Borosilicate glass

What the boron actually does

Ordinary kitchen glass is soda lime: silica with sodium and calcium oxides added to bring the melting point down. It is cheap, clear and perfectly good for a drinking glass. What it is not good at is changing temperature quickly, because it expands and contracts a lot, and a dish that expands unevenly tears itself apart.

Borosilicate replaces much of the sodium and calcium with boron trioxide. The result has a coefficient of thermal expansion roughly a third that of soda lime glass. Heat one side of it and the stress that builds between hot and cold regions is correspondingly smaller. That is the entire trick, and it is why laboratory glassware has been made from the stuff since the eighteen nineties.

What that means at the oven door

A borosilicate dish will take food from a 400°F oven and, once it has stopped steaming, go into a fridge. It will take a frozen portion into a microwave. Neither of those is a good idea with a soda lime dish. That is the reason most cheap glass storage is sold as fridge and microwave safe but stops short of claiming the oven.

The practical consequence is that one dish can do three jobs. You roast in it, store in it and reheat in it, and the tray you would otherwise have washed does not exist.

The two ways it still breaks

Direct heat, not oven heat

An oven heats a dish evenly through the air around it. A gas flame, an electric coil or a grill element heats one patch of it. No glass, borosilicate included, is designed for that, and a dish put under a broiler or onto a hob will crack. The same applies to a dish put on a wet worktop straight from the oven: the cold patch under it does the same thing in reverse.

Impact and chips

Thermal shock resistance is not mechanical strength. Borosilicate is if anything slightly less resistant to a knock than tempered soda lime glass, and a chipped rim is a stress concentration that will eventually fail under heat. A dish with a chip in it stops being an oven dish and becomes a fridge dish.

Living with it

• Let a hot dish cool on a wooden board or a folded towel, not on a cold stone worktop.
• Do not pour cold water into a hot dish to loosen baked on food. Let it come down first.
• Lids are plastic and stay out of the oven regardless of what the dish will take.
• Check the rim when you wash it. A chip is the point at which the dish changes job.

Is it heavier?

Yes, noticeably. A five pack of glass dishes weighs several pounds more than a five pack of plastic, and that is the honest argument against glass for anything carried a long way on foot. It is also the argument for it in a fridge, where weight is stability and a glass dish does not slide when the door swings.