Enamel blocks (4x4x3 mm) were obtained from unerupted human third molars recently extracted and kept in 0.1% thymol solution at pH 7.0. The blocks were cut using a ISOMET low speed saw cutting machine (Buehler Ltd., Lake Bluff, IL, USA) with two diamond disks (Extec Corp., Enfield, CT, USA), which were separated by a 4 mm thickness spacer. The surfaces of the blocks were ground flat with water-cooled silicon carbide discs (320, 600, and 1200 grade papers; Buehler, Lake Bluff, IL, USA) and polished with felt paper wet by diamond spray (1 µm polishing particle size; Buehler, Ltd., Lake Bluff, IL, USA). Blocks were cleaned using an ultrasonic device for 10 minutes and checked regarding the presence of white spots and cracks using a microscope (x40).
The surface hardness (SH) was determined using the average values of five indentations performed at distances of 100 µm from each other (Knoop diamond, 25 g, 10 s, Hardness tester from Buehler, US). Seventy-two enamel blocks with mean SH of 343.9 (± 15.5 KHN) were selected and sterilized by exposure to ethylene oxide gas.
The surface hardness (SH) was determined using the average values of five indentations performed at distances of 100 µm from each other (Knoop diamond, 25 g, 10 s, Hardness tester from Buehler, US). Seventy-two enamel blocks with mean SH of 343.9 (± 15.5 KHN) were selected and sterilized by exposure to ethylene oxide gas.