite to elevate their bulk thermal expansion to levels compatible with dental PFM alloys. The microcracks that form around these leucite particles when cooled during porcelain manufacture are a potential source of change in bulk porcelain thermal expansion during fabrication of porcelain-fused-to-metal crowns and bridges. The purpose of the present study was to determine whether multiple firings of commercial dental porcelains could produce changes in microcrack density. Specimens of six commercial porcelains and the "Component No. 1" of the Weinstein patent were fabricated and subjected to 1, 2, 4, 8, and 16 firings. The microcrack densities were determined by quantitative stereology, whereby intersections of microcracks were counted with a test grid. The microcrack data were subjected to linear regression analysis and analysis of variance. The microcrack densities of four of the six porcelains and the Component No. 1 frit were not significantly affected by the number of firings (p > 0.05). One porcelain exhibited a weak but highly significant positive correlation between microcrack density and multiple firings (r2 = 0.24, p = 0.0003), while the remaining porcelain exhibited a weak but statistically significant negative correlation between microcrack density and multiple firings (r2 = 0.15, p = 0.006). The results of this study indicate that even for porcelains that exhibit a measurable change in microcrack density as a function of multiple firings, the magnitude of the increase or decrease in microcrack density after several firings is sufficiently small to cause only negligible shifts in porcelain bulk thermal expansion.

Aluminum Silicates↗

PubMed1996

Effect of cubic leucite stabilization on the flexural strength of feldspathic dental porcelain.

Previous studies (Mackert and Evans, 1993) have shown that, when feldspathic dental porcelain is cooled, leucite undergoes a transformation from cubic to tetragonal, associated with a 1.2% volume contraction. This contraction leads to the formation of microcracks in and around the crystals and the development of tangential compressive stresses around the crystals. Our aim was to stabilize increasing amounts of the cubic form of leucite in a leucitereinforced dental porcelain, evaluate its effect on the flexural strength, and characterize its microstructure. The hypothesis was that in the absence of crystallographic transformation, the contraction of the leucite crystals would be lower, thereby limiting the formation of microcracks and minimizing the development of tangential compressive stresses around the leucite particles. We prepared 8 porcelain compositions by mixing increasing amounts of either leucite (KAlSi2O6) or pollucite (CsAlSi2O6) with Optec HSP porcelain (Jeneric/Pentron Inc., Wallingford, CT). Porcelain disks were made from each composition (n = 10 per group). X-ray diffraction analyses showed that the amount of stabilized leucite increased with the amount of pollucite added. The microstructure of the specimens containing tetragonal leucite was characterized by twinned leucite crystals, whereas no twinning was observed in the specimens containing cubic leucite. The evaluation of crack deflection showed that significantly less deflection occurred in the specimens containing cubic leucite. The mean biaxial flexural strength for the group corresponding to 22.2 wt% added pollucite, fired at 1038 degrees C, was significantly lower than that for the control group. The group corresponding to 22.2 wt% added leucite fired at 1150 degrees C exhibited a mean biaxial flexural strength significantly higher than that of all other groups that were not significantly different from the control group. Overall, the stabilization of cubic leucite reduced the flexural strength and the number of crack deflections in leucite-reinforced porcelain. Apparently, the development of tangential compressive stresses around the leucite crystals when cooled is responsible for a significant amount of strengthening of feldspathic dental porcelain.

Aluminum Silicates↗

PubMed1995

Isothermal anneal effect on leucite content in dental porcelains.

Porcelain thermal expansion behavior is a function of leucite content, and leucite changes can produce porcelain-metal incompatibilities. The purpose of the present study was to determine whether isothermal anneal treatments (such as are encountered during "post-soldering" operations) could result in alterations of leucite content of dental porcelain. Six commercial dental porcelains and the "Component No. 1" frit of the Weinstein et al. patent (1962) were studied. Twenty-eight coupon specimens of each porcelain were fired and divided randomly into seven groups of four specimens each. One group served as a control, while the other six groups were subjected to isothermal anneal treatments--three groups at 500 degrees C and three groups at 750 degrees C, for 4, 8, and 16 min, respectively. Leucite volume fraction was measured via quantitative x-ray diffraction. From an equation for the growth of particles in competition for diffusing material, an expression was derived for the potential simultaneous processes of leucite crystallization and reaction of leucite+glass to form sanidine or other compounds. This exponential growth/decay curve was fit through the experimental data for each porcelain by minimization of the sum of the squares of the residuals through iteration. For each porcelain, a statistically significant correlation was obtained between leucite volume fraction and isothermal anneal duration at 750 degrees C (p < 0.01), whereas at 500 degrees C, only CII exhibited a statistically significant correlation (p < 0.01). The control (non-annealed) leucite volume fractions for the commercial porcelains ranged from (mean +/- SD) 0.155 +/- 0.002 for WIL to 0.442 +/- 0.003 for Component No. 1. The 750 degrees C isothermal anneal increased the leucite volume fraction by between 6.1% and 21.3%.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum Silicates↗

PubMed1994

Isothermal anneal effect on microcrack density around leucite particles in dental porcelain.

Because of the large differential in thermal expansion coefficient between leucite and the surrounding glass matrix, microcracks form around the leucite crystallites during the manufacture of dental porcelain frits. These microcracks decouple leucite from the surrounding glass matrix and affect the bulk thermal expansion of the porcelain frit (Binns, 1983). The purpose of this study was to determine if the microcrack density in a dental porcelain decreased as a result of isothermal heat treatment. Ten specimens of a commercial dental porcelain that had previously exhibited an increase in thermal expansion as a function of isothermal heat treatment were prepared and divided into two groups. The experimental group was heated to 750 degrees C and held for 16 minutes at that temperature. The control group received no anneal. The mean microcrack densities were determined by quantitative stereology to be 575 cm2/cm3 +/- 75 cm2/cm3 (mean +/- SEM) for the control group (no anneal) and 231 cm2/cm3 +/- 25 cm2/cm3 for the experimental group (16-minute anneal at 750 degrees C). The specimens annealed at 750 degrees C had a significantly lower microcrack density (p < 0.001) than those that received no anneal. A model was developed to estimate the effect of microcracking on thermal expansion of the porcelain, and a 6% increase in the coefficient of thermal expansion of the porcelain was predicted from this model as a result of this decrease in microcrack density.

Aluminum Silicates↗

PubMed1992

Side-effects of dental ceramics.

Evaluation of side-effects to low-dose exposure of any agent is difficult, especially if the agent exhibits a low toxicity. The most common way to approach such evaluation is to define special groups which are exposed more than others. Studies of such risk groups may facilitate interpretation of information related to those exposed to a low dose. For dental materials, dentists, dental assistants, and laboratory technicians represent typical risk groups. In addition to receiving dental treatments and having restorations like anyone else, they handle the materials in their daily work. The exposure to the materials occurs more frequently and at a higher dose for these groups than for the patient receiving dental treatment. Thus, the possibilities for side-effects are greater. Some materials are handled more closely by laboratory technicians than by other members of the dental team, e.g., dental ceramics.

Ceramics↗

PubMed1992

Cariostatic and ultraconservative sealed restorations: six-year results.

The objective of this clinical study was to determine the ability of an ultraconservative, sealed composite resin restoration, without a traditional cavity preparation and without the removal of the carious lesion, to arrest Class I caries. Tooth preparation was limited to placing a bevel in the enamel. These restorations were compared, over 6 years, with (1) ultraconservative, localized, sealed amalgam restorations with no extension for prevention and (2) traditional, unsealed amalgam restorations with the usual extension for prevention outline form. Caries was arrested by the ultraconservative, sealed composite resin restorations for 6 years. Complete sealant retention on the sealed amalgam restorations was somewhat lower than that on the sealed composite resin restorations; conversely, partial sealant retention was higher for the sealed amalgam group. The marginal integrity of the sealed amalgam restorations was significantly superior to that of the unsealed amalgam restorations. The sealant also protected Class I posterior composite resin restorations against wear.

Adolescent↗

PubMed1992

Sealed restorations: 5-year results.

This clinical study determined the feasibility of a sealed resin composite restoration to arrest dental caries using a minimal tooth preparation: a bevel in enamel only without removal of the carious lesion. These ultra-conservative sealed composite restorations placed over caries (CompS/C) were compared with ultra-conservative sealed amalgam restorations (AGS) that had no "extension for prevention". The CompS/C restorations were also compared with the traditional (unsealed) amalgam restorations (AGU) with the "extension for prevention" outline form. Caries progress, as determined by standardized radiographs, revealed that after 5 years caries remained arrested under the CompS/C restorations; the marginal integrity was similar for the CompS/C and the AGS groups, and exhibited highly significant superiority to the AGU restorations (Chi square, P less than = 0.00004). Complete sealant retention over the amalgam restorations (AGS group) was less than over the composite restorations (CompS/C), and conversely, partial sealant retention was higher for the AGS group. Sealants also appeared to protect the posterior composite restorations against wear.

Adolescent↗

PubMed1991

Analysis of cromakalim-, pinacidil-, and nicorandil-induced relaxation of the 5-hydroxytryptamine precontracted rat isolated basilar artery.

The effects of the K+ channel activators cromakalim, pinacidil, and nicorandil were investigated in endothelium intact, 5-hydroxytryptamine (5-HT) precontracted rat isolated basilar artery. Cromakalim, pinacidil, and nicorandil produced concentration-dependent relaxation of rat isolated basilar artery precontracted with 5-HT with a rank order of potency of cromakalim greater than pinacidil greater than nicorandil. All compounds produced full or nearly full relaxation. The calculated Hill coefficients for cromakalim-, pinacidil-, and nicorandil-induced relaxation of 5-HT-precontracted rat isolated basilar artery were 2.20 +/- 0.36, 1.30 +/- 0.07, and 1.00 +/- 0.01, respectively. Under conditions of increased tone produced by 50 mmol/l KCl (which inhibits cromakalim-induced relaxation) pinacidil and nicorandil produced marked reversal of spasm, with pinacidil being more potent than nicorandil. In arteries precontracted with 5-HT, preincubation with glibenclamide (0.1-1 mumol/l) produced concentration-related inhibition of relaxation with calculated mean pA2 values (and slopes of Schild regression) +/- SEM of 6.84 +/- 0.20 (1.1 +/- 0.20) against cromakalim. 6.60 +/- 0.14 (0.95 +/- 0.23) against nicorandil, and 6.57 +/- 0.26 (1.04 +/- 0.18) against pinacidil. For cromakalim, pinacidil, and nicorandil the slopes of Schild regression were not significantly different from unity. Tolbutamide 10 mumol/l was without effect against the cromakalim-, pinacidil-, or nicorandil-induced relaxation. Tetraethylammonium (TEA; 1-10 mmol/l) produced noncompetitive inhibition of the cromakalim-induced relaxation, but appeared to produce competitive inhibition of the pinacidil- and nicorandil-induced relaxations. We conclude that cromakalim, pinacidil, and nicorandil produce relaxation of the 5-HT precontracted rat basilar artery by similar mechanisms to those identified in other peripheral vascular and visceral smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Comparison of cromakalim-induced relaxation of potassium precontracted rabbit, cat, and rat isolated cerebral arteries.

The effects of cromakalim were investigated in KCl-precontracted cat, rabbit, and rat isolated cerebral arteries with intact endothelium. Potassium induced contraction of all cerebral arteries studied, but exhibited marked vessel and species variation with no spasm to 20 or 30 mmol/l KCl in the rat basilar artery or 20 mmol/l KCl in the rabbit middle cerebral artery. On sustained tension to 20 mmol/l KCl, cromakalim induced concentration-related relaxation in the rabbit basilar artery and the cat basilar and middle cerebral arteries with Hill coefficients greater than unity. Cromakalim was more potent in the rabbit basilar artery precontracted with 20 or 30 mmol/KCl than in the rabbit middle cerebral artery or the cat basilar or middle cerebral artery. Elevation of the KCl concentration to 50 mmol/l inhibited cromakalim-induced relaxation and produced a decrease in the Hill coefficient. Preincubation of cerebral arteries with glibenclamide (100 nmol/l-1 mumol/l) produced concentration-related inhibition of the cromakalim-induced relaxation in the rabbit basilar, cat basilar, and cat middle cerebral arteries precontracted with 20 mmol/l KCl. The degree of rightward shift of concentration-effect curves by glibenclamide was calculated at the EC25, EC50, and EC75 levels. A good correlation was observed between the shifts at the EC50 and EC75 levels. However, the shift in concentration-effect curves for cromakalim produced at the EC25 level was markedly less than the EC50 or EC75 levels in the presence of 1 mumol/l glibenclamide. The pA2 values for glibenclamide calculated at the EC50 level were 6.6 +/- 0.09, 7.1 +/- 0.1, and 6.5 +/- 0.5 in the rabbit basilar, cat basilar, and cat middle cerebral artery, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Ultraconservative sealed restorations: three-year results.

The overall objective of this clinical study was to determine the feasibility of using a sealed composite restoration to arrest caries without the removal of the carious lesion and without the traditional cavity preparation. The minimal tooth preparation (a bevel in enamel) usually did not require any anesthetic injection and conserved tooth structure. These ultraconservative sealed composite restorations placed over caries (CompS/C) have been compared with ultraconservative sealed amalgam restorations (AGS) and with the traditional outline form (unsealed) amalgam restorations (AGU). This clinical study has shown that: (1) caries can be arrested by the CompS/C restoration for a period of three years; (2) the marginal integrity of the AGS restorations showed a definite trend toward improvement as compared to the AGU restorations; and (3) complete sealant retention over posterior restorations was similar in both the CompS/C and the AGS groups during the first two years; however, at Year 3 complete sealant retention was 16 percent higher in the CompS/C than in the AGS group.

Adolescent↗

PubMed1991

Effect of cooling rate on leucite volume fraction in dental porcelains.

Prasad et al. (1988) have shown that slow cooling of dental porcelain produces increases in thermal expansion sufficient to make a compatible metal-porcelain system incompatible. The present study was undertaken to determine whether the increase in porcelain thermal expansion might be attributable to crystallization of additional leucite during slow cooling of the porcelain. Eight x-ray diffraction specimens for each of six commercial dental porcelains and for the Component No. 1 frit of the Weinstein and Weinstein (1962) and Weinstein et al. (1962) patents were fabricated and divided into two groups. Specimens in the first group (termed fast-cooled) were cooled in the conventional manner by removing them from the furnace at the maximum firing temperature immediately into room air. Specimens in the second group (termed slow-cooled) were cooled slowly by interrupting power to the furnace muffle and allowing them to cool inside the closed furnace. Quantitative x-ray diffraction was performed on the fast- and slow-cooled porcelain specimens with standards containing leucite volume fractions of 0.111, 0.223, 0.334, and 0.445. Unpaired, one-tailed t tests were performed on the fast- and slow-cool data, and a significant increase (p less than 0.05) in the amount of leucite (as a function of the slow cooling) was found for each of the porcelains. The increases in the leucite volume fractions resulting from the slow cooling ranged from a low of 8.5% to a high of 55.8%, with an average increase of 26.9%.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum Compounds↗