I dug a hole in my back yard this weekend, because... as it turns out, this part of Georgia is full of clay. Not just red dirt, but actual clay. In fact, I live just north of the Fall Line here in Georgia, which is where kaolin is mined. Kaolin is a white clay with a high alumina content which makes it particularly useful as a heat resistant material. Where I'm at, we have pockets of kaolin. Just dig down a little, and you'll get a little here, a little there... and it's finely disbursed throughout the regular red clay as well. You can, if you're so inclined, manually sift through some dug-up clay and separate the kaolin from the red clay. Then you can refine both by mixing it with water and allowing to settle. Organic materials will float to the top, rocks and sand will settle to the bottom, and the clay will form a distinct layer in between. The local red clay is good for making fired clay pots, roofing tiles, weeping tile piping, and other uses. But the kaolin is an outstanding refractory material - it's an alumina-silicate capable of withstanding thousands of degrees. It is often used to make crucibles for melting steel, for example.
Speaking of steel - great material. I picked up some old leaf springs at a salvage yard. Leaf springs on Ford trucks are made from 5160 spring steel, which is a great steel for making things out of - things like knives, axes, chisels, etc. I cut a 16" section off to make a timber framing slick, but unfortunately my little forge couldn't heat the metal up to the white hot state needed for form the socket. I used wood charcoal, and this gets just hot enough to melt aluminum - around 1,230 degrees Fahrenheit, but I need to get steel up to 2,200 Fahrenheit to do a decent forge weld with steel. (Steel melts between 2,500 and 2,750. Hot!) The end result is that I basically mucked up the socket end of the steel slab - big cracks, and huge pits where the metal burned up. "Burned up, you say? But how do you burn something you can't melt?" Well, that's the thing... burning is an oxidation reaction, but steel also oxidizes at lower temperatures. Ever see rust? Basically, at 1,200 degrees with me pushing air through the forge inlet, I had accelerated burning. And that's the problem with steel - it's a terrible material for high temperature use.
But don't they make jet engines from steel? And pot belly stoves? Yes, and no. Jet engines are made from special high temperature alloys that include steel, but also significant amounts of nickel, chrome and other additives. The nickel specifically allows steel to avoid oxidation at temperatures up to 2,900 Fahrenheit, while the chrome keeps the steel hard at those temperatures. At the other end of the spectrum are metal stoves like potbelly stoves, which are made from cast iron, which is basically steel with high carbon levels and lots of impurities. It's an excellent conductor of heat, so it's used in stoves more or less to contain the fire, but let the heat out. These reach temperatures of 500 to 700 degrees. They last decades, but still eventually rust out.
Enter the latest craze in home heating, which is the "rocket stove mass heater". Rocket stoves get their names largely from the sound they make - it's a rumbling, wooshing noise like a space shuttle launch in the distance. The rocket stove basically consists of a J-shaped tube, and at the low end of the tube, you put wood fuel in and light it. The heated air goes up the smoke stack (the high end of the J), drawing in more cool air behind it. The volume of the tube where the fuel goes in is a little smaller, and this sets up a jet action that draws in air at a higher velocity. The result is like when you blow on a fire - it burns hotter. In fact, combustion temperatures inside of rocket stoves go well above the 900 degrees needed to completely combust the wood. The benefit of this is that rocket stoves have little or no smoke, and are much more efficient in generating heat - 85% to 95% vs 75% for high efficiency wood stoves, and 65% for typical wood stoves.
If you go on youtube right now, you can find hundreds of videos of people welding up all sorts of rocket stoves, and some of the designs are quite ingenious, but they all suffer one problem - they're welding them together from mild steel. I have burned up thin gauge mild steel in my forge, which tops out at around 1,200 degrees. Rocket stoves typically burn at this range. And I know what you're thinking - no big deal, once my stove burns up, I'll build a new one. Well, consider this: your first sign that your stoves is compromised may come with catastrophic failure. And if you've integrated your rocket stove into a mass heater design, that catastrophic failure can mean you or someone you care about gets badly injured, and your home burns to the ground - or both.
The best video out there about rocket stoves comes from the duo of Erica and Ernie Wisner, and if you get nothing else from their video, the one thing you should know is you should make your rocket stove not out of metal, but out of ceramic. They discuss commercially available refractories, and how important it is to keep the heat contained in the combustion chamber and the high end of the J portion in order to get complete combustion of the wood and its smoke. After that, you can vent the heat into an auxiliary chamber with a metal cook top, and even route it through a buried vent pipe to heat up your floor, and radiate warmth into the room. At that point, metal piping is okay either because the pipe is backed with a thermal mass that draws it away from the pipe, or because the exhaust has cooled way down.
Enter me, in my back yard, playing in the dirt. The kaolin can be molded just like modeling clay; hand sculpted, pressed into forms and smoothed with simple tools. Then it can be air dried for a few weeks or heated in an oven at 400 for a few hours to drive the moisture out. At this point you can check for contraction, cracking or other defects that weren't apparent during the molding process and if needed bust up the model and re-use the kaolin to make another. Or, if your model (in this case, a rocket stove "J" tube) passes the test, you can build your stove. You don't need to actually fire the clay because that will happen during the course of use. At typical rocket stove temperatures of 900 to 1,200, the kaolin will chemically change to metakaolin, where small crystals of alumina-silicate begin to form. This will make the tube stronger and more heat resistant. At higher temperatures of 1,700 degrees, the kaolnite will chemically change into spinel, and at the unlikely temperatures of 2,000 degrees, it will form mullite, which is basically a very strong porceline that can withstand even higher temperatures. You can basically build a bunch of rocket stoves out of metal, each costing about $80 in materials, or one rocket stove that will last you your entire life and is literally dirt cheap.