The second treatment to which the composites were subjected was a staining procedure which reflects the conditions in the oral environment, in which restorations are exposed to coffee drinks. Coffee was chosen in this study as a staining solution because it has been shown to have a strong staining effect on composites as well as on natural teeth.26,27 According to Um and Ruyter,28 discoloration by coffee occurs by both adsorption and absorption of colorants by resin-based restorative materials. The authors explained that this was probably due to the compatibility of the polymer phase of the resinous materials with the yellow colorant of coffee, which served to facilitate this adsorption and penetration of colorants. According to the coffee manufacturer, the average time needed to consume a cup of coffee is 15 minutes; coffee drinkers ingest an average of 3.2 cups of coffee per day. Therefore, 48 hours of storage in a coffee solution simulated an average of two months of coffee consumption.