الفهرس 
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Abstract
The purpose of this study was to evaluate the effect chemical aging represented in artificial saliva, citric acid, tea with sugar and tea without sugar on color stability and surface roughness of hybrid ceramic blocks (Cerasmart) and composite blocks (Brilliant Crios).
A total of 40 rectangle shaped samples of dimensions (14Χ12Χ1mm) were sectioned by a linear precision saw machine. Then all samples have been finished and polished according to the manufacturer’s recommendations and divided into 2groups according to tested materials. group A: hybrid ceramic blocks and group B: composite blocks. The samples in each group were randomly divided equally into 4 subgroups (n=5 for each subgroup) according to the type of immersion protocols.
• Subgroup A I and BI: artificial saliva for 24hr.
• Subgroup A II and B II: 2% citric acid pH=4 for 8hr, followed by artificial saliva for 16hr.
• Subgroup A III and B III: 2% citric acid pH=4 for 8hr, followed by artificial saliva for 13.5hr, followed by tea without sugar for 2.5hr.
• Subgroup A IV and B IV: 2% citric acid pH=4 for 8hr, followed by artificial saliva for 13.5hr, followed by tea with sugar for 2.5hr.
For each solution, pH was confirmed. Then all samples in each subgroup were stored in 20 ml of immersion solutions at room temperature for 28 days. All immersion solutions are refreshed daily. After each immersion, all samples were carefully washed with copious amount of distilled water and dried.
Color stability of all samples was evaluated by using spectrophotometer according to CIE Lab color system before and after aging and ΔE was calculated. Furthermore, surface roughness of each sample was evaluated three times at
different location by using a stylus profilometer before and after aging to determine Ra value.
The results of this study showed that there is significant increase ΔE values for all subgroups in both groups. Subgroup III and subgroup IV showed ΔE values higher than clinical acceptable level of color stability. The results of surface roughness showed that there is significant increase in roughness after immersion protocols compared to before immersion in all subgroups. Subgroup B III showed the highest increase.