Kelly in AK

What cone is that? Boo!!! My local concrete/masonry supply used to carry Lincoln 60. It was one the cheap place to buy a legit ceramic supply in bulk. Sadly two years ago they switched to some awful clay from Ohio that is nothing like good old Lincoln 60. For them it’s “the same thing,” for me, not so. Poorly milled, stinks of sulfur, no idea what’s in that stuff. Label tells nothing. So for over twenty years I could get Lincoln here like it was an ordinary hardware store item, then suddenly it’s done. Another material that may be going away. Dang. Looking forward to hearing any news on this.

Nice to see the professional/commercial interests have a similar response to what you hear here: Test it first, see if you’re happy with it. “You could fire the velvet underglaze onto a vitrified clay surface. However…” I’m constantly amazed at what commercial products pull off. Big engineering energy. If you’re not doing functional work, then if it looks good it is good. Sculptural work does not suffer the same beatings as functional pots, nor are room temperature finishes forbidden. On the other hand, nothing will last like properly glazed ceramic.

You weren’t kidding, those are definitely bubbles in the clay. Like @Hulk said, you ought to get some cones to see what’s really happening in there. Kiln’s getting too hot or soaking too long.

It’s odd, for sure. The only times I’ve had something like this is when some material in my glaze was not ground fine enough or not dispersed well enough. Sieving it well fixed the issue. I’ve had tin and wollastonite do this. But you’re not making your own glaze, that’s what catches me off guard. For the people considering this problem, could materials agglomerate or crystallize in a glaze that’s sat around long enough and cause this?

@Callie Beller Diesel Yes! The sturdy drill with a mixer. Indispensable tool.

An angle grinder, masonry grinding discs and diamond cutting discs. Cordless Dremel tool, diamond bits.

My impression, based on the photos, is that those cracks may get larger when the piece is fired to maturity, but I don’t see it falling to pieces, at all. A commercial bisque patch (from a reputable brand, such as Amaco) to fill the cracks was my first thought. Personally, I’m a do-it-yourselfer and would probably go to great lengths grinding up bisque ware made of the same body, add just enough clay to make it stick together, and throw in a little paper for good measure. Wet the piece, then fill the cracks as completely as possible, sand when dry and fire. Of course, you could do the sane thing and just buy some Bisque-fix. I do recommend reading the instructions carefully before using it.

I believe earthenware is very good for sculpture. Yes, you can use glossy glazes and bright underglazes. They’ll stay bright and colorful because of the lower firing temperature. It will not be as strong as stoneware or porcelain fired to maturity, but that doesn’t mean it’s not strong. Earthenware is usually only fired to bisque temperatures and is generally stronger than bisque fired stoneware. Taken just a cone or two hotter, some earthenware develops significantly higher strength. Those 8000 terra cotta warriors are still around, I say earthenware for sculpture is a go.

Everything mentioned so far will help reduce warpage. I’m glad Mark mentioned “memory,” I thought of it right away when you described how you make your work. Vince Pitelka has a great explanation of this seemingly nebulous concept: https://www.vincepitelka.com/wp-content/uploads/2021/06/Rolling-Slabs-Platelet-Grain-Structure-and-Clay-Memory.pdf The short version is you need to roll the slab after it’s out of the slab roller, in other directions than the roller first forms the slab. Both sides. Rolling it in only one direction gives the slab a “grain” and you’ll get different shrinkage in one direction than another. The glaze tension and how the feet support the form certainly play a role, but eliminating the grain structure will prevent at least the initial forming tensions from being set up.

Sort of echoing what’s been said. You’ve got to fill the hopper, mix well under vacuum, then pug that batch. It’s not a continuous feed sort of operation like a regular pugmill. It’s batch-at-a-time. I recently replaced a Shimpo dual auger pugmill with a VPM20 in my classroom. After figuring out the how to operate it, the thing is a dream. Tight, plastic, de-aired clay every time, at exactly the consistency I want. Clearing out the vacuum chamber is a regular part of using it (every batch, says right on the machine). It’s a learning curve.

All functional pottery is haptic. Feedback by touch.

Looks a little bubbly. Try and crunch those with your fingernail and if they pop you’ll have more insight into your problem. My off the cuff guess for a fix is sanding, followed by reglazing the area and a refire.

A lot of encouragement here. I could wax on about how single-fire is how most glazed pots ever produced were fired and how forgiving commercial clay and glazes are to “off label” techniques. I only caution you to do it for yourself at least once before trying in a workshop setting. To further embolden you, every now and then one of my students glazes a piece of greenware (Because they don’t listen! Haha!). I fire them with the rest and haven’t had a problem.

My first thought was baking soda, but ozone generator is tops. I de-mildewed stacks of old notebooks someone gave me that way. I put them in a closed container with the O3 generator overnight. It seemed pretty miraculous, they went from stinking up the whole room to smelling fresh as a flower.

If a piece is non-functional then pinholes come down to pure aesthetics (if it looks good it is good). If it is functional then its functionality is part of the aesthetic equation, and pinholes become a different “term,” so to speak. Either way, it amounts to decisions about craftsmanship, process, and beauty. Eliminating pinholes is a good goal, challenging at times, but achievable. Forgive my clumsy way of saying it, but getting pinholes, crawling, and craters right is a whole ‘nother level.

This reflects my current state: Hands down it’s Tony Hansen’s Digitafire website. He covers both the need for quick information and a more involved quest for understanding. That said, fifteen years ago when I was looking for a simple floating blue recipe, and knew a lot less, the website seemed like the last place for quick information. “I don’t need to know all this stuff, I just need a recipe!” A cute thought, haha! Next is Glazy, I take it for what it is, a huge user driven database with very powerful tools. I am astounded it’s a free resource at my fingertips. Finally, but not last because it is in its own separate class, is Pioneer Pottery by Michael Cardew. More of a “Curl up by the fire and read about pottery” kind of resource.

They’ve both got pros and cons. Earthenware temps are a lot friendlier on kilns, elements will last longer. You can get really bright colors. Warping is less an issue because the clay doesn’t go through the pyroplastic stage stoneware does as it becomes vitrified. And it shrinks less. Some earthenware bodies are very strong, much stronger than bisqued stoneware. Fired to the upper end of their range they can rival stoneware in durability. Then again, earthenware can get really hot in the microwave, if you’re unaware that’s potentially dangerous. It’s my biggest problem with it.

So many things. One thing I learned early on was to be very careful about doing anything on commission. I rarely consider it anymore, though I gladly take feedback, ideas, and suggestions. Some have gone spectacularly well, but they were made up for by a few that went equally bad. You’re aligning your client’s vision, your vision, and your abilities. Clear communication has to happen before you touch clay. The minute you need to do something over to get it right any money you might have made is gone (or at least halved).

I’m guessing that’s MDO panel. It’s available from lumberyards. I’m not sure how common double sided MDO is or what thicknesses it’s available in, but now you know what to look for.

Cool project! I believe the chimney also needs to be two layers of brick thick, at least to the top of the kiln.

A white glaze I’ve been using a couple years is based on glazes from John Hesselberth. It may be one of his glazes, I can’t remember. I just know I like it and it’s reliable. It’s stiff, stays put, glossy, but satin matt if slightly under fired. You’ve got to sieve it when mixing (should do that with all glazes anyway), the wollastonite likes to stay in lumps. The speckles in the photo are from the clay body, on lighter clay it’s white like milk. EPK- 32 Ferro frit 3124- 31 Wollastonite- 23 Silica- 14 add: Zircopax- 12

If your glazes look good you’re doing it right. I would say it’s a solid cone 6, probably slightly over. It’s clearly a tad hotter on the bottom, Some glazes (and clay bodies) benefit from a slightly higher temperature, others are hitting their limits at the stated cone. If you’re not seeing defects related to over firing you’re probably not over firing.

Without knowing Michael Casson, and his work, the average person seeing those videos now, …well… I have no idea what it conjures in the mind. To me, it’s a hero, gracing us dreamy clay makers with his expertise, along with the rebellious yet stylish attire and personal vibe. And doing so in an impeccable way. Definitely a hoot. Brings me joy. Definitely not a joke. Michael Cardew next.

Congratulations on the upcoming studio tour! That’s a beautiful looking tray. I also agree that your cracks are starting before the firing. @oldlady’s suggestions are excellent. Of course, the steps to alleviate stress during firing are worthwhile. For good measure I’ll add to never use or add water on the edges of work (use slip if you have to, the thicker the better). Some clay is very sensitive to this and it really shows up on the edges of slabs under tension.

I believe metal should be added after firing. I can’t think of any metal commonly available (as a threaded rod) that wouldn’t be ruined at cone 6, also the clay will shrink around it and most certainly crack. Epoxy or epoxy putty is a good choice for bonding metal to the clay, it cures very strong, is dimensionally stable, and fills gaps. Plan for the shrinkage of your clay, if you make a space for a rod it’ll be significantly smaller after firing. That’s going to be one cool table!