My foundry furnace works okay for firing small things, but I can't hope to fire anything more than four inches across without having way too much temperature difference across it. This is *not* satisfactory for firing, especially for crucibles which need to fit the furnace!
One early idea I had was to fire insulating bricks and build with that, but it would take 72 bricks, curved and about 4.5" cube sized, plus 56 spherically curved bricks for the dome ceiling. (At $2 per brick, this would cost me a whopping $512, plus tax, and they wouldn't even be curved like I need! To fire them myself in the crucible furnace would take two intensive months, firing about two bricks each day. It would also be about as expensive with the fuel and clay costs, if I can even fire an insulating brick evenly in the first place!)
Some interesting details about the above kiln, however, are: the efficient cylindrical domed shape*; four burners for very even heat; insulated flues, one with a right angle which reduces radiation loss; and two flues, giving the choice of upward or downward draft. Yes, convection works backwards! I don't care what gravity says, as long as I have a sufficient chimney to pull draft, it will suck the exhaust out the bottom. (The top flue is plugged.) Now this would be very good for firing a circle of something, such as bricks for instance. (At the bottom of the drawing, there's a shadow of how I might arrange bricks in it to fire them.) If stacked tightly, hot gas from the burners can't really get through to the center and leave, so it has to go allll the way up, concentrate at the top (since heat rises after all), then get sucked out due to its bouyancy as it climbs the chimney (sort of a gaseous siphon you could say). Since heat also concentrates around restrictions, the rim around the floor flue will be warmer than it would be otherwise, which combined with direct heat from the burners (around the outside of the bricks) and reflected heat from the hot gas at the ceiling, this thing would have a very consistent temperature inside.
*Actually, it isn't efficient. For one, it's relatively small - "only" 1.6 cubic feet inside. The walls are thick brick, though if made of light insulating material, will hold heat well; nonetheless, the outer volume is several times the inner volume. Lastly, the geometry is all wrong - it IS round, but it is very tall. The dome really needs to be a short parabolic profile rather than a round hemisphere, and the walls need to be shorter. An even cube, cylinder, or best of all, if impractical, a sphere, would have less surface area and thus heat loss.
Fortunately, I happen to have a good friend in Ohio, Lou, who recently came upon a cache of kaowool. This is the good stuff, 1.5" thick, 8lbs (per cubic foot, so each square foot weighs 1 pound) and 2300°F rated, 2600°F peak. The story behind how he got it is even more interesting than what I'm building with my box of it: apparently a friend's company was building a boiler (the kind of industrially sized thing that is oh 20 feet cubed and runs say cone 10 non-stop for years...), and it turns out they bought the wrong stuff and couldn't use *any* of it. The company didn't know what to do with it, apparently they can't sell it back, so they just gave it to this guy. All thirty boxes of it. There are 48 square feet on a roll in each box. He was remodelling his house, so he needed some insulation. Instead of going out and buying that expensive yellow fiberglass batting, naturally a deal is a deal and he used this pretty snow white stuff in his walls! No, he is not in any danger whatsoever of burning down...or at least the outside walls aren't. After even this splurge, he still had about twenty boxes left over, which Lou discovered and gained access to. (I say "gained access to" because apparently he didn't take any home, unfortunately for me as I had to wait for a second shipment to finish this project as his friend went on an unexpected vacation..) I can only wish I was there to see his draw drop to China when he saw what this guy had, around 20 grand of kaowool, almost half stuffed into his house! That night when Lou sent me a message I think he silently appraised it around $15,000...
But because he's a nice guy, and I'm a slimy bastard (or so he'd tell you, at least until I placate him with a few crucibles), he sent me about 20 square feet (okay...more like 30...) and I got to work.
So first I need a chassis to mount the stuff in. Well actually I came into two 1980s printers a month before that story and a kiln is a project I thought I'd like to do with one. Then Lou found the kaowool, I begged him for an amount somewhat larger than he was going to give out of friendship ("it's for a higher purpose" after all :) and I was on my way.
The chassis, shown with loose kaowool and a 5 gallon pail for scale, after stripping out the electronics, plastic chassis and some baffles, and about twenty pounds of gooey acoustic padding!
That said, I need some heat. I decided on two burners, at opposite bottom corners, for two reasons: one, the hottest spot in the furnace isn't at the top ("heat rises") but around the burner, because radiation dominates, not convection; two, corners simply suck the heat out. In the crucible furnace, the single burner causes a difference of about 200°F from tuyere to opposite corner - this is why I can't fire anything of reasonable size in it!
This is the prototype burner. I assembled this by walking through the hardware store, no parts list in hand or anything really, just the basis that I need a pair of 1/2" (pipe) burners for this kiln. Armed with my extensive propane knowledge, I grabbed a pack of 0.023" Tweco MIG tips (which I already determined are perfect for ~3/4" i.d. tubing) and pairs of (since I'm making two) 1/2 x 5" black pipe nipples, 3/4 to 1/2" reducers, needle valves and suitable compression fitting hardware to connect them together to my propane tank. Total price: $30 - rape if you ask me. I really should've cut some of these things from stock laying around.
Construction: Start by drilling and tapping a #6 or #8 hole in the end face of the reducer (to later attach the choke plate). Braze a few inches of 1/4" steel rod to the reducer, and at 3/8" from the end of the reducer, bend the rod 90°. Cut it 1/8" below the center of the fitting and smooth the cut with a round file, putting a concave face on the end to seat the Tweco tip. Carefully balance tip on the peg and braze or silver solder in place (being careful not to plug the jet hole!!). Screw on the pipe nipple and adjust position of the jet, centering it in the pipe as best you can. The needle valves are right angled 1/4" compression to 1/8" MPT. The inside of the MPT is slightly smaller than the o.d. of the jet tip's threads, so file them down a bit until it fits. Then to bond, well I don't have any silver solder on hand and braze, well the valve itself is brass DOH! I just soft soldered it with some 60/40 rosin core. This so far is plenty strong, just be sure the copper threads are clean so the solder sticks. Acid core solder wouldn't hurt, either. Finishing steps: cut some sheet metal for the flare and attach it with a hose clamp, or if you wind the sheet around a smaller rod, you can use its natural springiness to hold it in place sans clamp!
Add propane through copper tube and the valve's compression fitting and congradulations my friend, you now have a burner stable to somewhere around 1/4 PSI, all the way up to 20PSI or more. (The small diameter has a tendancy to blow out at high pressure and lean mixture. A larger flare should fix this, so I'm using a larger dowel in the cast refractory parts below.)
Since I'm going to embed these in refractory - wait, why bother at all? Well since I'm going with kaowool, a good insulator, just shoving a burner down into it will cause a steel flare to simply disappear. It will literally melt, and then the oxide that drips onto the kaowool will flux it (scale (black iron oxide) is a flux) and cause more trouble. I don't want to go without a flare because one, the intense heat will melt the kaowool, and two, it's a rather unpredictable flare. The acoustics alone are probably quite weird (and no doubt important to burner operation), not to mention kaowool is a very porous substance. The logical solution is to use a refractory flare, but why stop there? I need enough structure to support that as well, so I might as well make it two inches around, which is around 4 1/2" o.d. Reinforcing wire is demanded and here I used 1/2" hardware cloth and coat hangers, tied up with 0.020" stainless lockwire. The coathanger is brazed to the burner pipe at one end, locking the pipe in place. The sheetmetal wing is for mounting, it really should've been heavier, or about three of them (I used 26 guage galvanized).
For the flare, I've got a 1" dia. 2" long dowel attached to the end. When removed from the refractory it will form the flare cavity. Being larger than the water pipe it will provide a much harder pressure step than the straight sheet metal flare, which should hold the flame better, preventing blowouts at 35PSI (if I ever need that much pressure). My concern is it may make it unstable at low throttle.
Top end view. You can see where the coat hanger is hooked to itself and stuck to the pipe. Inside the pipe is a coat hanger, tensioned to hold the flare pattern in place. I'm suprised how well this half-assed trick worked!
Bottom end. I bent a short 90° on the end of the coat hanger, sent it down the drilled dowel, the pipe and hooked it on a cross wire (see above).
Forms: old ravioli cans! Thank you Chef Boyardee ;-) Yes, I had to cut down the side, bend out a flap, insert the structure and fold the flap down to allow for the mounting bracket.
After much poking, prodding and vibrating, it's all done, filled with Plicast KL3000KK.
As for the kiln itself, I have to do something about the floor. Kaowool has Zero strength and Zero rigidity, it will never support a hundred pounds of pottery under fire. Thus, I will pour a 1 1/2" thick slab of KL3000KK, and for insulation value, stuff underneath and beside with kaowool, where the wood is now. Add some bolt mounts, coat hangers, a square of hardware cloth, yet more lockwire and it's ready. Well except for the front mold face, which is held in place with the door, which is open right now as you SOOOO cannot tell...
All 35 pounds are in, totalling about 1 5/8" thickness. Phew, it's level too. I rock! 8-)
Slab is currently setting/drying, I am deciding on how to bond the kaowool to the walls and what to do for the ceiling (at the moment, I'm considering LWI26, attached, hung and insulated similar to the floor). My last kaowool test was less than encouraging, completely charring the paint and adhesive on a piece of sheetmetal I bonded it to. (Admittedly it was on the bottom and didn't have any air circulation, I'm going to try a fair test yet.)