الفهرس 
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Abstract
The purpose of this study was to evaluate the effect of repeated number of firing cycles (3, 5, 7 or 9) on color stability of 3 different shades (A1, A3 & A4) of Zirconia ceramic system. Then, color measurements of ceramic specimens were performed using a spectrophotometer before firing and after each firing cycle.
Seventy-five specimens (discs) were fabricated from zirconia ceramic system (Zircon Zahn) which were divided into three groups (Twenty-five discs) according to their shade. Then, each group was subdivided into five subgroups (Fifteen discs) according to the number of firing cycles done.
All specimens (except the control) were exposed to additional firing cycles (3, 5, 7 &9) at holding temperature 900C/15 min. of vaccum firing. Last, color measurements were done before and after firing by the spectrophotometer to detect the differences in color.
All data collected were tabulated and statistically analyzed.