Interesting. I know some glassblowers, but they use modern propane systems of course.

A friend spent a weekend (including Sat night, all night) trying to get a beehive glass furnace to turn some sand and ?phosphorus? into glass. He was only able to get it hot enought to make "proto-glass" pellets.

I'm guess the fuel costs in the middle ages were astronomical for making plate glass. You have to blow it large enough to form a reasonable cylinder, cut the cylinder while hot, and flatting the walls into a sheet. The tail and head are waste products. All done with forced-air charcoal (where humans are doing the forcing), which had to be made first in sufficient quantities.