Kelly in AK

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.

I use moppy glaze brushes (labeled as glaze brushes) for straight up application, they’re also my favorite for applying terra sig. They load up nice, lots of bristle holds lots of glaze. For applying slip and kiln wash I do like the cheap “chip brushes,” white bristle brushes you get at the hardware store. They often leave drag marks I find interesting. For decorating with slip, oxides, or underglaze I like the asian style calligraphy brushes. I can vary the thickness of lines, from detailed to bold, they hold a lot of material, and they’ve got just the right spring. If I’m after a wide, even coating of slip or glaze I’ll choose the wide flat Japanese ones @Mark C. mentioned. Oh, one more, if I want to stir something up, keep it in suspension, or mix it after it’s settled, I’ll use a stiff bristled brush like oil painters use. That’s for mixing small quantities, like a stain or oxide, not application. I love my brushes, keep them clean, and rarely have to replace any even after many years of use. Important tools. Now, always trying to learn, I’ve been using a bit of wax lately after years of eschewing it and haven’t found a favorite brush yet for that.

A thought I’ll share: As soon as I get the powder into the water bucket I put the lid on and go do something else for a while. It lets things get hydrated before mixing, but mostly I do it to reduce dust in the air.

A credit card, pinking shears, and some nice gloopy slip did this.

It’s a regular occurrence, as @Min said. I have done it many times. I have two experiences worth sharing here though. One is that a person who does workshops where I live asked that no porcelain be glazed and fired the same day because she had something blow up in a workshop. The second is in my wood kiln I had a freshly glazed piece blow up, it was near the bag wall and made of fine grained clay, porcelain like. If it wasn’t my piece and I hadn’t glazed myself I wouldn’t have believed it. I would have chalked it up to someone glazing greenware. I’d never seen a piece of bisque ware explode and haven’t since.

I’m happy to say that after a couple of decades in clay, having built several kilns and operated many more, I finally graduated into the class of “Has Successfully Replaced Elements” a couple weeks ago (I completed the “empty kiln to 04/oxidize the elements” firing, but still have to complete the final, “cone 6 glaze fire.” So, I haven’t actually gotten my diploma yet, but am confident.). I have earned many other (self-awarded) certificates over the years. “Scraping Shelves,” “Proper Application of Kiln Wash,” and “The Economy of Angle Grinders,” were earned concurrently over time. “Low Fire Clay in a High Firing” was a crash course. There’s a lot more fuel kiln stuff rattling around in my head than electric. I feel like most of what I’ve learned about kiln maintenance, operation, and repair has been so incremental it’s hard to place myself on a scale. The thermocouple goes out and you learn how to replace it, you don’t have to learn that now. Multiply that by kiln-life experiences and you wind up knowing a lot that most people don’t. We end up being experts by just making things work day after day, year after year.

Wedge a pound of each together. Make some pinch pots. Glaze to check for fit. See what happens.

I can’t say with certainty about a pre-firing, but I have had many students over the years who accidentally glazed greenware with cone 6 glazes (Naughty! Weren’t paying attention!). I always run those through the regular 04 bisque first and haven’t had a problem. The bisqued glaze can vary from dry to satin, but it’s never soft and comes out fine in the final glaze fire. I’m not sure I would go through the trouble, especially if they’re brushing glazes. As @neilestrick said, those have gum that hardens the glaze on drying. On the other hand, I have a couple glazes that just getting into the kiln require great care, I can’t imagine transporting them. Carefully, gently, wrap in bubble wrap. More is better, the pots should be well cushioned and immobile. Bring some glaze and favorite brushes with you.

A liner is a highly reliable unquestionably safe glaze that goes on the interior of pottery, allowing artists to use more outlandish combinations on the exterior. I must confess, I habitually overthink in my imagination of what clay and glazes do in firing. I’m strongly leaning towards what @PeterH, @Hulk, and @neilestricksaid about application and crawling bearing significance to your problem.

Zinc margaritas. I love this place.

Do some test tiles and put them on a cookie. A tile (or sacrificial piece) with a large vertical surface will help you interpret how the glaze runs. The only problems I foresee are a runny glaze that goes all over the shelves and an uninteresting result. A lot of raku glazes are specifically designed to become interesting on being thrown into a trash can full of straw, red hot. “Around cone 04” sounds a little vague. I would trust what the label says, no more, no less. Mayco is not exactly a fly by night operation. It does seem like they say cone 06 is the target for these glazes. Prepare for runs at 04.

We have an old crappy cupboard, edges sealed with vinyl tape, and doors sealed with a rubbery gasket. It maintains humidity well. A pan full of water in the bottom clinches the deal. Saves some time fussing with plastic. When that’s full, its plastic drapery and inverted storage containers…everywhere.

I’m late to the party, but glad you’re doing a brick chimney. It’s not that the metal will degrade fast, it’s that the metal won’t keep the column of air hot enough to provide draft. There’s a point where it’s throwing off heat fast enough to stall the kiln. Double brick the chimney at least to the height of the kiln, higher is better still.

Random armchair speculation, two cents, worth every penny. The ash glaze is bubbling. The defect is happening in the cups because, even in a vented kiln, the atmosphere inside a more closed form is different than everywhere else. The iron in your “red slip” is decomposing at the same time your Leach glaze is melting, and it’s doing it more vigorously inside the cups and mugs because there’s a slightly more reduced atmosphere there. On a different note, I have to agree with @Min about the Leach glaze. It’s melted and gorgeous, but some people may take issue with its durability based on the recipe.