Kelly in AK

Making the world a better place. Good man. I appreciate the sharing.

You’ve gotten tons of excellent advice here and provided great documentation for people to help you. My first impressions were: Beautiful kiln, you’re going to get there, just a few tweaks away. The first two are general principles, the third applies directly to your kiln, forced air or not. The firebox always has to be larger than you think, wood needs a lot of room to combust. There has to be significant space under the grate, air is what’s making the wood burn hot enough (air’s got to go under the burn, not over it). Double brick that chimney! Insulate it. At least to the height of the kiln. The higher the better. I’m not going to give you a long story, I’m just telling you in all sincerity it will change things dramatically. Air is key, and that column of air has to stay hot to create the draw needed to make wood burn at 2300.° You’re advancing along this learning curve rapidly, Olsen and Rhodes’ guidelines are excellent, but, as you may have noticed by looking at many kilns, not set in stone. The groundhog kilns of the Carolinas were my inspiration. I built a mini, and double bricking the chimney took it from cone 6 with difficulty to cone 11 with ease. 12 hours.

I don’t want to bark up the wrong tree, so I’ll ask this way: What’s changed? You’re using the same body, but is it possible the formula has been changed? The clay maker might not give you the formula, but they’ll probably volunteer whether and how it’s been changed. Have you used cones lately to verify the kiln is doing what you think it is? Which material you’re using is the newest purchase? That’s a of questions and no answer. My feeling is you’ve been riding on the cusp of this a long time. The drop and hold schedule suggests you’re already correcting for it (edit: I imagine the drop/hold and slow cool may also be to enhance mattness). Some single variable pushed it over the edge is my guess.

And here we come to it, the rub. Glazy has been a godsend to me, and I’ve yet to see anything bursting that bubble. I learned glaze chemistry and calculation thirty years ago in college, and used it loosely, only to evaluate recipes. Now, as then, I rely heavily on materials knowledge and gross ratios to come up with what I believe will work. Glazy allows me to put my guesses into a context of what has worked in other places for other people, and reduces (not eliminates) the testing. For materials that don’t have a published chemical analysis, or materials that have the same name but varying compositions (this talc vs. that talc, “ potash spar,” “boron frit,” or my local clay) there’s still guesswork. It takes much less effort to arrive at a data point that before would have been tedious to find, even with previous glaze calculation software. Glazy rests on the shoulders of everything that came before it and I don’t discount that, but we live in good times. Three cheers for Derek Au. I, like @Roberta12, look for that sweet spot on the UMF chart, nudge my glazes towards it and wait and see how the pots look after a few years in my kitchen. The only faster way to test seems to be alternating baths in strong alkali and acid (good old lye and that potent 30% vinegar @PeterH mentioned in another post). Like the weatherman, we haven’t arrived at perfect prediction, but it’s a lot better than it used to be.

The least amount of exposed skin. Long sleeves and full pants. Clothes you won’t cry about when an ember burns a hole in them. Cotton or wool, not synthetic. I mostly use ordinary leather gloves until late in the firing, when I switch to leather welders gloves with the high cuff. Comfort and dexterity are important until it gets too damn hot. Never synthetic gloves (though there are heat resistant synthetics such as Nomex, I just haven’t tried them). Leather shrinks and becomes rigid when exposed to extreme heat. Seams fail first because they’re synthetic (plastic) thread. I consider gloves a semi-consumable. I’ve used cotton jersey gloves, they’re very cheap and provide some insulation, won’t melt onto your skin, but they also wear out quickly. Don’t use any glove with a hole in it, anywhere, you will get a burn. Footwear is always closed toe, usually leather, not necessarily boots, but socks must go above the ankle. No exposed skin! There is a lot to be said for behavior over wardrobe. Tie your hair back. Never reach in, use a stick. Don’t stand in front of the firebox, stand to the side. A position five or ten inches lower can be 100° cooler.

I wasn’t reading carefully enough, and spoke before I knew what I was talking about. Sorry about that, it happens occasionally. I’ve had a concentrated share of picking up the pieces after deceased family members the past two years and my mind went straight there. I wish you (and Phil) the best of luck completing this arrangement.

This prompts me to share two microwave experiences in which pots cracked. Very different clays and experiences. The first was an earthenware bean pot I made. It had regular use on the stovetop. Always soaked in water before use, heated slowly over gas, and made the best beans. I was fastidious about cleaning it. Occasionally it grew mold. I would scrub with baking soda, fill it with water and boil for a couple hours, then rinse and dry, which kept it sanitary for a while. Once after this process I had the bright idea to put it in the microwave to dry it better, mold needs humidity. After a couple or three minutes I heard a clearly audible crack and discontinued that experiment. The pot made three or four rounds of beans after that before I decided it was ready to fall apart and tossed it. The next was trying to gauge how the microwave affected vitreous ware, mostly interested in iron content relative to heating. I tested a cup, iron rich clay, interior glazed only, for absorption. Heated in the (regular) oven at 185° to make sure the dry measurement was accurate. Two hour boil, then sat in the pot of water another four hours. Came out at less than 1%. Then I put it in the microwave (empty) to see how hot it got after two minutes. Before the time was up I heard an audible crack and stopped it. The cup was too hot to touch. It has a crack you can find if you really search for it in good light. I still use it. These are the only two times I’ve had a microwave wreck a pot. Both were empty and dry on the exterior but were also saturated, though one had below 1% absorption. No explosions.

It seems safe to suppose Phil of Bison Tools has either passed away or an invalid and it’s unknown who’s responsible for sorting out what’s left of his life. That said, it’s my opinion that politeness, civility, and compassion are always worthwhile until proven unwarranted. @Min is right, you have nothing to be sorry for, but as a reality check, the guy is probably dead. Sure looked like a one man show.

Tony Hansen enthusiastically champions this idea and has documented it working well for him. I can’t explain what’s happening, but I use it, a drop and hold strategy, and it’s reduced craters and pinholes for me. 30 minutes at 100° below peak temperature. At the same time, I’m constantly navigating other variables that could explain why things work out well or badly, such as adjusting glaze formulas and changing the schedule of reduction in my kiln, so I won’t flat out say it’s a cure all. I have my own peculiar issues. I don’t hesitate to say it’s worth a try.

+1 to the idea of altering color using stains rather than blending glazes. A small percent of stain won’t alter the chemistry much but can have a noticeable impact on color. Adding another glaze on the other hand can easily prevent those crystals from forming depending on its composition.

That is strange, because it’s the clay body that’s usually the determining factor if a pot leaks. I say usually because it’s possible with a perfectly fitting glaze to have a porous body that doesn’t leak (for a while, anyway). Are you sure you’re getting to cone 6? Some bodies show a significant difference between c5 and c6. As @Min says, the sure way to rule out the clay is to test it for absorption.

I once made bricks (a couple hundred) using scrap clay, some fireclay, and sawdust. They were used in a cone 10 kiln and performed reasonably well. I put in as much sawdust as I could without losing the ability to form the clay. I later made sculptural pieces with the same mix, mostly by cutting out rough shapes with a wire. Firing it was a smoky mess, must be done outdoors. The sawdust does nice things though.

I appreciate this question. As well as the notion that simple frit and clay glazes are best . In the 2016 paper, Matt refers back to the 2012 presentation, discussing results of 0.5 R20:0.5 RO samples in relation to boron. He concludes that paragraph with: “Thus the quality of glazes is not dependent on any specific temperature for durability. Chemistry is the only relevant factor.” Obviously, temperature does matter, if a glaze hasn’t melted it’s not complete. But once it has fully melted, why would a glaze that’s durable at cone 04 (say, 0.3 R2O:0.7 RO, 0.5 boron) be less durable if fired to cone 6? It might run all over, but that’s a different problem.

Definitely will smoke!

Calcining clay only has to get to around 1200° F, if that helps. Tony Hansen has some good info on it if you haven’t already seen: https://digitalfire.com/glossary/calcination

I use cement board for multiple things: wedging, ware boards, and working on. As a work surface I have to get it wet or my clay dries out too fast.

I saw Tom Coleman sift alumina on his kiln shelves. He said his porcelain would sometimes pluck (fuse) if he didn’t take that step. I had the thought of painting kiln wash thinly on pieces of newsprint and layering that between your porcelain sheets. It’s an idea, not sure how practical it’d be.

Thank you for the photos. I believe you made the pots too thick. The outside profile vs. the inside shape shows it, to me, clearly. If the bowl is in fact 20” wide, the foot must be well over an inch thick in some areas. There are many variables (relative humidity, clay body, temperature of candle, etc.), an eight hour candle ought to have taken care of most of them, but honestly it’s just too thick and blew up. Water did that.

We’re hearing a lot in the news about silicosis because people in the “cultured stone” business, countertops, are getting it long before a lifetime of exposure. This is a really specific scenario though. These people are cutting silica slabs all day with inadequate protection. The exposure is hard to fathom. That’s not to mention the unknowns about the engineered material they’re working with. You shouldn’t worry about this. You’re doing the right things now. Even if you had a few dusty years you didn’t ruin yourself, not even close. Keep the dust down, wear a mask when you’re playing with powders, you’re good.

If that’s the glaze, then wiping across the emblem with a sponge may do it. I’m with @neilestrick on stamped food surfaces. Do it on the bottom or find a glaze that fills it and still shows the details. I like that stamp. Nice looking work!

My BFA professor was a sculptor, her work was around 4-5 feet tall and probably averaged an inch thick. She used an Imco sulpture clay, it was pretty groggy. Also fired very slowly. Another thing I've seen is people building reinforcing structure inside the sculpture, extra walls where support is needed.

This is cool. You can have your own clay mine, with pool! You’ll hardly have to leave the house.

Thank you Mark, that explains it very well. A wide combination of factors creates the variation in price.

I was rearranging my glaze materials, making space, trying to cull out stuff I never use, and I came across twenty pounds of lithium carbonate. Something I don’t use, which is why I forgot I had it. So much talk about how expensive it’s gotten made me wonder enough to look up what it actually costs, you know… just in case. Now I’m totally confused. Prices from ceramic suppliers, some I’ve ordered from and some I haven’t but are well known, are all over the place. They ranged from $50 to $130 a pound. Does anyone know how much lithium carbonate costs?

I use two clay bodies, one for cone 6 soda firing and the other is a local clay that fires at cone 03. The cone 6 body is Laguna B-mix 5. After trying various clays I found it responds well to soda as well as hot and cool spots in my kiln. It seems easy to throw for me and is pretty tolerant when my craftsmanship isn’t perfect. The downside is it’s expensive compared to other white stonewares. The local clay is something I’ve played with for many years and gotten to know. It isn’t tolerant in any way! I add 1-1/2% Veegum to it, the biggest monetary expense. It cracks, slumps in hot spots, and is left porous in cool spots. I have to be completely on my game to make it work, which is part of its charm. I haven’t tried to calculate the cost, I’m afraid it would be too discouraging.