skuzbucket

Sorry, I unloaded a little before I remembered to snap a photo. the kiln was loaded pretty much as tight as possible given the way the dome curls downward at the arch- which is a slight pain in the butt but not horrible. certainly I was able to sneak pots put tight to the arch even at the top of the front stack... so there were two distinct sections to the stack- the back half like this:

and the front half composed 2/3 of pots on shelves with those two tall pillars off to the side- the front was the portion that got the most hot- was it stacked to a similar tightness as the back?- I pretty much think so, but the nature of the spaces was totally different- lots of choppy ones instead of tall columns as it was in the back... anyway- the front was massively more hot than than the back- cone 12. the back top was at about cone 8, bottom more like six.
this is so you can see what I'm talking about with the flue openings- lots in the floor, and a lot in the wall- so for this first fire, I had all the wall closed off, and most of the floor open. it was closed only on the right side near the firebox, because I anticipated using only one side of the kiln's potential fireboxes. but that's another story.

so it is clear that simply having most of the floor openings open in the back is not enough to move the heat back there-- ok, now we know. you can see what was open in the second photo. and remember that I yanked out the bag wall, which was a triangle-shape, taller in the back of the kiln. this would have been even worse, in hindsight. Stephen Mickmaster came to the unloading and suggested a small bag wall at the opposite of where I had had it even temporarily- a little deflector triangle at the front of the kiln- I may do that yet, but I like the rest of his suggestion better- open up some ports in the wall portion of the flue- and figure out a way to slow the flame- more of a skateboard of a target brick, or a rubble by Richard's idea, and of course more focus on air to the back in terms of flue openings... What I'll probably do is load a super-dense load of pots at that place where the bag-wall should be-

But here's the current bane of my existance: SCUM!! scum started when I asked my clay company to ad 5% redart to a "takamori" blend of porcelain. I had done this in grad school without problems.. big disclaimer there- so I had no qualms about ordering half a ton of the special blend from the clay company. they mixed up 2800 lbs for me, which I should have noted to them at the time... as it happened, something about the redart is causing a migration to the surface of the soluable calcium- as the pot dries, any oils on my fingers are making - not a resist- the opposite- a magnet for a scum to form. it shows up in the firing as a thin but distinctly gray layer. flowerpots below are from this firing. color is a little wacked but unintentional surface design is evident.

Below is a pot from a previous firing at Mt Hood college- in a salt kiln, and reduce-cooled, as this one was. Neely thought that perhaps the salt was aggravating it, essentially giving a light tacky surface on which the soot would settle and integrate with the surface. (I hope I got that straight- it was over a year ago). but there was no salt in this kiln... may I draw attention to the orange flashing at the center of the frame- I sanded some of these pots before firing them, to see what would happen- that orange is the most clean clay. then there's the scum, and an inbetween zone.

one photo shows the marks from my rubber rib- there at the top of the left-hand pot is where I pulled the rib upward. lower on that cylinder is a big gray band- why there and not elsewhere? anybody??


so here's the left hook: while I was at Penland, a friend used my studio, making pots with her own porcelain. here's one of hers:


there's no glaze on that pot. there is pretty much no chance for soluable soda ash or any of those quirky elements to be responsible for that mark. turn the pot right-side up in your head- looks to me like water dripping from the lip, absorbing into the clay, like she had washed the rim (with water from my tap...!), and then trimmed it- trimming off the bottom half of the "active ingredient"-- anybody? does this make sense? clearly, the marks on her pot are not scumming but flashing. but is is possible that the minerals in my water are reacting to the redart in this batch of clay to create an unusually subtle problem?? comments, please?!
I'm going to throw with distilled water for a while and see what happens-
next firing scheduled for the end of september.

Ist firing- Aug 2, 2008

Damage assessment reveals that a mere 20 gallons of honey-gold veggie oil, not more than 1/2 cord of wood, and 33 beers were consumed during the first firing. Given how little of the first two were used, I am forced to assume that the primary fuel required for this firing was alcohol. This is not a boast. The first firing tested all manner of variables. One of them was how I lead. I was sober, of course, as was the person "on shift". beyond that, it gradually turned into a fuzzy line- there were hangers-on who made themselves useful in a time of need and we could not have done it without them... I prepared as best I could given the wide array of unknowns, but we suddenly needed a pile of side-stoke wood, and lo! it was created, without loss of finger. Later in the firing, we needed to carry on side- stoking wood, and helping hands were still there. My gratitude to all who helped in many ways... Richard, Jack, Jamie, Oliver, Tim and Travis. hats off, my dears-

The first firing was excellent. Not at all how I expected it to be, excellent in its own way. It helped enormously to spend a few hours warming up the chimney before beginning the candle. most pots were green, large, and porcelain. six hour candle, slow rise to 900*, and about 100*/hr after that. this made for about 24 hours up to 900, 12 hours to cone 04, six more to cone 12, and four in reduction cooling to 1850. There was a certain point in the evening when Timmy and I were puzzling over the correct path and Richard confirmed what was becoming a growing realization: this kiln really loves the woodfire. But he loves the oil fuel too. we found ourselves side-stoking, directly into the firebox, through a port I had imagined would be used mostly for soda, or reduce-cooling with wood. it was remarkably painless, for woodfiring.

The whole story: Here's a photo of he floor of the kiln, from the front.there is a firebox channel 9x9 in cross-section that runs continuously from the front of the kiln to the back. at one opening, we stoke wood. at the other is an oil burner. the kiln is symmetrical- there are two such channels at each side. there are vertical dampers that control the opening to each fuel source, and a space by which the channel opens into the chamber. In the center are also two parallel 9x9 channels of floor flue, making it a downdraft kiln. those channels are spanned by these huge bricks with holes in them. there is also a large opening in the wall of the chamber so that the entire floor flue area can be closed off, including the place where it enters the chimney, and the whole kiln can be fired as if a crossdraft (except it isn't really a crossdraft since the fuel sources are more like a downdraft configuration- from the back sides and exiting the back, but it might give a more crossdrafty look than not)

I think that bit about the wicking action of the bed of coals was accurate. in the pre-firing, I had managed to turn on the oil when the chamber air temp was about 600*, meaning much much lower than that at the point where the oil burner was located. of course I was hugely excited and watched the temperature soar as I turned on the air and oil together. I managed to find a setting that was about 100* an hour, a safe heat gain for the hypothetical pots. I watched it as such for about an hour before heading to bed, thinking an hour was sufficient proof that the burner worked at that temperature because of its efficiency among other things... all well and good.

BUT the coals had not fully burned off in that baby firing. I think now that if I had observed the oil burner for two hours instead, I would have seen what we saw during the real firing: as the coals burned off, the wick burned away. the sudden temperature soar was the sudden rush of air from the burner over the coals. the slow gain was the adjusted air flow but basically still the same action. without the wick of coals, the oil could not burn properly at lower temps?? something- it needs a porous craggy surface from which to burn. maybe a rubble of soft brick would work as well. but i think it's not just a wick but also a radiant heat action taking place-- what we observed, to our dismay, is that at 600*, at 1100*, at 1600* on the pyrometer ( located again, far away from the burner port,) at every point, we would switch on the oil, observe a sudden spike in temperature, a gradual tapering-off, and then a slow but inarguable decline --as the coals burned off---. at 1700* we tried yet again but by this time, night was falling, we would have kept the ladies of the house awake with any more wood splitting- (oh- yes, so up until 1700* we basically gained temperature by side-stoking wood.)--- so we stood around staring at the pyrometer like some folks stare at a television set hoping and praying that the oil would catch and stay caught. But before we dared find out that it wouldn't, we just kept on with light side-stoking while keeping the oil on.

so, to be clear, before about 1700*, we gained temp with wood, occasionally trying oil, finding it ineffective, and shutting it off, resuming wood. by 1700*, we'd pretty much realized what was going on and decided to keep up a light side-stoke for the primary purpose of maintaining a proper bed of coals into which the oil would spray. It was as if the oil was super-charging the coals, and the amount of wood required to gain temperature was minimal- 3 or 5 short sticks every few minutes for a cyclic gain in temp as woodstoking tends to show.. Oil flow to each burner was at full throttle. air passing through the blower was at about 2/3 open. we figured out a pattern that seemed effective, averaged out the heat gain at about 80* an hour including all the noodling around and failures, and calculated that we were only three hours behind an optimal schedule without mishap. at 12 am, I took a cone reading in which 1s were down, and passed out for three hours, anticipating a long and intense night. Richard closed off the last 15 square inches of passive damper and wakes me up a bit later- "um, we have good news for you- cone 12 is down almost everywhere..." -- the consistently cool spots at about 9. so Jamie and Oliver took off (thank you two ever so much!) as Richard and I sealed up the kiln and began reduce-cooling. he slept for a few hours as I continued, and the firing was complete at about 7 am monday morning.

other tidbits: 1: at a certain point mid-afternoon, we had tried the oil for the second time (1100*), and found it was smoking badly, acrid smell, as if a heavy reduction. kiln was behaving as if he were choking. I leaned a ladder against the chimney and removed the piece of expanded steel that I had installed on the suggestion of a fire marshall. it was about 3/4 blocked off with soot. I added a foot onto the chimney. I will add another foot, for a total of 14 from floor level. I've decided to do this because through out much of the firing, we pretty much had the active dampers wide open and the passives all in. I would like to have a little more elbow room than that.

2: for this firing, the "cross-draft" flue opening in the kiln was completely blocked off, and 10 of the 12 "down-draft" holes in the floor were open, primarily on the left side of the kiln, and towards the rear. this would make for 70 square inches of flue opening in a 65 cubic foot kiln, about twice what Nils Lou suggests in his book. I opted to double it for two reasons- mostly being chicken, and wanting the control of quantity of air flow to come from dampers which could be changed during the firing. but those holes in the floor intrigue me- how to move the heat more forward or back by which ones are open-- as a side-note- no bag walls in this firing. I loaded about 40 cubic feet of pots into the space, densely packed.

thoughts for the future: build an after-market bourry- box on each side of the kiln for more effective and physically comfortable wood-stoking in the beginning of the firing. a fine idea from richard. the only catch is that we need to be able to push the coals into the channel near the oil burner so that we can use them as the wick. I'm sure I can figure that out... one big question is this- what if we push coals forward at, say, 1000*, and continue to "side-stoke" above them lightly as the oil droplets soak into them. will it work? or will we need to continue with wood somehow. next time, we will find out! same bat-place, same bat-channel......