Objective: This study is a part of a comprehensive project to improve the thermal stability of dental porcelains.  The present study examined the mechanism of coefficient of thermal expansion (CTE) changes in a dental porcelain via quantitative X-ray diffraction (XRD) measurement of leucite content.

Methods: Eighty dilatometry specimens were fabricated from Vita porcelain and randomly assigned to receive 0, 1, 3, 7, or 15 additional firings (16 specimens per group).  Dilatometry runs were conducted on first heating following a thermal pretreatment to avoid the interference of porcelain structural relaxation (Williams et al., J Dent Res 2002;81:A-484).  Leucite weight fraction was measured via quantitative XRD using an Al₂O₃ internal standard.  The thermal treatment afforded by dilatometry runs has been shown to have no effect on porcelain leucite content (Mackert et al., Dent Mater 1996;12:8-12).  The data were subjected to nonlinear regression analysis. 

Results: Significant effects of additional firings on porcelain leucite weight fraction (p < 0.0001) and CTE (p < 0.0001) were found.  The plots of leucite weight fraction and CTE versus additional firings were both concave downward, with peaks at 3 additional firings.  The leucite weight fraction was found to increase from 0.245 to 0.265 after 3 additional firings and then decrease to 0.208 after 15 additional firings.  The CTE curve exhibited similar behavior.  X-ray powder diffraction revealed crystallization of sanidine after 15 additional firings.

Conclusions: These results indicate the crystallization of leucite during the first few additional firings, followed by a partial conversion of leucite to sanidine after 15 firings.  During typical laboratory firing (involving only a few firings), this porcelain would be expected to exhibit a slight increase in leucite weight fraction and a corresponding increase in CTE.

The support of NIH/NIDCR Grant No. DE07806 is gratefully acknowledged.