Kelly in AK

I refire things, usually for very similar reasons described. Rarely do I add more glaze. Either they work or they don’t, but they do often enough I keep doing it. If there’s a piece I’m on the fence about I will hit it with 1000-1500 grit wet sandpaper first. It’s abrasive, but more like a polish, knocks down the grit and helps me know if something is not up to par or if I’m just being finicky. When I find myself sanding for more than one minute then I stop and reflect on my choices. Seriously, use a stopwatch. As to re-firing used dinnerware, I’ve done that too (with 20 year old plates no less!). There wasn’t much to lose, those plates were heading to the landfill. It was a sentimental move. Very happy, it was like getting new dishes . No additional glaze, just a refire. I agree with being cautious, slow enough to be positive all water is gone. I’m not advocating it, I just want to be a voice that says “I did that once and it worked well.”

Shivering is generally considered a clay body problem, the exception being one glaze out of many that shivers while the rest fit. With slip (the added kaolin is not helping), underglaze, and glaze, you’re working on making four things play nice. If you’ve dropped the slip, you've narrowed it down. The application and gum are unlikely to be the source or solution of your problem. Adhesion at room temperature counts for very little in this equation. It’s the clay bodies. Wildly different coefficients of expansion from either the glazes or the underglazes. Continue eliminating variables, you’ll solve it.

Commercial glaze makers have tuned in to the cone 6 market very well in the past twenty years. You can be assured the claims they make hold true most of the time. I’m impressed to see color charts and samples fired to cone 6 of products originally marketed as low fire. The formulas are proprietary so there’s no way to know if or how they’ve changed them to accommodate the higher temperatures. They “just work.” Cone 10 is different. In fact, they don’t make many claims about that, other than “it might work.” This kind of firing usually happens in a gas kiln and a reduction atmosphere. Cone 10 firing in an electric kiln is unusual. The atmosphere makes a difference. Finding a different clear glaze may be a worthwhile chore. Another avenue to pursue is using slip. The easiest course is to use your clay body and add a significant amount of black Mason stain (6600 is my go to). Unfortunately, I can’t suggest a percentage because I haven’t personally fired those test tiles yet (Coincidentally, they’re slated for Saturday, 20% 6600, 80% B-mix5. It’s in a soda firing at cone 6, rather different from what you’re experiencing, but I’m still shooting for clean blackness like you. I’ll let you know), fortunately, you shouldn’t have to worry about fit issues because the slip is mostly the clay body, unfortunately, you’ll have to apply it in a wet or leather hard state to be safe, fortunately, you’re doing sgraffito which suggests you’re already doing that, unfortunately, you’d have to mix up the stuff yourself, fortunately if your current clear glaze works with the clay body it should work with a slip made mostly from the clay body, unfortunately, maybe those bubbles were there already and you didn’t see them until they were over a large black surface and it is just the glaze… Fortunately or unfortunately, this is ceramics. It only gets better.

True artist. Brava!!! Brava!!! It’s a special place we have here. Thank you for the saga. Inspiring.

Build it on a shelf. I think that’s a great idea. Taking rings off the Skutt is a solution to the next problem, you’re thinking it through (You really need a car kiln or front loader for this!). Wheeled carts are brilliant. I struggle with this next bit, because I would rather do things on my own and screw them up (It’s a blessing and a curse) than have either help or an audience. This is a large piece. Having another pair of trusted eyes and hands is indispensable, enlist help for the critical move if you haven’t already considered it.

Not wanting to veer off topic, but I tried, and settled on, kyanite as the groggy/temper portion of my stovetop earthenware clay body (It’s for personal use man, don’t hassle me!). I agree, it’s something special, worth a look. I can’t say anything about the color response, as I was using red clay. I see Tony Hansen has made a low shrinkage bisque patch recipe using just that and sodium silicate: https://digitalfire.com/picture/SpqnhMR7kv

It’s a good question. People end up finding their absorption happy place, some are unsatisfied with anything over 1%, others with 0.5%, I am good if the mugs don’t weep or get wickedly hot in the microwave. I believe that’s below 2%. The ASTM standard for vitrified is 0.5%. My understanding is similar to yours, about firing range. I could be wrong about this, but my general observation is the lower the maturation temperature of the clay, the narrower the firing range. It’s certain the closer you get to vitreous the less wiggle room you have, at any cone. In his book Clay and Glazes for the Potter, Daniel Rhodes says, “Fired stoneware should have an absorption of 3 percent or less.” That’s on page 42 of my edition. I was taught, a few decades ago, that absorption below 2% makes reliable pottery but you should shoot for below 1.5%. Talking to potters over the years gives me the impression that the desired number has crept down since Daniel Rhodes wrote his book. Now, back to low fire, is it suitable for everyday use? Not without some careful consideration. There are a lot of variables, so many it seems like a subjective question. It depends on what you’re willing to put up with, or what you’re asking of people who use your pots. I use a lot of earthenware in my kitchen, I wouldn’t expect someone buying my work to deal with that, so I don’t sell anything unless it’s non-absorbent. The issues are strength, absorption, and glaze fit (which really goes back to the first two). The solutions are finding/making a clay body that works and then glazes that fit it.

I second looking to glazy.org. Another option is to find a pre mixed powder that suits you. That’s a good middle way. If making glaze from scratch, you will have to decide if learning about glaze chemistry is worth more than buying a pint of glaze. Your time and mental real estate are the true costs, raw materials are cheap. And you’ll need a scale. And probably a sieve. A few other things too.

This is it. I’m not sure what you’re worried about. I understand the need for consistency and control, but if the glaze firing is working it seems like you’ve got it. On the other hand, I do appreciate the desire to know what’s going on in there and how what’s on the display relates to reality. A packed kiln will fire slower and take more energy than one sparsely loaded. A bisque load with pots stacked and full top to bottom, for example, is a significant mass to heat. The controller handles it as programmed, using the thermocouple. I’ve found the programmed cone firing schedules more useful, easier, than anything I’ve concocted trying to program schedules myself. Believe me, I’ve tried second guessing cone fire modes. Use physical cones for when you want to know with confidence what’s happening in the kiln. I use a Skutt and a Paragon for bisque. The manuals for both are helpful for understanding how the programming works. If the kiln is jam packed “slow” speed works best. My Paragon’s elements are near the end of their lifespan and if I try to fire a bisque with it fully loaded (think hundreds of tiny pinch pots made by little children, stacked and packed) at a regular speed it’ll error out. To put a finer point on it, if it errors fatally and shuts off it’s done, usually a bit under fired. If it errors and continues to the programmed temperature it will plod along soaking everything for hours near the target cone and things get over fired. Simply choosing “slow” avoids this trouble. Not a second spent thinking of segments, rates of climb, or offsets, just a single button press. As @High Bridge Pottery mentioned, the cone firing modes are dynamic and adjust themselves as the firing progresses. This is a good thing, because the density of your kiln load and the wear of your elements are dynamic too.

Damn @neilestrick!!! Best low fire work I’ve seen in a long time. So good I could eat it. To @Biglou13, bisque anything will tolerate that kind of abuse better than a clay close to maturity. Lots of grog, kyanite, or even spodumene (gotten a bit expensive lately) will help. Unglazed flameware sounds better than any other clay body, but I haven’t tried that. It can be done in the studio, ordinary clay products that handle the extreme temperature swings of cooking, but that’s far from saying it should be. I’ve made several earthenware pots that survive the stovetop, experimenting with clay formulations and bisque temperatures. Though I used them regularly, the maintenance was unforgiving (mold will not be denied if given a habitat to grow), and sooner or later they will fall apart despite all best practices. You didn’t bring this up, but I would never try to sell such a thing. Too complicated. Pottery used for cooking is largely disposable, in places it’s actually used. Manufacturers like Le Creuset and Wiliams-Sonoma create ceramic ware that’s functional in a contemporary kitchen, but they are carefully engineered first world conveniences. If people can afford to use metal they do. I’ll confess, I’ve never had better beans than those I cooked in clay pots. I don’t want to discourage anybody, I just think they should know what they’re in for.

I make terra sig regularly with my local clay and follow something similar to @Magnolia Mud Research’s post. I don’t go to great lengths to extract any remaining “finest particles” from the dregs. I played with that a bit but found it was counterproductive. Letting it slake a few hours then mixing it really well gets the all good stuff in suspension. I’m sure this is derived from Vince Pitelka or maybe Pete Pinnell, but it’s what I go with: 1 quart water to 1 pound of clay. 2.5 grams deflocculant per pound of clay. My deflocculant is half soda ash/half sodium silicate.

The two photos are from Luxurybonechina.com and narumi.co.jp. A link to another photo is here: https://simple.wikipedia.org/wiki/Porcelain#/media/File:Transparent_porcelain.jpg I get the sense a greenish tinge to bone china is not so unusual. The green of the piece @Jarman Porcelain posted is tilted away from a yellowish I would expect from iron, but not so much to make me think iron’s not primarily responsible. The clay body is a hair’s breadth from being a glaze at maturity (the ultimate clay/glaze interface: the pot is the glaze). Grolleg and Standard both have over 0.5% iron in them, perhaps that’s enough to show when it’s all in the melt? NZK has half the iron of those two, so I don’t know. I found this interesting, since I feel like I hardly know what I’m talking about: https://www.jonsinger.org/jossresearch/tjiirrs/017.html I certainly imagine getting some into a reduction firing, being able to compare, would provide useful information.

A quick glance tells me Iron in oxidation sums it up.. See other thread for more details.

Support for the pyroplastic clay seems the name of the game.

Damn! Who needs glaze when your clay does that? It’s already glass. Beautiful.

I’m going to comment here not because I have a great answer, but because I’m keen to know how this unfolds. I have some ideas to share. My initial thoughts are calcined kaolin will counteract shrinkage and be white, so some percentage of it will be helpful. Kaolin or your current clay body is the best way to achieve color match, some percentage of that is necessary for a paste that’s plastic, can be pushed into cracks, and remain cohesive. The body clay is better because it’s going to mature at the right temperature. Strength is the tricky one in my mind. One critical thing is the “keying” of the paste into all the little nooks and crannies of the crack, pushing it in. Filling the crack, not just covering it (I’m preaching to the choir here, I know). Paper, as you noted, complicates that. It has its virtues, in general it can mend things many experienced clay people consider un-mendable. I could go on about it, suffice to say I’m not sure paper is a helpful ingredient in your case. It may be, I’m just not sure. Soda ash and/or sodium silicate I think of as helpful. ***DISCLAIMER: I am no ceramic engineer*** One effect they can produce is to reduce the amount of water needed in the mender to make it fluid, that allows you to better fill the cracks and reduces drying shrinkage. The other effect is as a flux, to fuse the paste together and ideally adhere it to the bisque as well. I have no good idea about the amount to use, other than it’s small. These materials have a melting point, both around ordinary bisque temperatures. Your firing temperature and how they interact with the other materials will affect their usefulness. Finally, considering the clay portion, if you use only kaolin and calcined kaolin then it’s going to be weak. Your body clay is going to produce something stronger, and soda ash/sodium silicate will compensate in fluxing power what you’re losing by adding calcined kaolin. Your work is great! Wishing you luck with testing.

That’s a beautiful piece (kiln) of compact design. I’m delighted you worked through some bugs. The rebar will sag and spall, you’re aware, but the thickness you chose will make it last longer and they’re a consumable. The next hundred firings will really teach you something.. Bravo.

The photographs suggest multiple techniques. Slip casting or a two part sprig mold to capture the finer details seems more appropriate for some parts. Paper clay is a good choice to get something long and skinny to survive the process. I imagine it’s possible to slip cast paper clay Coating with slip progressively diminishes surface detail, and you've got to build up a little just for structural integrity. It’s also going to shrink around the branch. Paper clay is miraculous, not sure it’s this miraculous. Good luck on your challenging project!

A small wide plastic container on the wheel deck for throwing (just need a few ounces of water there), and a five gallon bucket full next to me.

two layers of 1/2” Hardibacker board will give you some peace. No mixing plaster, woohoo! Great to wedge on, dimensionally satble, easy to get. I still use it for wedging and working surfaces, as well as ware boards. Pulls water out aggressively. It will saturate eventually, the same issue happens to terra cotta planter plates which I also use for small batches. Of course it happens with plaster too,

20° C is 68° F. If that number is the temperature and it’s going down when you start the kiln, I agree it seems like the thermocouple is wired backwards. It would naturally error. Perhaps it was replaced and the kiln not used afterwards, so the problem wasn’t apparent until now. Thermocouples are a regularly replaced kiln part. Unplug the kiln, switch the two wires and try it again.

Nah, not necessarily a mixing fault. I can only guess your clay has a lot of iron in it. It’s bloating because it’s becoming vitreous at the same time something is decomposing in it, that’s usually iron around cone 6. But, I’ve had white clay bloat when I fired it too high too. One thing’s for sure, if you get that pug mill in shape there’ll be less wedging, and that’s something to look forward to.

This mallet. I make most of my tools, but this one was a long labor of love. Sculpted to fit my hand from a beetle killed spruce and a bone I found. I use it often, to pound out slabs before I roll them.

That’s a very cool way to make a plate! Are the bottoms glazed? If not, it could be the glaze causing it. Glaze with a tightly compressive fit will bow a flat test tile glazed on one side. It would be magnified on a thin plate.

The gas/soda kiln (finished work) gets fired 4-6 times a year. Sometimes it’s jam packed, others it’s a little more roomy. I had to scroll through photos to double check my guess. That’s two of us making pots in the studio. No big operation, but steady. Of course at school it’s different, the kiln’s going constantly, feels that way anyway. Though it’s not my work or kiln I still squeeze some tests in there and things that work for me I’ll use with my kids. It’s been a great opportunity to learn and be a better ceramics teacher.