Solid Arch Kiln Forrest Gard

Instructions

The only limit to using castable refractory 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.

I built an experimental soda kiln that is adapted from a design by David Herrold and the book Building Your Own Kiln by Itabashi Tamura Kawabuchi. My design uses a catenary arch made from 1½-inch-thick, hard castable mixed with stainless steel pins (Hank Murrow’s idea). This reinforced the strength of the hard castable, which is usually cast three inches thick or more. The cast arch was then backed with five inches of 2000°F (1093°C) fiber held down with diamond lath and coated with a top coat (See top coat recipe on page 13).

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 (the top 2×4 is visible in image #1). Fill this form with the polystyrene arches.

Mix the castable, paying specific attention to the manufacturer’s instructions. I used a commerical castable, Pryor-Giggey Phlocast 30 Low cement, and added Rib-Tec 400 1-inch stainless steel pins. Use approximately four pounds of steel pins per 100 pounds of castable refractory. 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.

This article was excerpted from the March 2012 issue of Ceramics Monthly.