This is based on my experience of doing large pieces on thinfire or other fibre paper with a relatively fast rate of advance. What seems to happen is that the edges of the glass soften enough and early enough that not all the binder in the fibre papers can burn out and the combustion gasses escape from under the glass. The resulting haze is the remnants of the combustion product fired to the surface of the glass.
I
have found that flipping the piece over and taking the glass to a low
temperature fire polish is enough to return the glass to its usual
appearance. You can, for extra insurance, apply a devitrification
spray, although I have not found it necessary.
You
could, of course, work the back of the glass with pumice and cerium
oxide to bring back the original shine without firing. But my
impression is that the areas with haze are fractionally depressed into the back surface. This means that a lot of glass has to be removed
to reach and polish the hazy areas.
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