Effect of multiple firings on optical and mechanical properties of Virgilite-containing lithium disilicate glass-ceramic of varying thickness

Objectives: To investigate the effect of multiple firings on color, translucency, and biaxial flexure strength of Virgilite-containing (Li_0.5Al_0.5Si_2.5O₆) lithium disilicate glass ceramics of varying thickness.

Materials and methods: Sixty discs were prepared from Virgilite-containing lithium disilicate blocks. Discs were divided according to thickness (n = 30) into T_0.5 (0.5 mm) and T_1.0 (1.0 mm). Each thickness was divided according to the number of firing cycles (n = 10); F₁ (Control group): 1 firing cycle; F₃: 3 firing cycles, and F₅: 5 firing cycles. The discs were tested for color change (ΔE₀₀) and translucency (TP₀₀) using a spectrophotometer. Then, all samples were subjected to biaxial flexure strength testing using a universal testing machine. Data were collected and statistically analyzed (α = 0.5). For chemical analysis, six additional T_0.5 discs (2 for each firing cycle) were prepared; for each firing cycle one disc was subjected to X-ray diffraction analysis (XRD) and another disc was subjected to Energy dispersive X-ray spectroscopy (EDX) and Scanning electron microscope (SEM).

Results: Repeated firing significantly reduced the translucency of F₃ and F₅ compared to F₁ in T_0.5 (p < 0.001), while for T_1.0 only F₅ showed a significant decrease in TP₀₀ (p < 0.001). For ΔE₀₀, a significant increase was recorded with repeated firings (p < 0.05) while a significant decrease resulted in the biaxial flexure strength regardless of thickness.

Conclusions: Repeated firings had a negative effect on both the optical and mechanical properties of the Virgilite-containing lithium disilicate glass ceramics.

Clinical relevance: Repeated firings should be avoided with Virgilite-containing lithium disilicate ceramics to decrease fracture liability and preserve restoration esthetics.