Fiber kilns are constructed of rigidized fiber to keep the walls nice and stiff. Some kiln companies pre-fire their fiber at the factory to a satisfactory temperature so that any excess rigidizer burns away while others do not. As you can never be 100% positive that all of the excess rigidizer is gone, a good fiber burnout is usually necessary.
With that said, before you put anything in the kiln, fire your new fiber kilns up to 1500°F and hold for one hour. Make sure to do this in a well-ventilated area, like an open garage, near an open window or your back porch because as the heat increases, the excess rigidizer may begin to steam, and well, yes even get a bit stinky. We get a lot of calls about this but, don't panic as it is totally normal.
As the process continues, the nice white fiber board may even begin to turn brown and discolor. But, once the temp reaches 1500°F the toasty looking area should return to pristine white. If there is a lot of rigidzer, the area may remain slightly discolored or seem stained. Again, don't worry, as this will not effect the firing of the kiln, or later, the glass inside.
Discoloration happens a lot when you use fiber papers like Thinfire or Papyrus. It’s just happening inside the kiln, so you don’t see it. If you vent your kiln while using these papers, the burnout process can discolor the edges of your kiln lid. Again, this just isn’t a real problem, just a cosmetic blemish. Think of this as smile lines from the joys of using your kiln!