Thanks for the encouragement Eric, and the link to the S-portal. But try as I might, I'm not sure I get it. If I figure it out, or if one if you can give me the short course, "S=portals for dummies", I might try that. I did find this, [The S-Portal is a stone or steel frame around the exit to the heatriser. The frame is hollow and fresh air, entering into the frame from outside the firebox, is injected through slits in the frame into the flames and smoke gasses. The S-portal can have any shape and is positioned vertically in one of the sidewalls of the firebox.] but still need to think about what it would look like in this instance.
In the mean time, I think I will take Peter's advice and try to fit in the P-channel in the manner Peter suggested - through the edge of the barrel or cheat it under the edge of the barrel.
A downside of the efficiency of this thing is the damage it has experienced from the 5-10 fires that I have made in it. I have made some repairs to the major damage with cob patch, but I'm considering repairing it a little more permanently with furnace cement. After that , I will be casting a new one or using bricks to build a more robust one.
Here is the damage to the side wall:
And damage to the throat:
You might try mixing wood ash in with the mud.. I do it this way:
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.
