Passage:
During the precision lens manufacturing process, newly molded optical glass blanks undergo a multi-day annealing cycle inside an insulated kiln. While outer surfaces naturally shed heat rapidly when exposed to ambient room air, quick exterior cooling causes localized surface contraction while the interior core remains expanded and molten. To prevent internal stress fractures that permanently distort optical clarity, technicians lower the kiln temperature by less than half a degree per hour until the entire glass blank reaches thermal equilibrium.
Statement: Optical glass blanks fracture during rapid cooling because unequal rates of temperature change create conflicting physical strain between the interior and exterior layers.
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