B) Specimen preparation:
Eighty recently extracted maxillary central incisors were collected. The crowns were separated from their roots using a diamond disk. The crown segments were fixed, with the exposed enamel down, on double-sided adhesive tape on a glass slide. They were then embedded in self-curing resin by placing a mold that was open on both ends on the adhesive tape and filling the mold with the mixed resin. The labial surfaces were ground flat using 600-grit sandpaper on a polishing machine. The specimens were randomly divided into four groups of twenty teeth each based on the bleaching