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I've been using the Norpro silicone measuring cups for microwave heating for a while now. Out of the box, they still have a few problems. They are a bit TOO flexible so you have to fill them no more than 1/4 to 1/3 full. That's not a problem for me because I only pour in small batches anyway. Another problem is they do get pretty hot, especially at the bottom where the plastisol is. I started playing with ideas for insulating these cups. Here's a pic that shows the test subjects. On the left is the first triel in which I used some high temperature insulation (expensive). While good for much higher temps, it's really overkill and would need to be much thicker to be ideal. The cup was still "floppy" so I added the large bead of silicone at the top in an attempt to make it easier to grip. That's just ordinary silicone caulk from the hardware store, good to about 400 deg and has worked fine in the microwave. For the middle cup I found this fiberglass "paper" that's about 1/8 inch thick. The black thread is wrapped around to hold it in place since I didn't want to make it a permanent addition this time. The insulation is much better, two layers on the bottom half and also two layers on the bottom of the cup were used. This cup barely gets warm during use, but the way it's constructed left it still too floppy and the tension on the thread actually starts to close/deform the cup when it gets heated up. The right-hand picture is my latest and probably final solution. For this one I first made an "insert" to put inside the cup while applying the insulation. I just used cardboard and then some paper to get a fit. First I glued (with silicone) two layers of the same 1/8 inch insulation to the bottom. Then used strips about 1 inch wide, starting from the bottom and overlapping each previous strip. This give a 2-thickness (of insulation) everywhere and creates those ridges to make the cup easier to grip. The strips were held in place with rubber bands until the silicone cured. After it was al done, I added "drip protection" using some old RTV mold making silicone. While using that second cup I poured from the wrong side a couple of times and if the insulation isn't covered with silicone liquid plastisol soaks into it and makes a mess. This last cup is pretty solid, not at all floppy, and is still flexible. It also keeps the plastisol HOT much longer than the bare Norpro cup. I've been able to pour, allow the mold to cool, remove baits, and pour a second time without re-heating, and this is with a very small batch of about 1/4 cup. For those of you using pyrex cups on a griddle, you could probably use this same insulation material, just leave the bottom of the cup exposed so heat comes through. I still have some glass beakers that I plan to insulate for microwave use as well. I got the fiberglass insulation material from mcmaster carr. They call it "fiberglass paper", but it's 1/8 inch thick material. They have other fiberglass paper that's not this thick so check carefully. This stuff comes in a 10 foot roll for around $20.