Build Your Own (90% Efficient) Fireplace
(Page 2 of 3)
November/December 1980
By David Gustadson
HOW THEY DID IT
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The Southwesterners' secret is not an intricate interior labyrinth (their molded refractory firebox is almost Spartan in this respect .. . though its metal flue pipe is somewhat convoluted), but the fact that cast masonry stores and liberates heat better than does metal. The team opted to fabricate the stove from Kast-Set, a refractory clay (available locally, or by writing to A.P. Green Co., Dept. TMEN, Green Boulevard, Mexico, Missouri 65265), because [1] it's easy to work with, [2] it's strong, [3] it can withstand 2500°F temperatures, and [4] it doesn't require kiln drying.
First, the two gentlemen made a mold — using 3/4" plywood and 2 X 4 braces — and cut it in half horizontally. Next, they lined the form with thin-gauge sheet metal and covered the inner "plug" in the same manner. Since the rear section of the firebox supports the flue pipe, they also had to devise a reinforced cardboard bung to provide for that piece of exhaust hardware when casting the "aft" portion of the stove. Then they worked up a mold for the simple arched slab that forms the back wall of the firebox.
The trickiest part of the stove's construction, its designers found, was the actual casting and curing. The refractory clay must be mixed with enough water to give it the consistency of wet sand that will hold its shape when squeezed. According to Bob and Alex, it's best to be thorough when dampening the mix . . . in fact, they found that three gallons of water per bag — though it's more than the manufacturer recommends — makes the strongest blend.
Since the "mud" is too stiff to pour, it has to be carefully spooned into the molds (which ought to be thinly layered with grease beforehand to make them easier to remove later) and firmly tamped between "servings" — being sure all the corners are filled — with a piece of 1 X 2. The mixture starts to set in about 20 minutes, so it's best not to dawdle at this stage.
When all the molds are full, excess clay can be skimmed off the top and a sheet of plastic placed over them. The castings should be allowed to stand for one week like this, at temperatures over 50°F. Then — since proper curing requires that the Kast-Set be kept slightly damp for at least two weeks after forming — gently remove the molds after the first seven days, and seal each "green" piece into a plastic garbage bag. Allow the sections another week in that moist environment. Finally, remove the shrouds altogether and sundry the parts for a fortnight or two ... or until they're completely air-cured (naturally, exposure to rain and condensation should be strictly avoided during this final stage). To save time, it'd be a good idea to order or fabricate an adjustable, vented, 3/16" X 16" X 16" steel firebox door while waiting for the molded components to harden.
