Castable refractories are easy to mix, easy to use, and reasonably priced. Choosing to cast a floor or an arch, rather than cutting and mortaring brick, gives you a monolithic piece that is joint and crack free, therefore eliminating heat loss and seepage of salt, soda, and ash.
Defining the Terms
Refractory—Resistant to high temperatures. Chemically and physically stable at high temperatures (from 1000°F–3200°F).
Soft Fire Bricks—Insulating Fire Bricks (IFB) are made from refractory clays and various other ceramic materials along with an organic filler that is burnt out, leaving small voids in the brick that help it to insulate. They are lightweight and absorb about half the energy of hard bricks. They come in a variety of temperature ratings, from 2000–3300°F, are easy to cut with a hand saw, and the common ones are called K-23’s (for 2300°F) or K-26’s (for 2600°F). High-fire potters often choose K-26’s for their kilns.
Insulating Castable—Castable refractory with insulating properties similar to IFBs.
Hard Fire Bricks—Refractory brick that can withstand and hold heat without fusion. Used in ceramics kilns, lining furnaces, and fireboxes. They are hard and durable, have good thermal shock resistance and can withstand various firing atmospheres. These can only be cut with a tile saw or brick saw. They absorb a lot of heat which adds to firing costs. The percentage of alumina in the bricks for ceramic kiln use varies.
Hard Castable (Dense Castable)—Castable refractory with similar properties of hard-fire bricks.
Spalling—Cracking of castable/ brick in a kiln. Can be caused by rapid temperature change, a wide temperature difference between the hot face of the castable and the exterior, or the result of improper heating of castable where water turns to steam and causes the castable to peel, pop out, or crack off.
Ceramic Fiber—Fibers of molten materials are stretched and collected to form a blanket or pressed into other products such as fiber board, fiber paper, ropes or custom shapes. Ceramic fiber comes in a variety of weights (density), temperature ratings, and thicknesses. Ceramic fiber is a dust hazard.
Castable Refractory—A refractory mixture with heat resistant hydraulic cement and binders that can be cast in place. Castable refractory has many applications in industry as well as studio ceramics. It is often used in kiln construction because it is easy to use and mix, and reasonably priced.
Castable refractory has many benefits over traditional bricks because it can be mixed on site and poured into molds of almost any shape. The forms are also relatively easy to construct. This leaves fewer joints that can leak heat or air. And, unlike hard-brick construction, you don’t need a chisel and hammer or a wet saw to make hundreds of cuts.
This material is often used by potters in kiln building to cast the kiln floor (both car and standard kilns). This is easy because you just make a square or rectangular form, seal it with something like Vaseline, and pour in the castable, giving you a level and smooth floor for your kiln. It is also commonly used to form the key block in standard sprung arch and anagama kilns. This eliminates the need to make complicated and precise cuts, which saves time and reduces mistakes. The key block is cast in place by sealing the top of the arch form and pouring in the castable. The walls and gravity hold everything in place while it cures. Some people cast the entire kiln, including the door (made of cast brick shapes). Others only cast portions of the kiln—for example, building a kiln body with standard bricks and then casting just the sprung arch to save the expense of buying specialized arch bricks.
Castable refractory is also simply mixed in a wheel barrow with a mason’s hoe. It can be bought commercially or homemade. All you have to do is add water. Getting the water ratio right is the most difficult part; too much water may cause it to shrink and crack. The manufacturer’s instructions are precise and easy to follow and have specific directions for curing and firing. The castable must be fired very slowly so that it doesn’t blow up.
High-alumina castable can be used to make the trough in the bottom of a salt kiln fire box. This helps contain the salt that is poured into the kiln and preserves the floor from its destructive effects. This trough can then be periodically removed and replaced to extend the kiln’s life.
Knowing the properties of the materials allows us to exploit their positive characteristics. For example, hard-dense castable is strong and durable but uses a lot of BTU’s to heat up, while ceramic fiber is very fragile but is inexpensive and is a great insulator.
The downside of using castable is making a form for everything that needs to be cast. Although wooden forms can be used repeatedly for multiple bricks and arch forms, you may only make one kiln. Many potters are already familiar with the mold-making and slip-casting process; this type of building is similar, being either enjoyable or tedious depending on how you feel about the process. Building a kiln often requires making a form for the brick arch, castable forms just need to be a bit stronger and must be sealed so that they will release from the castable easily.
Castable refractory can crack or blow up if it isn’t fired properly during the first firing (after that it is the same as brick, so essentially you are firing your own custom bricks). It will definitely crack if the mold is made incorrectly. Additionally, poured castable cannot be allowed to freeze when casting and before it is fired, but once fired it is just like regular brick. Cast kilns are not easy to disassemble and move and so require a permanent location.
Castable is only fired from the inside, so the interior may reach cone 10 but that heat doesn’t necessarily penetrate a 5-inch-thick wall. Thus, spalling is possible if the walls are cast too thick and the kiln is fired unevenly. This is generally more of a problem with homemade castable as many commercial castables products are spall resistant.
Commercial castables can be stored up to twelve months in a dry place. You must wear rubber gloves and eye protection when preparing the material because it is caustic. It should be allowed to cure for a minimum of twenty-four hours so that it can attain maximum strength before firing.
John Britt’s 20-Cubic-Foot Hard Castable Soda Kiln plan. Drawing: Martha Peiser.
Building Forms and Casting
A catenary arch form is a simple arch; start by establishing two points, say 36 inches apart, and hanging a chain of a certain length from those two points. The curvature of the chain translates to the arch form of the kiln to be cast. Next cut two identical pieces from this form out of 1⁄2-inch plywood. Establish the length of the kiln, say 45 inches, cut 2×4 lumber to length and attach them to the plywood arch forms. Then cover the plywood with 1⁄8-inch masonite and screw it to the 2×4s. Cut two more catenary arch forms out of 3⁄4-inch plywood the thickness you want the kiln wall to be (e.g. 4 inches thicker on each side, so 44 inches instead of 36). Screw these to the ends of the first form and coat the form with Vaseline so that it releases from the castable refractory. Next attach a 3⁄4-inch piece of plywood approximately six inches high to the end arches, mix the refractory, and pack it into the hollow. When you reach the top, make a V-shaped groove onto the top and let it harden. This is the “key” that holds the next layer in place. Then put plastic wrap over the top. Separating the parts with plastic creates an expansion joint and avoids cracking during firing. After it is hardened, put another 6-inch board on and put in castable. Repeat. When you complete the form, all you have to do is unscrew the forms, drop the interior form, and you have a hollow catenary arch kiln.
Firing the Castable
The first firing is a very important aspect of castable refractory construction. All potters are aware of the hazards of greenware blowing up in a kiln if it is fired too fast or too wet in the early stages. Now imagine that the pot is five inches thick and weighs 2 tons. If that blows up, it could easily destroy your studio and cause great injury.
Following the firing protocols ensures both success and safety. Consult the supplier for their protocols. A typical manufacturer’s instructions suggest to fire approximately 100°F per hour with a specific hold for every ½ inch of thickness at 250°F and 500°F and then to finish the firing to peak temperature. The instructions include the degree of rise per hour for each section of the firing.
When firing homemade castable, err on the side of caution. There are sample firing guidelines found online. But remember that each situation is unique and there are not guaranteed firing cycles, nor do they cover every type of situation. One online source lists the following guidelines: start by raising the temperature to 225°F (107°C) and holding it for 24–48 hours to be sure all the water is out of the castable, next raise it 25° per hour up to 600°F (316°C) and hold it for 12 hours, then climb 50° per hour up to 1200°F (649°C) and hold for 12 hours, and finally climb 50º per hour up to peak temperature. This is just one potter’s sample firing for his particular castable and is not necessarily appropriate for all castables. In addition to causing explosions, excessive steam created by heating the kiln too fast can also damage the bonds of the castable and cause problems later like pop-outs, cracking, or spalling. If you have excessive steam, reduce the temperature and hold until the steam dissipates.
Recipes (in parts)
This article was excerpted from the March 2012 issue of Ceramics Monthly, which can be viewed here.
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