Ceramic for Stoves Part 7- Ash/clay-- Modifying, Shaping, and Drying

Richard Boyt, July 2005

Greetings Stovers & Gassifiers!

In a 6/20/00 entry to stoves, I ended by suggesting that, “while char/clay makes a pretty good high temperature insulation, I think I’ve got a better one.” That better one is ash/clay. Five years later, I have finally learned enough to report on it. I believe that some of this information might be useful to both stoves and gasification.

Clay, when properly fired, can make a hard, strong, long-lasting, heat resistant material, but it is brittle and tends to crack when subjected to high, sudden, uneven heating as would be experienced in the lining of a stove’s combustion chamber. Wood ash is a splendid lightweight loose-fill insulation that remains dimensionally stable up to about cone 10 (1,250C, 2,300F) when it becomes a viscous fluid glass. A combustion chamber made of ash/clay should make a strong, high temp, rigid, low to no cost insulative material with very good thermal shock resistance. However, for many common earthenware low fire cone 06 (1,005C, 1,840F) clays similar to the ones I dig, the addition of wood ash acts as a flux actually lowering the temperature at which the clay can be fired without softening, sagging, or bloating.

Rhodes’s book Clay and Glazes for Potters, page 188 states “wood... ash... contains 10% to 15% alumina, 30% to 70% silica, up to 15% potash [a potassium compound]...” Baldwin’s Biomass Stoves page 175 writes that wood ash contains about 20% potassium oxide. Potassium is so strong a flux that it is often used to lower the melting point of ceramic glazes. Fortunately, these potassium compounds are solulable in water and so can be removed by repeated washings. I find that I can remove up to 11% by weight of solulables in ash by soaking in water and draining several times daily for about a week or until the wash water becomes clear and colorless. This liquid feels slippery and can be used to make good old-fashioned lye soap.

Firing a mixture of 2 parts washed ash to 1 part clay to cone 04 (1,050C, 1,920 F) shows no sign of softening. Firing the same clay mix to the same temperature using unwashed ash shows unacceptable slumping. The washed ash/clay mix is a bit lighter in weight and color, more insulative, more refractory, and more thermally shock resistant than clay only. However, when fired, the mix is also a bit softer, and so less resistant to abrasion. I have tested various mixtures ranging from 1 part washed ash and 2 parts clay, to 2 parts washed ash and 1 part clay. Results of each mix are a trade-off of strength, thermal shock resistance, high temperature tolerance, and insulative properties. I suggest that the addition of washed wood ash to many clays other than the one I use may increase their ability to withstand high temperatures. Both char/clay and sawdust/clay fire to ash/clay, but their potassium content cannot be removed

But yet another problem. Ash/clay is difficult to form. When wet, it has very little cohesive strength, and sticks to everything except itself. I make a fat donut out of ash/clay, slide it into a small tin soup can and carefully spread to an even 3 to 4 mm layer. This method tends to exclude air pockets and minimize the lamination of layers that can separate as they dry. It is very difficult to dry ash/clay without forming longitudinal cracks as it shrinks and tends to cling to the inside surface of the can. This can be minimized by coating the inside of the can with a very thin layer of lubricant (oleo, Vaseline, grease). Very slow controlled drying over a period of several days helps. However, the time can be greatly reduced by wrapping the can a bit loosely in several wraps of polyethylene grocery bags and placing in a warm (less than 100 C) oven. Heat speeds up the movement of water molecules in the damp clay and the bag greatly restricts but does not totally sto p the escape of water vapor.

Periodically during the drying time, I carefully compress the ash/ clay layer with my fingers, crushing and expanding it back against the inside of the tin can that forms it. This reduces the final percentage of drying shrinkage and so helps control cracking.

Another way to minimize drying cracks is to add numerous short (1/8”) pieces of various fibers to the clay. Less than 0.5% by weight of cotton and wool work well, but of course burn out during firing. I’ve tried Kaowool, asbestos, and Kevlar, but the best so far is a special high temp fiberglass (Pyrex?) used as door gaskets for home heating stoves. Expensive, but provides great strength-- green or fired, hot or cold.

The ash/clay/fiber mix still shrinks about 7% to 10% as it dries, leaving a gap between the clay and the can. Small cans develop small gaps which can be firmed up by poking in pieces of thin metal (tin can, aluminum, stainless steel, iron wire, or mesh). In larger diameter castings, the shrinkage may be so large that it is necessary to cast the clay in an oversize can and abrade it to the final dimension after drying. If properly sized cans cannot be found, cylinders of plaster can be cast over removable styrofoam plugs to serve as molds into which the moist, plastic ash/clay/fiber can be pressed or cast. This is one way to fashion homemade riser sleeves. In any case, if the ash/clay/fiber shrinks enough to permit its removal from the mold, it can be fired seperately in a kiln to high temps that would destroy the mold. If the superior strength of high temp firing is not required, the ash/clay/fiber may be fired in place in the can by building a good hot fire inside. T he ash/clay/fiber provides good enough insulation to protect the can.

Yet another forming method plasters a thin (1 mm) layer of ash/clay (no fiber) to the inside of a can with a very clean roughened surface. It will crack when drying, but so long as the pieces cling tightly to the inside surface of the can, you could build a fire in place in the can. While the high temp moldable material I describe here is principally intended for combustion chamber liners for stoves, I believe that variations in compositions using superior commercial clays might prove useful in gassifiers. I have experimented with A. P. Green insulating castable 22, and it is excellent. No shrink, light weight, very hard, thermal shock resistant, but I believe it is no longer available. Norbert Senf wrote about it 2-1-99. I think A. P. Green is in Chapter 13 bankruptcy under asbestos lawsuits. China may be making it now. There are others that are available, including Kaocrete, Kaolite, and Litecrete. See Olsen’s The Kiln Book pages 13-15.

The use of ceramics to solve problems of high strength and high temp is as old as primitive civilization, and as new as the tiles on the wings of the next shuttle flight. Closer to home, admire the porcelain body of a fine spark plug.

Feel free to use the information here in any way you wish. Credits are not necessary. Here is a chance to explore some new territory in stove design and materials.

I am working now on designing survival stoves for the most desperate victims of wars and natural disasters. I think it is better not to send them stoves, but rather provide information on how they can make stoves from native clay and old tin cans.

Hope this someday may of some use to someone.


Richard Boyt

Dick Boyt
20479 Panda Rd
Neosho, MO 64850

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