Effects of glaze application and extended firing on the mechanical and optical behavior of a leucite-based ceramic

Extended dwell times during self-glaze firing have been shown to improve the mechanical behavior of glass-ceramics; however, their effects when combined with overglaze procedures remain unclear. Therefore, this study evaluated the effect of firing protocols (conventional or extended) on the mechanical and optical behavior of a leucite-based glass-ceramic subjected to self-glaze or overglaze. Disc-shaped specimens of a leucite-based glass-ceramic (IPS Empress CAD) were fabricated and divided according to the glazing protocol: self-glaze with conventional firing (1 min 30 s at 790 °C) or extended firing (15 min at 790 °C) and overglaze with conventional or extended firing. Surface roughness (Ra and Rz) was measured using a profilometer. Translucency (TP₀₀) and color change (ΔE₀₀) were evaluated with a spectrophotometer and calculated with CIEDE2000. Biaxial flexural strength was assessed using the piston-on-three-ball test. Crystalline phases were analyzed by x-ray diffraction. Data were analyzed using ANOVA, Tukey's test, and Weibull analysis. Extended firing and overglazing resulted in higher flexural strength values. Overglazing increased both Ra and Rz roughness parameters, whereas extended firing reduced roughness values. TP₀₀ and ΔE₀₀ were affected by the firing protocols; however, the mean differences between conventional and extended firing remained below the clinical acceptability thresholds. Leucite crystals were identified in all experimental groups, with no evidence of crystalline phase changes after firing. Therefore, extended firing and glaze layer application improved the mechanical behavior of the leucite-based glass-ceramic without clinically compromising its optical properties.