Studies are sparse on how rapid heating and cooling negatively affect optical properties, including color and transparency, and mechanical properties, including hardness and durability, that affect esthetics and shorten the clinical usage period of the ceramics.

The purpose of this in vitro study was to determine the effects of repeated firing on the color difference, mechanical properties, and phase formation of different ceramic materials.

A total of 160 disks (12×1.35 mm) were produced from 4 different ceramic materials: lithium disilicate glass-ceramic, zirconia-reinforced lithium silicate ceramic, zirconia core, and monolithic zirconia. Specimens from all groups were divided into 4 groups (n=10) with a different number of veneer porcelain firings (1 to 4) by simple randomization. After the firings, color measurement, X-ray diffraction analysis, environmental scanning electron micrograph analysis, surface roughness evaluation, Vickers hardness evaluation, and biaxial flexural strength tests were performed. Data were analyzed with analysis of variance (2-way ANOVA) (α=.05).

Repeated firing did not affect the flexural strength of the specimens in any of the groups (P>.05), while the color, surface roughness, and surface hardness were significantly affected (P<.05). The specimens fired 4 times showed the highest mean Vickers hardness and ΔE00 values but the lowest mean surface roughness values. Zirconia core specimens showed the highest mean ΔE00 and flexural strength values, and lithium disilicate glass-ceramic specimens had the highest mean Vickers hardness values.

The increase in the number of firings affected the color, mechanical properties, and phase formation of the specimens; this change differed according to the ceramic tested.