Thanks for your help.
My limited experience of recycled glass (i.e. glass recovered from bottles and other glass waste) is that the impurities picked up along the way - or using a mixture of types of glass with different characteristics - can cause bubbles to form in the raw "metal" in the furnace.
As I understand it, recycled glass isn't usually used alone, rather, a proportion of it (already molten and then processed into low quality cullet) is mixed with raw batch ingredients (sand, potash, soda lime, lead oxide etc, in whatever permutation you decide to use) or high quality cullet.
This is supposed to help stabilise the mix, make it more malleable, and help it anneal more evenly. Even so, I've heard that it is very difficult to achieve a high quality mix.
I also understand that it is difficult to work and anneal glass containing a large proportion of recycled materials, which is why most "recycled glass" items are thicker and less complex.
Is that an accurate view? I'm very curious about the "alchemy" of glassmaking, but I think I'm getting into way beyond my level of competence... :-)