The funny business started when I went to set the lid back down on the kiln: it did something that I would, in computing terms, describe as deprecated:
The joints between the firebricks had aged out, and the bricks themselves (modern IFB [“Insulating Fire Brick”], which is soft and porous) had cracked. In addition, the steel band that the handles are attached to (it also helped to hold the lid together) had rusted out, and it parted. I couldn’t say what went first, and it doesn’t matter: the lid broke. When I went to remove the pieces they came apart even more, and many of them fell into the kiln, where they knocked everything about, doing a bit of damage to the glaze coats on some of the test tiles in the process. Argh.
[This kiln is quite old, and has seen very heavy use, so the loss of the lid was not entirely unexpectable; I just didn’t expect it to happen today. This initial mess, however, was not my doing: I disclaim all responsibility except for having fired the kiln lots of times.]
I picked the pieces of firebrick out of the kiln, put the cones and test tiles back in place, and thought about what I could do. As I write this I have a pretty good idea of what I should have done, but it didn’t occur to me at the time. First mistake; if I hadn’t been all stressed out and in a hurry I probably would have done The Right Thing, and about now I would be firing the next set of test tiles.
I initially tried building a makeshift lid from some pieces of high-temp fiberboard that are heaped up behind the kiln. They mostly aren’t very square, and that lid would have leaked like a sieve, but it gave me an idea. I went inside and looked for some intact boards. There were four, just as I wanted, sitting on two cartons of type “M” high-temperature boards, on top of a cabinet. I went and got a stepstool and took them down. (“M” board is only rated for 2300° F / 1260° C, and I was planning on taking the kiln somewhat higher; but a single brief overtemp is not necessarily a big deal.) I should have dragged down one of the boxes of high-temp boards and made a comparison, to be sure that was what I had, but I didn’t. Bad move.
[[Cue the sinister organ music so you’ll know that the horrible kiln-eating monster is hiding behind the door. Don’t let those poor innocent test tiles go in there! Nooooo! Don’t ...aaagghhhh.]]
I am not ordinarily quite so stupid, and I even had a vague feeling that I was looking for trouble; but I was stressed and rushed and a little bit out of sorts. (I am having a mild reaction to something, perhaps the ’flu shot I got yesterday or maybe something I ate.) I took the boards outside and built a temporary lid for the kiln from them. These boards are only a foot wide, and the kiln is about 17" across, so I used a pair, and put the other pair crosswise on top of them. Because there were seams between them, for a bit more stability, and also to provide more insulation, I put some of the pieces of high-temperature board on top of this assembly. So far, so good, ...or so I thought. In consideration of the lowish rating that I thought I was dealing with I decided not to push things too hard I figured I would go for cone 10 instead of cone 11. (I didn’t want to go any lower than 10 because the glazes were designed for the 10-11 range.)
After one false start caused by something being out of position, the firing began quite well. As usual, when the temperature reached about 750° C I put the kiln into reduction. (Long explanation omitted here. If you really wanna know, please either comment or send email.) The new boards had not previously been used, and as they got hot the binder in them burned off, releasing plumes of delicately colored smoke that would not have been out of place in a Jack Vance story. [Phandaar slowly turned to face the intruder. His carefully manicured hands were clasped together, Mazriel’s Tinted Effluvium wafting out from between the fingers. Clune, instantly convulsed in agony, fought to reach the door.] Needless to say, the only photo I got that shows the colors reasonably well is totally out of focus; it was that sort of day. Here’s one that’s in focus, and shows at least a wee bit of color:
As the kiln started to get fairly hot I began to see orange glow between some of the extra pieces of board, so I took it out of reduction at about 1216° C instead of waiting for it to reach 1220. It usually takes off quite nicely when I do this, but today it responded sluggishly, and it never got any hotter than 1226°. When it got back down to 1214 I declared the firing to be a failure, and turned off the gas. (Less than an hour later it was already below 750°, which should probably have suggested that things had gone rather far astray.)
After I gave it more time to cool I went to take the test tiles out of the kiln so I could reserve them to be refired elsewhere it is likely that the largest difference would be a much slower cooldown than I get in my little kiln, and that would provide a good comparison. When I removed the extra pieces of high-temp board from the new boards, however, this is what I found:
There is no way that could be “M” board. It is, rather, almost assuredly a type of “backup board” that is rated for use only up to 1900° F, which is less than 1040° C, ...and I had exposed it to more than 1220° C, whereupon it did about what you’d expect. Sigh. Not exactly what I wanted, and it will be a real bear to clean up.
I managed to remove most of the test tiles from the kiln. Here are some of them:
The odd thing is that I got a little bit of usable information out of a couple of them, but I will have to redo the firing to be sure I can trust it. Before I can do that, I get to remove everything from inside the kiln and replace it, which is annoying and time-consuming. This kiln is on its last legs in any case, and I’m not sure how many more firings I will be able to get from it, so I am not sure whether it’s really worth the effort. OTOH, I desperately need to fire more things in order to get ready for SFContario, so I may not have the choice.
I also have to borrow a lid from one of the other kilns here if I can find one that isn’t too large and heavy, which is what I should probably have done in the first place. (I think I have one that will do. If not, I'll have to find or buy a bunch of IFB and make one, which will delay me even more. Sigh.)
I really hate being a moron, and in case it isn’t entirely obvious I also hate admitting when I've been a moron; but if I can provide a cautionary tale that helps prevent someone else from committing a similar piece of idiocy, maybe it’s worth the pain. Maybe. Still hurts, though.
(15 October, 2012, evening)
I removed the hearthplate, the shields that stand in front of the outlet port to control the flow of the flame, and the stand for the cones ...all of which have become a single unified object:
Then I removed nearly all of the high-temp fiber blanket lining that I had put in place when I first converted the kiln from electric to gas. (The kiln was rated only for cone 8, and I knew I’d be getting it hotter, so I was prudent. This now turns out to have been The Right Stuff, as the blanket caught quite a bit of the melted board and prevented it from getting on the walls.)
There was still an unhappy amount of glop on the floor, some of which had actually melted into the blanket badly enough that I had to chip a bunch of it out (doing some damage to the bricks in the process), but I did eventually get just about all of it out. Mirabile dictu, there was very little actually stuck to the walls, and almost none on top of the wall where the lid sits. The one bad issue is that there’s a nasty drip stuck to the ceramic tube that holds and protects the thermocouple. With some luck I should be able to grind it off, so I can continue to use that tube. If not, I will have to get a new tube and cut it down to the correct length, which will take a while. Fingers crossed.
I vacuumed out the remaining debris, and put some ITC-200 patch material into the larger divots and also into the inlet port where it has taken damage over the years...
(16 October, 2012)
I applied more of the patch material today, and I removed the thermocouple with its protection tube. I attempted to grind the melted glop off the tube, but the vibration was more than it could handle, and it broke. I will have to acquire a new one, which will take a few days. In the meanwhile I can probably borrow the Type K tc that is normally on the big electric kiln, though it is going to be squirrely: Type K is known to be nonlinear above about 1150° C, and I have been firing this kiln to about 1305 C lately. This is yet another reason to be assiduous about using cones. (The main reason is that they provide information about the actual amount of heatwork that has been done on the pieces in the kiln, not just the temperature.)
I also found enough IFB to make a new lid. Haven’t put them together yet, but it isn’t particularly difficult.
I still need to make a new fiber blanket liner and install a new hearthplate (I believe I have a spare), and we’ll see whether I can get some more mileage out of this kiln before it finally gives up the ghost. [I will probably add more photos as things begin to take shape.]