Well I've already got fire and plenty of temperature in the foundry, but it's a bit like a lead-lined baseball bat. Eh, more like a car crash. Reason being, ceramics demand very slow heating and cooling schedules, so they don't crack and to develop the crystalline and/or glassy structure characteristic of ceramics. We're talking 500°F/hr absolute maximum, period. More sane rates are in the 50 to 200°F/hr range. Cone 10 (link is a complete list of Orton cone temperatures) spans 2284 to 2426°F (depending on heating rate; I'll get to that later), which takes a run of 11 hours just to reach temperature! That's 11 hours of continuous, gradually larger fire. How does one maintain such relaxed rates?
As we all know, foundry furnaces are designed to melt something as fast as possible. I can do 10 pounds of bronze in half an hour with the propane cranked. That's around 2100°F in 0.5hr = 4200°F/hr, ten times faster than is safe to fire pottery. Not to mention the big chunk of cold metal consuming heat in that time. Without it, the furnace would probably melt in that time (since it's only rated to 2600°F, and appears to melt soon after, given the tuyere is glazed).
Well obviously, the first step is to turn down the gas. But just how far? Well, turn it down to like, 5PSI (or whatever is a medium to low idle in your estimation) and leave it sit for a few hours. HOURS? Yes- it takes that long for the heat to conduct through the walls and reach equilibrium (a state of no change). I guarantee you, your furnace will be glowing a good yellow inside. Maybe even yellow-white if you covered your lid (more on this below). You'd be amazed how much temperature you can get with just a touch of gas and a good insulating refractory (this stuff is rated for 3 BTU.in/hr.ft2.°F = 0.5 W/m.°C).
But that's not the end-all. If you toss "dry" pottery into the fire, it WILL explode. Even though all the "wet" water evaporated, there's still water (and other gasses) chemically bound in the pottery that has to come out. Especially if you used pure clay, especially a fine one such as ball clay. I once rolled a 1/2" thick slab of KT#1-4 for busting up into grog, I was drying it by slowly and intermittently playing the torch (my medium burner) over it. Well despite my best efforts to go ever so slowly, it EXPLODED. Right in my face. POP! Fortunately it's just soft, unfired clay, and mostly exploded away from me as well. To prevent this, you can predry your pieces in the oven, or just bring up your furnace slowly (at the time, I was alternating the burner between the exploded piece and some similar pieces that were in the furnace, none of which ultimately experienced any damage).
There are other points in the 400-500 and 800-1200°F ranges (among others I'm sure) where other decompositions take place, but primary water is your biggest concern. After that, the usual heating rate is necessary because the particles melt and sinter together, reducing volume, shrinking all of the object's dimensions. (10% is typical for straight clay, while shrinkage under 1% is attainable with fillers (grog, silica, etc.); some things such as kyanite actually expand on heating, causing an overall expansion - these mixes can even be used to patch cracks!) Now, heating rate comes in here: only the surface of the object is being heated. For the center to heat up and likewise shrink, it has to be heated by the outer surface layer. This means it will always be cooler than the surface, and that means the surface has shrunk more. If it shrinks too much, it seperates from itself and you get what is known as a crack. The goal is to prevent this, and there are only two ways - reduce thickness, or increase heating time.
Now that we've got the ground rules down, I'll return to the point I left hanging above, and throw in these pictures that are kicking around waiting for a home. ;)
Here's my crucible furnace, plus a few key additions. I've added firebricks everywhere to seal in the heat. It is simply amazing how much heat is lost through that three inch hole in the top. For the lid, since I don't feel like making a new solid one, I've just slapped an insulating firebrick (made of spare LWI-26, of course) over the hole. It works reasonably well, but a plug that slides down into the hole would be better. To allow airflow, well at low pressures, you can leave it like this - there's enough gap around the lid and brick to let out the air. At hotter burns, there's too much backpressure and I have to add a pebble of refractory material under the lid to prop it up. Since the tuyere is much larger than the burner (my medium burner of course), I also added some chinking around it. Some loose brick fragments work fine.
I already mentioned this must be brought up to temperature very slowly, this simply can not be overstated. As such, I use my most stable burner, which is solid down to probably half a PSI when very rich (choked off). Run it down, way down, until it looks like a candle. Indeed, candles have been used to start small kilns, and of course the verb is "candling". Once you are sure it won't burn out or do other bad things, go watch TV for half an hour, come back and give the regulator an ever-so-slight nudge. Increase it like this over the next hour or two. At the two hour mark, it should be no more than red hot. Yes, two hours for only 1000°F - insanely fast for pottery, yet nail-bitingly slow for anyone who wants to see fire. The biggest enemy here is the temptation to sit by and watch it. I guarantee you'll putz with it and end up at red heat in 45 minutes, destroying all your work.
Letting out a lot of heat for a picture, after 3 hours or so I think. You can't really see anything inside, that's the fault of the camera but you shouldn't be able to see anything anyway because it's all the same temperature; black-body radiation doesn't know the color of its skin. (Well it does, but not much for matte pottery at 2300°F.)
Letting out an atrocious amount of heat, but hey, I don't care, this is a cheezy load. I'll show 'ya below.
It's pretty easy to see here what I've got going on. Since the tuyere supplies the heat to the whole furnace, it also gets the hottest. If I were to put a pot in the crossfire from the burner, it would split very nicely because it is 400°F hotter on one side. That's not a whole lot, but it's the difference between orange and yellow heat, and remember that "bright yellow heat" at best spans cones 04 to 2 (circa 1900-2100°F), a massive difference in ceramic terms. (See how precise this stuff is?) Well, my solution was (and is) to place a firebrick on edge, facing the tuyere. This shadows the high heat at the tuyere (and the facing wall) from the stuff on the bottom. I actually placed it so the burner hits the very corner of the brick, allowing some heat past to the left side, countering the cold spot always present only 90° away from the tuyere. In addition, I added a block on the floor, to elevate the ceramic above the heat-sinking furnace floor. (You can see the very corner is actually dark, whereas the tuyere area is yellow - avoid placing pottery in these areas if at all possible!) To complete the load, I also placed another vertical brick, hidden on the left, and a flat one laying on top of this and the bag wall (the term for a wall blocking the intense heat from the burners) to place some other items I wanted to fire. These are a test scrap of clay and a clay/vermiculite mix firebrick.
Counter-clockwise from left:
As near as I can tell, this ended up a bisque fire, maybe cone 06-04, meaning the clay formed ceramic-like sintered bonds, but it's all very loose and porous. It sounds clinkey like ceramic, meaning stiffness has gone up, as well as strength as I found testing the broken scraps.
I have since done more intense firings, faster, slower and to different bodies. One small stoneware bowl I made was supported unevenly and, combined with a rapid firing of only three hours to cone 6, not only cracked but warped as well! Other ware from that firing also came out warped, so you can see the value in firing slowly, allowing the glass melt to form mullite crystals which hold the structure together. Other firings include the bars-of-hell slabs of raw clay which I fired to a good toasty cone 10 (this is yellowish white hot, so bright you cannot make out any shapes with the unaided eye) and later quenched and ground into grog, some mullite compositions, which so far are looking promising, other firebrick formulations and yes, even local dolomite (which, having been calcined, grind up for use as lime).