RESUMEN
The use of bleaching agents for the sake of whitening teeth has extended for many years. On the other hand, due to the simultaneous presence of a series of tooth colored restorations and existence of few research studies on the effect of different whitening materials on the tooth restorations, the present study with the aim of determining the efficacy of carbamide peroxide on the hardness of Glass Ionomer cements had been emerged. Forty numbers of glass ionomer cylinders were prepared and preserved for 48 hours in 100% humidity and in the room temperature, then were divided into 4 groups each contained 10 samples. The bleaching gel 16% VARI-SHADE [Lochemco USA] was applied for groups I to IV as respectably o, 21, 42, 63 hours. Finally the microhardness of each sample group was measured by Vickers's machine. Microhardness mean was respectively 27.37 +/- 2.4, 25.98 +/- 2.7, 24.41 +/- 3.6 and 26.02 +/- 3.7 in group I to IV, and there was no significant difference between them. Vari-shade bleaching gel did not result to reduced microhardness of Vitremer glass-ionomer in the studied time