Figure 6. SEM photomicrographs of an enamel surface without (A) and with (B) exposure to a bleaching procedure. The enamel of an extracted human tooth was cleaned with water-spray, and half of the surface was covered with nail varnish (the control). The tooth was then exposed to 10% carbamide peroxide gel for 1 hr two times daily during 3 wks. After each bleaching procedure, the gel was removed by water-spray, and the tooth was stored in water between treatments. At the end of the bleaching period, the nail varnish was removed, and comparative sections of bleached and unbleached enamel were prepared for direct scanning electron microscopy (Holmen et al., 1985). The enamel microstructure of the bleached-enamel surface (B) illustrates an obvious enamel etch caused by the bleaching agent, compared with the unbleached surface (A).