Kelly in AK

It is coincidence that @davidh4976 mentioned a clay body so high in iron that a magnet will stick to it and I received in the mail today the strongest magnets I’ve ever seen. So, of course I had to play.. My clay is a local iron rich earthenware that vitrifies at cone 03. Not all the pots do this, apparently only the ones that are reduced heavily and fired to maturity.

Yes, @davidh4976, ASTM C554 is the “prerequisite” test for test C1607. And I agree, putting pots through the worst I can imagine a reasonable person doing in their kitchen gives sufficient information as to how they’ll hold up. I believe it’s important to use your own work, it helps you understand what you’re expecting of people.

Thank you all for the insight. And thank you @Min, I was very curious as to the nature of that test.

Amen to that!

Microwave safe. What does that mean? I’ve been puzzling over this a while, a few years, at least. Lately an event, which I’ll get to, pushed me into posting the question here. My understanding was that vitrified clay wouldn’t get particularly hot in the microwave, but there was some question when it came to high iron clay bodies. Experience in my kitchen over twenty years bears this out, the semi-vitreous pots could be counted on to get hot, the really porous earthenware could get dangerously hot while the food remained cold. High iron clays don’t seem to behave differently, but I remain open to the possibility they can. The other experience I’ve noted is that any dish fresh from the dishwasher, even cooled to room temperature, will get hot in the microwave. My assumption is adsorption (water that intimately sticks to the surface), not just absorption, plays a role. The event that gets me seriously wondering centers on a diner plate. My dad was in the catering business and when he retired I snagged a few restaurant dishes. They’ve been in service for over twenty years in my home, unbelievably perfect ceramic engineering. Plain white, no chips, cutlery marks, or cracks, despite regular use and abuse (They’re diner plates, I don’t treat them like handmade pottery!). They don’t get hot in the microwave. My son burned his finger pulling one out of the microwave the other day. That didn’t make sense. Tonight he tells me he observed the plate closely and saw a craze and discoloration, and that it looked like a glaze crack, not a clay crack. I’m proud as a dad, he nailed it, though I doubted until I saw for myself. That discoloration is at about 11:00 in the photo, poorly visible I’m afraid. There is another blotch around 2:15. So finally, after all these years, one failed. Time for destructive testing. I threw that plate in the microwave, one minute. At 50 seconds, I added another minute. At one minute and fifteen seconds it shattered cleanly into three pieces. What did I learn? These plates, tough as hell, never got hot in the microwave for twenty years, never chipped, are in fact made of porous clay. I put a drop of water on the newly bared ceramic and it soaked right in. This rocks my world a bit. I honestly didn’t imagine it was possible to fit a glaze that well to non vitreous ware or that non vitrified ceramic could be so tough. The ASTM has a standard and a test for “microwave safe,” I’m not shelling out sixty bucks to read it. I’m not sure my pots would even pass the prerequisites to perform the test. I figured “vitrified”, as in 0.5%, was a prerequisite. What does microwave safe mean?

It’s below the lowest temperature I would consider bisque fired. It’ll be fragile. More absorbent, glaze will go on thicker than you're maybe accustomed to. Some people glaze greenware, it works. This seems beside the point. I don’t even know that you’re planning to glaze your work. Put a piece underwater overnight to see if it’s ceramic yet, shouldn’t be able to scratch it with your fingernail. It doesn’t matter if you fire 6 hours, 10 hours, or 50 hours, the clay has to get hot enough to change. If your kiln fails at bisque, how are you going to do a glaze firing? I agree with @Bill Kielb, troubleshoot everything from the breaker to the kiln. Fix that before making any other plans. Get some cones if you don’t have any, don’t rely on the pyrometer alone. There are no ceramics without a kiln (or a very very hot fire). You are ok, by the way. Things will work out.

If it’s bad I’ll use a needle to even the rim, but I might try that hacksaw trick. For the foot, trimmed stuff takes care of itself. Untrimmed pieces will often get a rolled foot, hold the (leather hard) piece at a 45° angle and roll it around on the edge of the foot. If something sits unevenly when dry I’ll get a ware board wet and slide the piece around on that to knock down the high spots. This occasionally happens even on trimmed work.

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.