A frequent concern about fibre board and fibre paper that appears brown when peeking into the kiln is that something has gone wrong.
Ceramic fibre papers and boards have organic binders that have to burn out during the firing. What starts as white later turns brown as the binders are burning out – often the smell is like burning paper. As the binders burn away, the fibre paper or board returns to its original colour.
The amount of smell or smoke is dependent on the amount of ceramic fibre you have in the kiln. If you have a thick board, you should think of venting the room, as there will be a lot of smoke.
Also if you are firing a large piece of glass, you should think of firing the binders out of the ceramic fibre before placing the glass on top. If you do not, the binders may not fully burn out, leaving marks of the smoke fired into your glass.