First of all, I haven't done it myself, so don't listen to me too much. My friends and I have made successful mirrors from some pretty scuzzy glass, so that bodes well for inexpensive remelting junk glass. Also, one can go to the trouble of building molds for ribbed blanks, and 'gluing' thin pieces in the kiln to build up rigid open frame blanks. I know that Hextek and others have done this, though the resulting mirrors may not have always been capable of amateur optical demands. If I were starting to research this, I would contact people centered on Univ of Arizona. The principle drawback for me personally has been that the cost of the kiln, and the cost of bringing in the kind of electricity needed into my shop has greatly exceeded the cost of buying the glass from United Lens and others. Also, frankly, Tom Waineo has me convinced to try a very large metal mirror for my next project at a substantial savings in moola.
Hope this has been of passing interest, Mel Bartels