Yeah, someone else already mentioned it, but historically, the way ancient glass production worked was that you had a small number of sites that produced huge amounts of raw glass, which they then exported as a trade good to large numbers of smaller glassworks over the nation (which would never produce their own glass, but focused entirely on working the raw glass they purchased into finished materials instead.
See Wikipedia on
Roman glass making, for instance. (Note that recycled glass was also very common, to the point where Roman writers of the time commented on its importance.)
Wikipedia's article on
Anglo-Saxon glass again notes the importance of a distinction between glass-
making and glass-
crafting -- historically, it seems England imported most of its raw glass, since it lacked the raw materials necessary to make glass itself. (Sound familiar? In fact, the fact that you could do this so easily is probably part of the reason why glassware became so widespread. Making glass from scratch is comparatively difficult and requires ingredients not found everywhere as well as fairly high temperatures, but crafting simple things out of raw glass is comparatively easy and can be done anywhere with a furnace.)