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°C and three groups at 750°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°C (p < 0.01), whereas at 500°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°C isothermal anneal increased the leucite volume fraction by between 6.1% and 21.3%. These increases in leucite could be responsible for the porcelain fracture that sometimes occurs during post-soldering.