What is that stuff that the tech at the optician’s sticks the stems of my glasses in to soften them up to bend them to adjust the fitting?
It looks like hot sand to me. This seems like a unique way of warming something up. Why not a hot air blower, or hot liquid? Is there anything special about the material itself or is it as ordinary as it looks?
Sand pours like a liquid, has a high heat capacity, so even a small pot of it maintains it’s set temp well, and is not wet. What could be better ?
Note that sand is a poor heat conductor (which is why the surface sand at the beach can be blazing hot but quite cool just an inch under the surface). So it can be made hot enough to quickly heat up the frames but not continue to heat them up to cause damage. I.e., the frames get just a “burst” of heat. This means that a total idiot trainee is unlikely to harm the goods. (They can if they “stir” the frames in the sand a lot, but it takes some doing.)