The only limit to using refractory castable is your ability to design and build a form. If you know how to make molds and cast plaster it’s a short leap to using this versatile material.
Casting an Arch
Determine the size of your arch and cut enough expanded polystyrene arch pieces to form the entire arch. Set it on four-inch “legs,” which can be dropped out after casting. Make the exterior wooden mold form by cutting two identical arch pieces out of 3⁄4-inch plywood that are longer than the foam arch by the thickness of the kiln wall. Establish the length of the kiln and cut three 2×4s to that length, attaching two to the bottom sides of the plywood arch forms and one at the top. Fill this form with the polystyrene arches.
Mix the castable. Use approximately four pounds of steel pins per 100 pounds of refractory castable. When mixing, wear a respirator and use thick gloves—the castable is caustic and the pins are sharp.
Pour the castable in 4-inch-high sections made from 3⁄4-inch plywood strips so they don’t bow, being sure to pack down and smooth out each section. Form a V-shaped groove at the top of each section and let it harden. Separate each section with plastic wrap before repeating—the plastic creates an expansion joint. After the castable has cured for 24 hours, take the outer plywood off and drop the legs out from underneath, allowing the interior expanded polystyrene forms to drop out.
Cover the castable with five inches of ceramic fiber to insulate it and put diamond lathing on with the cover coating to keep the fiber from becoming a dust hazard. You can add iron oxide to the final coat to make it look good if you’re not into battleship gray.
STANDARD COATING
Alumina …………………………. 50%
EPK Kaolin …………………….. 25%
Calcined EPK Kaolin ………. 25%
– Apply very, very thin as needed or it flakes off. (a.k.a) kiln wash or wadding if thick.
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Article Source : John Britt (ceramicartsnetwork .org/daily/article/How-to-Build-a-Solid-Arch-Ceramic-Kiln-with-Castable-Refractory)