Kelly in AK

I agree with @Bill Kielb, glaze fit is notoriously hard at low fire temps. It’s easy to imagine quick cooling is the culprit, but it’s unlikely that’s the cause. Industrial producers have astonishingly fast firing cycles, literally three or four hours from green to glaze fired, no crazing. It’s not deep magic, it’s because the glaze fits (they do have some tricks, but they’re mostly centered around avoiding other defects). I’m happy to confirm that most of the things you’ve tried will have no bearing on your problem, none. The one thing you mentioned that might is trying a variety of clays. The glaze is shrinking a lot more than the clay. The other avenue to explore is temperature. In the range you’re firing at glaze fit can change radically within a cone, even half a cone. I say that from experience because I fire my work to cone 03, low fire. The clay is unique, and the glaze is formulated to fit, but a cone lower and it’s going to craze, a cone too high and the pots will dunt. This narrow range is true for many red earthenwares and clays designed to vitrify at low fire temperatures. Also, I should note that one of my kilns is fiber board and cools very quickly. It’s recently retired, but I fired dozens of glaze loads in it. Shut off at 10:00 pm and cool by morning. No crazing. That’s another reason I’m confident quick cooling is not the root of your problem. I suggest you approach the two variables of clay body and glaze firing temperature (Some people fire their bisque quite high and do a lower glaze firing, I can’t speak to that with experience so won’t suggest it. It may well work.). Your cone 06 glaze might do fine at cone 04 and actually fit. I regularly fire commercial cone 06 glazes to 04 and 03 without problems. There is the avenue of glaze formulation, but that’s an entirely different can of worms.

Not long after I got really into clay I realized I needed my own kiln. It’s the one thing that you can’t just whip up on the fly. If I have fingers and clay I can make pots, all the other tools and gadgets come along easy enough. No kiln, no pottery, though. A big hot fire works, but it’s not long before you see that putting a ring of bricks around the fire works better. Then making inlets for air, then a top, then a chimney… Next thing you know (many years later and many kilns later) the fire pit is a 15 cu. foot downdraft and instead of burnished terra cotta I’m doing cone 6 soda. Yes, I have my own kiln. I’ve made several from fiber and a couple from fiber board. I’ve gotten away from that miracle material though, I have enough risks in my life without worrying about respirable silica. My kiln is fairly low tech and firing requires that I babysit most of the time. Along this evolution was a cone 6 wood/soda kiln, which was a blast. The occasional columns of thick black smoke and endless need to replenish the woodpile nudged me to gas though. Every firing produced anxiety the fire department was going to show up. An electric kiln is on my wishlist, I have access to a few and that’s how my bisque is usually done. I can bisque fire in the gas kilns, it’s not so bad, just adds a lot of time to an already long process. Pushing a few buttons and going to bed is hard to beat.

My main concerns are about the foot bearing some relation to the form. It’s the conclusion of a line that starts at the rim and ends at the point it touches the table. The feet on my pots vary. Some don’t have “feet,” proper. Others are distinctly footed. I just want them to look right, it’s not always successful. I usually have an unglazed exterior, soda firing takes care of the outside, or terra sigillata on the local clay, so I’m seldom fighting glaze drips. Nirvana is when the foot is shaped right, I’ve preserved the wire marks from cutting the pot off the wheel, and I’ve managed to carve a lovely deep logarithmic spiral from the center to the inside of the foot. All while removing the right amount of excess clay to make the pot balanced. I aim for it every time, happens about once every never. Two things I directly avoid are the suction cup effect @Callie Beller Diesel refers to and sharp/crisp angles (“That’ll chip in the sink.”).

Very cool! I find using locally available clay both deeply rewarding and challenging. Since I don’t have issues with lime or calcium (That I’m aware of, haha!) I can’t speak to that. My clay is probably very different from yours. I have some thoughts that may be relevant though. The first is, I wonder what you’re expecting of the clay in terms of “settling out properly.” I too use a wet or slip process to make what I collect useable. When I get the water/clay ratio how I want, it mixes in the bucket as a whole circulating vortex. It’s fluid. Not necessarily watery, but it flows enough that I’m not working any harder than I have to with the mixer. I may have understood this bit wrong, but there’s no way I could fit 60 pounds of clay and all the water in a five gallon bucket. After I get it mixed it has to go through a sieve, it’s got to be fluid. I think 20 pounds of clay is about all I could fit in a five gallon bucket full of water. Maybe. Long way to say, perhaps you’re not using enough water for it to settle out, no matter how long you wait. The next thought goes back to flocculation. For a while I flocculated my clay with epsom salts to improve plasticity. It went from pourable to pudding state instantly. So I can’t say about vinegar specifically, but about flocculation generally, I wouldn’t expect flocculated clay to settle. It’s the very reason I flocculate my glazes, to keep them from settling. If I wanted clay to settle out that would be the last thing I did. (It was, literally, the last thing I did before clay went into drying trays, after letting it settle a couple weeks then siphoning off the water). I could ramble on… I’ll just finish by saying don’t underestimate the potential information you can find through observation, record keeping, and testing. You will discover new things as you run the clay through the kiln, there’s a trove of data there. Many sound conclusions about the chemical composition of your clay can be made through testing you can do personally. Good luck, and hoping to hear more about how things unfold.

I have a small gas kiln (8 cu. ft) made from fiber board. Two layers of 1” board. Has no problem getting to cone 6. It was cheap and easy to build but I won’t build another. I agree with the points @neilestrick made and have a couple more. Fiber board has its strengths, being rigid and super insulating, but it has some drawbacks. One is that it will shrink. Once you fire your kiln (unless you pre-fire the boards) you’ll have to re-fit it because big gaps will form. The other is that it’s really friable. As bad as fiber blanket, in my opinion. Even though it’s rigid it’s not strong. Every time you lift the lid or open the door, you’ll be abrading it. The wear and tear is one thing, all the respirable silica it releases is another.

Large straight vertical surfaces encourage the glaze to run. I agree with @Pres‘s suggestion to consider design along with finishing methods. The texture of the clay is a feature, quite beautiful, what portion of the bottom can be left unglazed? Can the continuous curve that now goes from rim to foot be disrupted in a way to elevate the pot above the drips, or catch runs in pools? I love the scallop the wire leaves, I’m not encouraging trimming that away. My tool of choice for smoothing out bits that will cut someone is a Dremel with a diamond bit. I dip the part I’m grinding on in water, keeping it wet and rinsed as I go. It works fast. I aim to remove as little material as possible, after realizing how quickly my “touch up” can turn into a scar. The running glaze is one problem, the gritty feel another. Sanding at bisque, perhaps. If you address this at leather hard it may save you time. Use a hard tool, like a wooden rib to press the grog back in. Slide a hard tool across areas fingers and lips touch. Wiping, even with thick slip and a smooth chamois, only pulls away the fines and pronounces the grog more. Pressing in is what you need. It’s like what you’re already doing with your finger on the bottoms. Post glaze fire, say on the rims and handles, a very fine grit wet sandpaper may take the edge off enough without marring the glaze too much. I’m thinking anywhere from 600-1000 grit. You won’t be removing much material, just polishing the edges of the sharpest bits. Finally, to a point, there is a place for roughness. Being able to feel the grit is not all bad. Just when it hurts.

All the stroke and coat glazes my students have used are opaque. I wouldn’t expect underglaze to show through them.

One more tidbit, I’ve had glazes crawl that otherwise don’t when I stick them in the kiln right after glazing and fire it up. These would be glazes that are bought premixed dry, so I haven’t a clue to the formulas. Applied too thick (usually) and/or fired too soon after application, they’re candidates for crawling. Student work, the defect is fixed by counseling on glaze application and time management.

Good to know any old off the shelf latex might not work. I tend to think all room temperature treatments will totally volatilize in the firing, obviously this is not so! I also recall a potter showing me where she had marked a piece of bisque with a regular pencil that somehow (faintly) persisted through cone 10.

I’m glad you took the time to share this and the resources you’ve found. China painting has become a rare art form. The more that gets written, the less people wanting to try it will have to reinvent the wheel. About ten years ago I was gifted several dozen slip cast glazed mugs from a paint-your-pot shop going out of business. I figured the best way to use them was have students china paint them. I was sure there’d be a learning curve, but china painting wasn’t rocket science. As I looked into it I found the wall that this thread expresses. China painting is not nearly as accessible as it once was. I settled on some bake-on paint pens that made through the dishwasher, but it felt like a sad substitute. Better than decals though.

What a treat to see and hear how this unfolded. Thank you for sharing it. It takes time to do things right, but always less than doing them over. Beautiful kiln!

I have agree with @neilestrick here, and would amend my earlier post to say "It's gotta stay out of the weather. Period." Upon reflection, and a good night's rest, I'm going to backpedal on casters a bit too. It could be managed, they'd have to lock, but still seems a bit dicey considering you're connected to electricity. My main reservation was that low ceiling, but again Neil's ventilation suggestion is apt. As is the idea to put cement board on the ceiling, from @Denice. The windows will help a lot.

Pretty clear you’re need an outdoor solution. It’s likely to solve more problems than it creates. An enclosure is doable, not rocket science. You will need good casters and a very smooth surface to roll that kiln. Kiln fires in open air, is retreated to its kennel/shelter when cool. With enough space around it elaborate leaf catching schemes may not be necessary. A hose is necessary. It’s gotta stay out of the weather when not in use.

The soda kiln gets vacuumed every firing. There’s plenty of stuff flying around in there, I don’t need anything extra to ruin my pots (While I’m at it, shelves get scraped and lightly washed every time as well. Silicon carbide shelves.). Electric kiln, very seldom. Once a year. Glazes get sieved when I see chunks while glazing. Some glazes do form agglomerations. When the glaze dries and there’s little bits sticking up, that’s my sign. Those bits don’t always dissolve into the melt.

On another side note, verging into off topic territory, yet not, is the copper red crackle that can happen on re-firing. I didn’t take pictures of my own pots this happened with, but I assure you from experience it happens. Crazed copper red glaze can go red in the craze lines. I believe reduction cooling or “strike firing” is a purposeful way to make it happen. I found a picture from Christie’s auction house that shows it.

Re-firing to bisque temperatures won’t stress your work with cone 5/6 clay. You will be loading and unloading it one more time, which is a slight risk. The only risk I see in glazing over unfired underglaze is screwing up the glazing to a point you have to wipe it off. Underglaze has ingredients (gum) that harden it on drying, it’s pretty robust in terms of applying glaze without smearing or bleeding. Especially if you’re dipping to glaze.

You run a higher risk of having blisters or pinholes unless you re-bisque to the temperature you know works.

Neil, you kind of beat me to this, but since I already wrote it... Minnesota Clay's website says the range of MB6 is cones 6-9. At cone 6 they say it's "vitreous and porcelain like." Also at cone 6 the absorption is listed as 3.7%. Huh? At cone 9 absorption is 2.1%. Their cone 10 version "MB Stoneware," lists absorption of that body at cone 8 as 1.4%. Yeah, the high fire clay is more vitreous at cone 8 than the mid range clay at cone 9. Something funky there. Back on topic: The studio head, if they are mixing glazes, uses a recipe. Ask them for a copy. If they are using premixed dry glaze you should be able to learn the supplier and the supplier will have some data on the glaze, though they don't often share formulas. It's possible with a recipe to adjust expansion of a glaze to fit a clay body (not always, but generally). This clay body though, I'm not so sure about it. Doesn't look "mid range" at all on paper. I do have issues with suppliers who bill clay as mature over a span of four cones, but I'll stop the rant here. Asking this question directly to the folks at Minnesota Clay would probably be the most direct solution. I suggest, however, you get some clay that is actually vitreous at cone 6 if you're making functional pots in that range.

I second the veegum, you have to hydrate it in advance though. Not something you should add dry.

My steps are much the same as others have mentioned, with one exception. After soda firing all the wads have to come off. They mostly fall off easy, sometimes they need a little persuasion. The next step is to scrub any residue of wadding off using a Scotch Brite pad under running water. Only after that do I consider what has to be sanded or ground. That scrubby pad takes off a surprising amount of material and gets into little divots. All feet get at least a once over with wet sandpaper (320 or 400 grit) so they wont scratch your great aunt's grand piano.

I dug up “John’s Noxema,” it’s from John Britt’s book The Complete Guide to Mid Range Glazes. Some similarities, some differences. Lots of feldspar, but soda not potash. No wollastonite or Gerstley borate, rather strontium and 3134. An abundance of cobalt, for sure. Kona F-4 spar 50 Frit 3134 24 EPK 11 Strontium carb 11 Silica 4 Add: Cobalt carb 3 Bentonite 2

I expect people more knowledgeable than me will check in, but for my initial reaction, I suggest you’ll want to look into various boron based frits. They will give you some modicum of certainty, that is at least one of your glaze ingredients can be depended on to be exactly what the analysis says it is. You’ll get the boron content you need to flux things at cone6. I lean heavily on Ferro frits 3124 and 3134, but the are several others you can use to tune your glazes with specificity. As to blue at cone 6 my thought was including an opacifier (zircopax, tin) would help even things out and reduce that “breaking” quality. That’s just a guess. I have used a glaze from John Britt called “John’s Noxema” that fired very evenly. I didn’t like it for exactly the reason you might seek it: it didn’t break enough on edges and texture.

I visited David MacDonald’s studio in Syracuse last summer, he said if there was one thing he could change it would be to get in-floor heat. With forced air it’s a constant battle to keep his giant platters from warping. The drain in my floor is a lifesaver for cleanup.

Outlet/venting for the kiln is tied for first with plumbing a utility sink. Once those two are settled, other things find their place. Clay is close to the door with wedging table nearby. Glaze and all my lovely powders are near the sink. Once it’s exactly how I want it, I’ll tweak it for for a few months and probably rearrange everything. Next I will somehow run out of space. Haha! I’ve learned enough about myself to know that anything that can be put on wheels should be (Locking casters, I’m in an earthquake zone).

I’m sorry to see what you’re going through. It’s one of the worst things, after getting everything right every step of the way, the last move blows the lot. Other than seconding advice already given and offering encouragement, I’ll say what I would be looking at if I were there (note: been at it a while, still learning) The decal firing is just above quartz inversion, a nice clear clue. The clay is not liking it. Perhaps it’s a rushed firing cycle, but there’s clearly some tension in that clay/glaze going on. I wouldn’t hesitate to do a test on a piece that survived: put in ice water, take it out and pour boiling water in. Half full, you want to stress it. It just takes a few minutes and one way or another, this test brings great peace. In addition to the suggestions already mentioned, consider using a different clay for the large platters or maybe glaze firing them slightly lower. I’ve had a few bad dunting days myself. Note in the attached photo most shattered pieces are one glaze (the red one) and also flat, plates and trays.