Tempered Glass

Chemically identical to whatever glass was tempered, so the food-contact story is the same one told on the soda-lime and borosilicate pages: inert, non-leaching, nothing to migrate. Tempering changes only how it breaks. The point of a safety glass is that stored compression makes the whole pane release at once into small blunt dice rather than sharp shards, which is why it is mandated for shower doors and car side windows. Its one distinctive failure is spontaneous breakage: microscopic nickel sulfide inclusions sitting in the tension core can change phase years later and burst the piece with no impact at all, which is why architectural tempered glass is sometimes heat soaked at 250C for two hours to force those failures in the factory.

Tempering buys real but limited thermal-shock headroom, because the surface compression has to be overcome before a cooling surface can go into tension. It does not change the expansion coefficient, so tempered soda-lime is still a soda-lime-expansion material and sits well below borosilicate. The important caveat is that "tempered" on cookware is not always true: when Bradt and Martens examined commercially bought US glass bakeware in 2012 they found minimal evidence of the heat treatment manufacturers claimed, not enough to substantially raise thermal stress fracture resistance, and not enough to produce the dice break pattern - sharp shards remained the outcome. Treat freezer-to-oven and hot-dish-onto-wet-counter as off limits regardless.

Architectural and appliance glass carries a permanent bug or etched mark in one corner naming the standard, which is the only reliable identification. Kitchenware rarely does, so tempered pieces are usually identified only by the words on the box. If breakage has already happened, the fragments answer the question: small squarish blunt dice mean it was genuinely fully tempered, large curved shards mean it was annealed or only lightly heat-strengthened. Ask a brand that says only "tempered glass" two things: what the base glass is (soda-lime or borosilicate), and what temperature differential the finished piece is rated for. A brand that cannot answer either is repeating a marketing word.

Strong in the face of impact and flexing, and much more forgiving of knocks than annealed glass of the same thickness. Its weakness is edges: once tempered the piece cannot be cut, drilled or ground, and a chip that reaches past the compression layer into the tension core can set off the whole panel, sometimes hours later. A chipped tempered lid or jar is not repairable and should be replaced.

Effectively unrecyclable through normal channels. The tempering is a heat treatment rather than an additive, but tempered pieces are almost always bakeware, appliance or architectural glass, which container-glass recyclers reject for melting-behaviour reasons. In practice it goes in the trash. Inert once there.