As I promised, here are the instructions for making the firing box.
When handling the fiber blanket it is recommended to wear a protective mask and gloves. An alternative to the fiber blanket will be discussed below.
Cut a square out of the fiber blanket, 2 inches longer than your kiln shelf on every side.
Cut off a 2″ square at each of the four corners of the blanket.
Fold the flaps upward and pinch the sides together.
At first the walls may not stay upright. When placing the box in the kiln, support each side with a post. With every firing the box will become more and more stable, and eventually you won’t need the posts.
Alternatively, as Mary Ellin D’Agostino suggested, you can stitch the sides with high-temp wire.
It is important that the walls of the box are a little away from the walls of the kiln chamber.
This box leaves the kiln perfectly clean. It can be used many times. I haven’t replaced mine yet.
In the past I have tried ceramic boxes. I used bisques and they broke after one firing. However, this ceramic box will not break.
All it is is a ceramic kiln shelf with posts arranged around it. The carbon stays contained in the box and does not spread in the kiln. You can add more posts to build a deeper box.