Heating enamel in the temperature range 200°C to 600°C resulted in poor crystal packing due to void formation, permanent change in the sign of its birefizngence (from negative to positive) in some areas, and an altered crystal morphology. Transmission electron microscopy of enamel heated in the temperature range 200°C to 400°C revealed that the distinction between the positively birefringent regions and the negatively birefringent regions (which were present up to 350°C and occasionally up to 400°C) at the tooth's surface was due to the greater volume of intra- and inter-crystalline voids within the positively birefringent regions. There was a significant increase in void volume at 400°C, and above this all of the enamel was positively birefringent and opaque. Large remineralized crystals of β-tricalcium phosphate (β-TCP) phase (whitlockite) were initially formed at 400°C, and their size and number increased at 500°C and above_ Both the greater solubility of β-TCP crystals and the increased surface area due to the presence of voids would increase the rate of demineralization of heat-treated enamel.
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