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.
My experience working with glass is that you _don't_ want it cooled quickly. It will shatter or get internal stresses that cause it to shatter later. You anneal it in a slowly cooling oven over time. Some steels and brass can be annealed slowly and that heat could, I suppose, be captured. Tempering probably not.
> Still, even somewhat lower grade heat can be upgraded back to high grade heat with high temperature heat pumps.
Which requires energy and incurs losses. If you have energy, assuming electricity, that's cheap enough to scavenge low-level waste heat you probably don't need to recapture the heat.