I bow before your experience and humbly ask for your recipe for insulating cob. I wish to cast j tubes using vermiculite or perlite and was considering adding wood ash. Straw probably for fibre. I know experimentation is in order for exact results but I am working off a friends soils so will be chucking the whole thing together and hoping it works. If not I get to pull it apart and try again! As always your input would be gratefully received.
This is for people who plan to use natural materials to cast (or build) rocket stove parts and/or rocket stoves. These materials can be used in SO many places, it's worth knowing this stuff even if you never build a rocket stove. This is for higher temperature applications. For reference, I've developed this approach, using natural clay-rich soils that can be dug up on my land and/or locally, near to me. Your particular soils will have different properties and requirements, so mixes will vary. None of what I've laid out here should be seen as a hard-and-fast recipe. ALWAYS test rigorously, NEVER assume that my proportions will just work for you.
I think that what is generally needed are 3 different kinds of mix, though they will be made of the same basic materials, the different mixes will perform different functions. You will need a hard material for places where it will experience abuse (the feed area), banging wood, fire poker, etc; a softer material that provides resilience to heat and better insulation but does not need to resist physical banging; finally, you need a "plaster" material for sealing porous surfaces, smoothing over shapes, patching dings and other odd jobs.
First, prep the basic materials: Make clay slip and screen out the rocks, roots and other junk. Basic Slip should have the consistency of a heavy-duty milk shake with just-melted ice cream. Dip your hand into the slip, it should come out looking like a thick rubber glove of mud. I toss raw dirt into a barrel, add water, mix with a drill motor/paddle mixer and then screen it into another barrel. Wood ash should be screened as well. I crush in sintered ash along with the powdery stuff and rub it through the screen. I think the sinter makes good aggregate. Remember when screening, the size of the screen you use is rather important. It's a big factor in determining the size of your mortar joints and things like that. Smaller screens make for harder work getting the stuff through, but big grained materials can be a total pain in the neck for some uses (mortar joints should be as thin as possible). Learn to know when you need a smaller screen but work with the biggest one you can. You will need a chopped fiber, I toss rice straw through a chipper/shredder, it makes a nice 1/4 inch minus fluff. Alternately you could use rice hulls, horse manure, maybe pine needles or something similar. The fiber (closest to big heat) will mostly just char away, either way it really helps with workability, prevents cracking, etc. You generally want sand that does NOT have a crystalline structure, crystals tend to expand differentially when heated, expanding along an axis, breaking the clay and crumbling the mix (There are exceptions to every rule). Ideally, your sand should include as many different sizes of grit as possible and be rough/crushed rather than round/polished. I've heard people say to only use the construction grade perlite, which is coated to keep out moisture; agricultural perlite will hold in moisture and is missing the coating. My experience is that the only difference between the two is that the uncoated perlite takes longer to dry out completely.
The key note of all of my mixes here are going to be wood-ash/clay, so the basic mix: Mix together the slip and ash, adding (by degrees) dry into wet until it gets difficult to add more ash; it often resembles a heavy, well floured bread dough. To this dough, we can add the other ingredients. It's a good idea to make tests of this with your clay selections as early as possible, put out a couple hockey pucks or adobes and see what it does. With my clay, the base mix can crack a little (best if it doesn't) but it won't just crumble. My dried product looks like slightly cracked (clay rich) but very hard, warm-grayish hockey pucks.
I should point out that wood ash has lye in it, which is caustic. I've been told that the hardwoods have more lye than the soft but either way, wear gloves and watch your eyes.
To make hard, resilient, (but less insulating) feed box mix; take the base mix and add sand and chopped fiber. You will most likely need to add water to work these into the mix, clay slip can be used instead depending on the stickiness (and/or expansiveness) of your clay. More of the fiber will make it more insulating, less knock resistant; more sand the opposite. (Always, there are exceptions to rules) Using my soils, (by volume) one part base mix with one sand, from around half to as much as one and a half fiber, depending. Alternately, we could go with one part base, two parts sand, half or less fiber. Test!
To make high heat insulation mix (that's fragile), I have most often just used perlite/slip. Ash can be added to the mix, when you do, it's easier to mix the dry together first before you add the slip. Wear a good mask when you work with this stuff, you don't want to breathe the dust; when everything is all wet from slip you can take it off (the mask). The slip (usually) should be wetter than what was used for base mix, add water and mix well. You should pull a thinner glove and it should still look distinctly glove-like, you don't want it to run clean(ish) off and expose skin; the difference between "slip" and dirty water. Pour the dry out on a tarp (sometimes in a wheelbarrow), perlite and other dry (ash), then sprinkle slip over and gently fold them together; try not to squish too much of the perlite (it's fragile) when you stir it. You can pick up the corners of the tarp and roll the mix around, this works but not as well as turning it and mixing by hand. You want just enough clay slip to thinly muddy everything and hold together the mix, no more. Pack a ball of it in your hands, it should form relatively easily into a little glob and stay together. When you squeeze this ball between your fingers, it will pop apart. If it smushes without popping, it's generally wetter than I would pack a heat riser with. The mix can be made deliberately wet (with slip) for a more sculptural, less insulative material. Fiber can be added, alternately you could replace the perlite entirely with organic materials. Wood-chips, forest duff, straw, manures, etc. can be used to make a fairly insulating material that isn't very strong or long lasting but is absolutely free.
For the plaster/slickum material, take the base mix, add water (or slip depending on the clay) and (often) fiber for workability. You can "add up" to whatever property is needed, sand/fiber for more sculptural mixes or wet down for thinner wash layers. Washing can fill pores and lock closed the surface (inside of perlite/clay heat riser), make 'em thicker or thinner for the need. Often, I make a creamy mix like toothpaste and smooth it on thin with a trowel or sponge to seal cracks. I plaster the inside of perlite/clay heat risers with it and I think that a layer of this can help to hold together sawdust/clay parts that would otherwise disintegrate over time. Base mix is the mortar that I use with fired brick and I use as little of it as possible, then I plaster the whole thing, over all of the brick seams, etc. with a thin coat of watered base.
For casting, none of these mixes are going to be "pourable". Making solid parts is going to be about packing, not pouring.. This might complicate the process of using some molds (batch box) but at the same time it will facilitate a lot more free hand sculpting, which makes it possible to make shapes that would NEVER come out of a form in one piece. More than that, several material types can be combined in one go and dried together. For instance, build up a layer of hard mix with insulation over it followed by a cap of hard mix. The combinations are endless so you can let your imagination and your intuition go a little.
Thanks again donkey, I am planning to build up around metal tubes to form my first 6"/100mm j tube and pull them out before firing. After if it looks like its going to fall apart. Hard for the core surrounded by lots of insulating then more hard I think, as this will be outside. Staw for fibre, must find a way to chop it up, cat litter for perlite. Its happening in two weeks and I can't wait!
Weather permitting, I am going to begin casting my stove core tomorrow using Matthew Walker's mix which is very similar to Donkey's but includes refractory/furnace cement. Matthew's video shows him mixing the refractory cement throughout the mix, but I'm considering making a couple of mixes, one "richer" in cement than the other and using it on the interior surfaces of the j-tube and layering the other over the top -- similar to Donkey's description of hard and insulation layers. Seem like it should maximize the benefit of the cement which is kind of expensive. I'm figuring to make the cement-rich layer about 1" of the total thickness of 4 inches.
Last Edit: Jan 12, 2014 12:46:56 GMT -8 by mbxxxxxx
Yes, a hotface is what that is called. I know for a fact that when mechanical forces are applied to insulating materials, that they will begin down the slow road to destruction. I cast various metals in my backyard foundry and have found the 3/4 hotface I put in front of 3" of kaowool to be adequate. Granted these are two different applications, but the principle remains the same whether your hotface is 3000* refractory or a clay based ceramic.
You generally want sand that does NOT have a crystalline structure, crystals tend to expand differentially when heated, expanding along an axis, breaking the clay and crumbling the mix (There are exceptions to every rule). Ideally, your sand should include as many different sizes of grit as possible and be rough/crushed rather than round/polished.
How is the right type of sand identified? Is there some particular grade of sand that should be requested to avoid one that has a crystalline structure? Do we need to look at a handful of sand with a magnifying glass to look for the absence of a crystalline structure?
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