 Solution

An unstressed glass is optically isotropic, so it does not split incident light into differently phased in-plane components; it remains dark between crossed polars. A natural explanation of the colored regions after heating is devitrification: anisotropic crystallites have nucleated in the glass. Edges and surfaces provide heterogeneous nucleation sites, making their greater concentration there plausible.

Each approximately uniform crystallite can give a nearly uniform interference colour. Regions with similar thickness and effective birefringence have similar retardation and therefore similar spectral colour, while differing azimuths multiply the intensity by ${\sin }^2(2\alpha )$ and change brightness. Identical hue does not prove identical three-dimensional crystallographic orientation.

Stress-induced birefringence is a possible alternative in a heat-treated glass. The observations alone do not uniquely prove crystallinity; rotating-stage extinction, morphology or diffraction could distinguish the explanations. The crystallite interpretation is the straightforward phase-transformation explanation, without inventing a composition or heat-treatment schedule.