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. Th e purpose of the present study was to determine whether isothermal ann eal treatments (such as are encountered during ''post-soldering'' oper ations) could result in alterations of leucite content of dental porce lain. Six commercial dental porcelains and the ''Component No. 1'' fri t of the Weinstein et al. patent (1962) were studied. Twenty-eight cou pon specimens of each porcelain were fired and divided randomly into s even groups of four specimens each. One group served as a control, whi le 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 v ia quantitative x-ray diffraction. From an equation for the growth of particles in competition for diffusing material, an expression was der ived for the potential simultaneous processes of leucite crystallizati on 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 s ignificant correlation was obtained between leucite volume fraction an d isothermal anneal duration at 750 degrees C (p < 0.01), whereas at 5 00 degrees C, only CII exhibited a statistically significant correlati on (p < 0.01). The control (non-annealed) leucite volume fractions for the commercial porcelains ranged from (mean +/- SD) 0.155 +/- 0.002 f or WIL to 0.442 +/- 0.003 for Component No. 1. The 750 degrees C isoth ermal anneal increased the leucite volume fraction by between 6.1% and 21.3%. These increases in leucite could be responsible for the porcel ain fracture that sometimes occurs during post-soldering.