How to Build a Solid Arch Ceramic Kiln with Castable Refractory

Kilns can be built out of many things and castable refractory is one of the materials we rarely consider. Perhaps it should be considered more since it is reasonably priced, easy to mix, and easy to use.

As John Britt explains in today’s post, if you are comfortable with casting plaster and making molds, you can handle building a solid arch kiln with castable refractory. – Jennifer Harnetty, editor.

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 Standard Coating recipe).

This post is one of the many handy tips and tools shared each month in Ceramics Monthly.

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Casting an Arch

1 Individual packing section for castable over top of expandable polystyrene form. 2 Remove the expandable polystyrene form after the castable has cured for 24 hours. 3 View of cast shell before the fiber and lath coating, brick door, and flue are installed. 4 View of kiln firing.

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. 

  • Peter M.

    Bravo! I like this. I have used refractory clay to make a mini smelting furnace so as to smelt bronze bits that I add to some of my ceramic sculptures. I hadn’t thought of working it on this scale.

    The catenary arch is an ancient classic, figured by hanging a chain (catena) to the desired width and depth. Nice to see it in this application.

    I am looking in time to come to build a large kiln, approximately two metres by two metres, and 1.5 metres in height, and I need to consider using propane or butane instead of 3 phase. I also would like to think about the possibility of having it adaptable, so that I can shut down or close off some of the volume for smaller firing loads.

  • Subscriber T.

    I found this post very informative and want to try this technique. Can you let us know where to purchase these materials, the quantities and cost? Thank you.

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