Types of Kiln Shelves
Kiln shelves are made from highly refractory materials.
- High-alumina shelves: Dry-pressed high-alumina shelves are tougher and last longer than slip-cast high-alumina shelves. Generally speaking, high-alumina shelves are best used in electric kilns. They especially should not be used for raku, salt, or wood firings.
- Silicon carbide shelves: Silicon carbide is a conductor; I strongly recommend they never be used in electric kilns. If silicon carbide shelves are used in an electric kiln, the kiln should be disconnected to all power before the shelves are placed or removed.
Kiln Shelf Storage
Like glass sheets, all kiln shelves do best if stored on their edges. Flat storage makes it more likely that they will break or develop stress cracks. Infinitesimal cracking in the shelf during storage may result in catastrophic failure of the shelf during firing.
If you don't already have one, build a kiln shelf rack in which you can safely store your shelves. All they really need is a sturdily-built box or railed enclosure where they can rest on their edges without danger of being knocked over. I also like to line the bottom of my rack with packing foam sheets or other cushioning material.
Make sure that all kiln shelves remain dry at all times. If they do get wet, heat them slowly up to 300° F and soak four to eight hours.
Kiln Shelf Usage
Kiln shelves should be set using a three-post method. Using three posts instead of four or more gives the most stable set and reduces the possibility of the shelf moving during the firing.
For optimum shelf life, kiln shelves should also be flipped over after each firing; the top surface should become the bottom surface for the next firing. Along with this, posts should be placed so that they connect with the kiln shelf in the same area (albeit on the opposite side).
Both the rotation and the consistent post placement will ease stresses and warpage caused within the shelves' structure and lengthen the life of the shelf.
Kiln Wash Your Shelves
Kiln wash protects your kiln shelves during glaze firing. Note: even unglazed pots can stick to the kiln shelf if the clay body vitrifies enough. Always either kiln wash the shelves themselves, or set your pottery on kiln washed, bisqued setting slabs or shallow dishes.
If you wish to flip your shelves, do not use commercially available kiln washes. They will bond to the shelves and make using both sides problematic. (Kiln wash can flake off during firing and land on glazed pottery below it, as well as potentially bonding the shelf to the kiln post it is resting on.)
Commercial Kiln Wash Alternatives
Instead of commercially available kiln wash, you can use
- setting slabs, making sure to kiln wash them first,
- alumina hydrate mixed with water to the consistency of heavy cream (after the firing the alumina hydrate will be powdery and can be brushed off the kiln shelf and reused, but do not use this method if there is a possibility of heavy drips or glaze running off pots), or
- a thin coating of 10% kaolin, 90% alumina hydrate mixed with water, which can be wiped off shelves with a damp sponge (but it offers only minimal protection of shelves).