Kelly in AK

Soda ash (sodium carbonate) is also a deflocculant, possibly easier to come by. You can actually make it by putting baking soda in the oven, but I suggest you try and find some first. People use it in swimming pools to adjust ph and fiber artists use it in dyeing. The long bisque firing is a good plan. I know you use local clay, if you have any hand in the processing it may be worthwhile to get more coarse particles out. Those are like little gas factories, as well as providing a path for gasses to escape. I agree with @Min about trying zircopax in your slip to whiten it. It will also make it less susceptible to getting soaked up by the glaze. I see from those test tiles you’ve been through a lot! Sometimes it feels like a miracle any of this stuff works at all.

As clay reaches its maturation temperature (becomes vitreous, changes from bisque ware to stoneware/porcelain ) it becomes “pyroplastic.” Soft from the heat, in other words. I feel that has a significant bearing on your problem. People solve this in various ways. One is to adjust the form to compensate for the sag, another is to add structure in the design to support wide flat areas. Still another is to calculate thin and thick areas for greater support. One very direct way would be to use a clay that matures a little higher than you’re firing it, or, better still, a clay known to keep its structure at maturity. I suppose even some kind of supports could be fashioned to make it work. Porcelain and highly vitrified clays are notoriously pyroplastic. The weight of a handle will pull a mug into an oval shape. Some peoples favorite ceramic word is eutectic. Mine is pyroplastic.

@Hulk, I can only attest to the complexities of starting a vehicle at -30° F. Sometimes batteries do not perform as the label leads you to believe. Usually the rest of the car isn’t cooperating either! Also have to mention, no visible monks, but some squirrelly little hoarders here. Any pot left in the open will be mysteriously filled with spruce cones by late fall. And there are the not so secret critics. Magpies that disassemble any areas packed with ceramic fiber. They like to strew it all about, not happy with the neatness of my work.

The bisque piece looks like you sponged it over while it was greenware. The sandy surface is from wiping away the clay and leaves grog sticking out. Like Bill said, smooth surfaces with a rib or flat smooth tool and push the grog in. The gritty surface multiplies odds of glaze defects. At cone 04 putting a layer of clear glaze on top of Stroke and Coat glazes has worked well for my students, but I have to say they use it on its own without problems too. None of them have played with it at cone 5/6, so that’s unknown territory for me.

Maybe nylock wing nuts or locking washers?

Yes! My main supplier has at least one clay body they tout as being “resistant to thermal shock.” It’s more porous than the regular cone 6 stoneware I use and has some glaze fit issues, so asking about compatible glazes is a must too. Also, Min’s comments about the shape are important, no sharp angles, avoid wide flat areas. Keep some curve everywhere, smooth transitions in form, if you (must) trim a foot ring keep it low profile. It is certain some clay bodies handle thermal shock much better than others. Like Mark said, stuff sold in big stores is a whole different ball game. Definitely some ceramic engineering involved there.

Yeah, that’s a lotta flux! Like Min just pointed out, it’s probably not a very durable glaze. Not good for the insides of functional pots. It was pretty fascinating how it came out on the piece you posted though. I think more clay in that recipe wouldn’t hurt. Your work is gorgeous, by the way.

I see a slip fit issue here. (I’ve never been clear about when a slip becomes an engobe, or the other way around, by the way. For that matter, why aren't underglazes called engobes?) Your clay is going through a dramatic shrinkage as it vitrifies at around cone 02. The slip is doing its own thing, looks like it’s shrunk quite a bit more from the crackle patterns evident. These are happening at different stages of the firing. The glaze has to fit all. My gut feeling is straight borax is not helping you, leaching out of the slip and into the body and glaze, exacerbating fit problems. The “do it yourself” ethos I pride myself on has its limits. While I would never give up on a lovely locally sourced clay, I would give a commercial white underglaze a try here. Not sure I see any lime pops as I’m familiar with them. I see strong glaze to slip interface and weak slip to clay interface. A slip/engobe recipe tested for cones4-6 on midrange stoneware may not be suitable for you firing range. Slips designed in the 04-02 range may be more promising. More zircopax in the slip will reduce the amount of slip absorbed by the glaze. A little less flux to raise its maturity.

Glaze it inside and out. I think Pres and Dick nailed it here. If you’re using stoneware the critical point is to educate whoever is getting the casserole to only put it into a cold oven and then on a towel or something when it comes out. And for heaven’s sake, definitely do your own trial runs first. If you ever needed an excuse to make some delicious cheesy au gratin potatoes, this is it. Pres mentioned unglazed casseroles that are soaked in water before use, that’s really an exception for earthenware cooking pots. Important to be aware of, but outside the realm of where most stoneware potters work.

I’m no stranger to taking junk electric kilns to salvage refractories. Also no stranger to being cash strapped. I’ve built several gas kilns from abandoned electrics.@neilestrickis spot on with a kiln from this era. The insulation behind the firebrick may well be panels of asbestos. Not fun when I’ve run into that. They should be paying you to haul it off.

Absolutely gorgeous setup Mark, and beautiful work! There’s a lot to learn in those photos, thank you for sharing them. Congratulations! You’ve earned the right to work at your own pace. Merry Christmas!

The wire will not support those, even if they’re light as a feather. Using stilts is how some people manage to glaze the bottoms of things. Generally stilts are rated for low fire (cone 04 or lower). As mentioned above another solution is to leave the bottom rims unglazed. Still another is to use a room temperature finish, paint, varnish, etc. Since they’re not something people will eat off of it’s worth considering.

Tina! You just made my day.

I really get a kick out of visiting other potter’s houses and seeing pots from potters I know on their shelves. It always feels like some wild unexpected connection, a sense of community. I have a few pots made by people I don’t know anything about, but most I’ve either met or I know their work. Got some real gems from the NCECA cup sales. One rare thrift shop find, a mysterious little beauty nestled in among all the “Beginning Ceramics” pots that wind up at the Salvation Army. No idea who made it, but that didn’t keep me from buying it. Probably helped that it was $3.

Right now it’s a Nissan Frontier pickup truck with a camper shell. My regular day to day vehicle. The eight foot shelves stick out the back, but all the furnishings fit in the bed, and I pack the seats with boxes of pots. I don’t travel any great distance to do shows, just a couple a year, in town. Many years ago it was a Ford Aerostar minivan. That thing was awesome, I took out the passenger seats and it was huge. I could fit a whole sheet of plywood in it.

Most clay sellers (all I’ve seen) publish specifications on their clay bodies tend to that include: fired color, shrinkage at the stated maturing temperature, and absorption at the stated maturing temperature. If a clay is sold as ranging from cone 6-10 they should show absorption at cone 6 and cone 10. There are many cone 6 clay bodies available that I consider vitrified (1.5% or less absorption). There are some with higher absorption, say 2%, sold as good for functional pots, and still other clays made for non functional work. It’s a slight complication that fired strength and porosity don’t always correlate (mostly they do). You just have to draw the line somewhere. Some potters accept more porous clay as long as it’s sturdy, usually with the caveat “microwaving not recommended.” As cone 6 oxidation firing has become more of a standard over the last twenty years issues of non-vitreous clay and problems with glaze fit in that range are much less a problem than they once were. By the way, I’ve never seen a clay that’s vitreous from 6-10.

The speed of cooling has a great deal of effect on mattness of some glazes, no matter what kiln you’re using. Given time, micro crystals will grow into that matt the surface.

Haha! If I had any sense I’d be making snow globes! Natural gas piping is in the plan for spring, and looking forward to it heartily. That, and baso valves. Two 100 pound tanks, if full and tied together, can get me to cone 6 in the 20° F range. I have to keep a big pot of hot water on an outdoor cookstove in case they start to freeze up. I also have to give the regulator a tap once in a while to wake it up. More nerve wracking the colder it gets.

I finally got the kiln dug out. Three snowstorms in a row, 41” in eleven days. I go through a lot of gyrations to fire in the wintertime and it got me wondering where people draw the line, temperature wise, for firing with propane. What is the temperature that you just say “no?”

There are many potters who skillfully use this effect to their advantage. A couple of Matt Kelleher’s lovely pots, white slip on dark clay:

Could you post a picture of the broken pieces? I had some students try this. The first pieces were small, coaster size, and very shallow. They came out great. Next load had larger pieces, saucer sized, some with deeper rims and a thicker layer of glass. Most of those were in pieces when I unloaded the kiln. Broke to bits on cooling.

ADHD, Flow states, having the world disappear when hands are in clay, those all hit home here too. With clay I don’t have to think about what to do, I just do the next important thing. All clay activity feels that way, not just the making of pots. Glazing, measuring materials, stacking bricks, digging clay, all of it. It’s one part of my life I feel total focus and connection. Otherworldly. My normal state of being does not feel the same. @kswanThere is a climbing gym in my town, it’s been my savior for fitness. I can see why it’s difficult to find something you like as much. I’ve never ever been a gym/athletic club guy (till now). It has some mild routes I haul my portly self up three times a week. Definitely worth looking into if you haven’t.

Thank goodness you used cones! I agree with Bill here, he really said it all. I imagine your cones touched down quite a while before the kiln shut off. I have one glaze in the school studio (cone 6 electric) that blisters if it soaks too long or is applied too thick. Many years ago I had a clay body that disproportionately produced blistered glazes. Same glazes, same firing, side by side, pots from that clay blistered, others didn’t. It took a few kiln loads to see without a doubt it was the clay. I was trying all kinds of things to solve it, the clay body wasn’t even on my radar. Bisque at 04 and try a shorter glaze firing. See if you can isolate certain glazes prone to blistering. In my home studio I struggled a while with pinholes (not blisters so much). Dropping 100° at the end and holding 30 minutes helped more than anything else to solve that.

Oh, and then this happened. Skipping the gym this week.

Isolating fine particle sizes by allowing clay to settle in water (elutriation, I had to look that up) was known to potters historically, actively used in places where clay had to be “washed” to make it useful. Terra sigillata is an extreme extension of this principle. Natron, naturally occurring soda ash, was well known to the Romans and had many uses, including in ceramics and glass. To me, it’s a stretch to imagine Romans didn’t use a deflocculant of some kind. Looking at the artifacts, I think it’s also important to consider the impact of firing on the finished appearance. Roman terra sig appears different than Greek ware, but it may not all be due to how they made their slip.