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?
Kitchens – What temperature can tempered glass withstand
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Best Answer
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.