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I have an induction cooktop, and sometimes I want to cook over another glass so the pots are not on the hob itself.

For that I thought to use a piece of tempered glass cut in the desired sizes.

What heat is tempered glass capable of withstanding?

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Borosilicate glasses such as old Pyrex and good laboratory pieces do well with both temperature and thermal shocks. Soda-lime glasses such as all current cookware only do well with temperature. Spilling cold additions to a hot pan onto glass is always going to have some risk of shattering.

The tempered glass you by in a store will not contain boron. So while it will easily endure high temperatures it will not withstand rapid changes in temperature. For example, you're sautéing some onions in a skillet and the bottom of the pan heats the glass to 600 degrees. You proceed to deglaze the pan with a nice white wine when a dog barks causing you to pour it onto the glass, which rapidly cools and breaks into a bunch of shards.

The root problem is that conventional glass undergoes a fair amount of thermal expansion. Borosilicate glass only expands about 33% of the amount another type of glass would when heated, so it's much less likely to blow up in your face when rapidly cooled. It's also more expensive, which is why Pyrex cooking goods are no longer made from it. (I'm guessing induction cooktops aren't either, probably using mass and fancy acrylics.) Nothing is stopping you from buying it (or a comparable glass) from a specialty supplier, just be sure you're clear with them about the use case.

A better bet would parchment paper or food grade silicone. Companies such as Silpat sell pan-sized solid mats made from fiberglass and silicone that you can use to prevent scratching the induction top. I believe you can also get carbon fiber pads sized to fit particular model ranges, for a whole-stove solution.

If you're committed to shopping at the hardware store rather than restaurant supply, how about a soldering heat shield? Metal grommets are out, but the flame retardant synthetics they're made from more than fit your bill. Pyron is good up to 2,500F and you can roll it up and stick it in a drawer.

One final thought, the typical temperature on an induction stove is well below the worst case. An empty cast-iron skillet, for example, will convert around 90% of the electrical power directly to heat. The cooktop itself has a bunch of mass and is a thermal insulator, you'll lose at least the mass with whatever you use. Silicone for example is good to 500F but can get gooey on high heat.

  • Thanks for your ideas. Definitely enriched my thinking. Anyway I prefer a solid thing. I didn't understand what Pyron means, but a tin sheet could actually work. Anyway I prefer a glass. Also because it's more appealing. So will a standard tempered glass you buy in a glazier shop withstand a normal cooking heat (>500-600°F)? – Shimmy Mar 19 '17 at 18:27
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    Pyron is a flame resistant synthetic fabric made of carbon fibers, so the pot won't burn it and the cooktop won't heat it. I don't know why you'd consider tin, odds are good it'd be heated up by induction except you'd have no extra cooking mass in it to absorb the heat. Sounds like a great way to get a serious burn. – Matthew Gauthier Mar 19 '17 at 22:00
  • I'm actually not locked on on glass, but fabric isn't good because it's not solid. It has to be a metal, rubber, plastic, paper or anything that avoids the transition of fluids in between. My budget is about a $100, but since I will be using it for 1 week during the year, I prefer it to be much cheaper. Any other ideas except from glass is welcome. And since I don't live in the US and English is not my native language, I'd appreciate if you can attach pictures of what you mean. – Shimmy Apr 2 '17 at 5:15
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    I'd suggest "parchment paper," which is a baking paper impregnated with silicone. It comes in rolls, stops fluids, and withstands oven temperatures. Slightly more expensive but more durable are "silicone baking mats" which are the same but plastic rather than coated paper. – Matthew Gauthier Apr 2 '17 at 15:37
  • Thanks for your detailed answer, I now decided I don't want ahh glass. I just want a silicone mat, the problem is I want to cover the entire cooktop (80x50cm) and I'll cut a hole for the control pad, induction is magnetic so losing heat should be different. Can you recommend on a roll/mat that is large enough and should be good for my scenario available for online shopping etc.? – Shimmy Mar 29 '18 at 15:36

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