> What they'd want to do is try to recover and reuse heat. In principle, there's no reason "new" heat has to be added each time they heat a batch of glass, if heat can be transferred from cooling glass back to the input materials.

Have you worked in any industrial or craft setting involving molten glass or metal? Walked around a workshop? There's no way the heat is going back into the process.

I know there are industrial processes where heat is efficiently recycled, but I agree there are serious practical problems, particularly if the molten glass must be cooled quickly. Still, even somewhat lower grade heat can be upgraded back to high grade heat with high temperature heat pumps.