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LeeS

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  1. Hi all, I'm working on some porcelain pieces with Clay Planet's glacia and pier porcelains. However, I'm having a lot of trouble with finding a glaze to fit. So far I have tested a few starting recipes, all around 6.5 - 6.8 calculated COE. I was most happy with this one: However, all the tests crazed. Predictably, the glazes closer to 6.7 COE were a bit worse than the 6.5 glaze. From there, I tried reducing KNaO and CaO and increasing zinc, silica, and alumina: However, this also crazed badly on the Pier and moderately on the Glacia, and is slightly undermelted (slightly matte, didn't flow well), so it feels like a bit of a dead end. I was hoping for some help with figuring out what to try next. I'm thinking of starting with a boron-fluxed glaze to get some more room to play with melting, something like: I would drop much of the dolomite to reduce MgO matteness, and increase the boron (probably with a frit replacing the gerstley) to correct the melting from removing dolomite, and increase silica and alumina to reduce expansion. Is that a good idea? Is it possible to just improve the one I already have? If anyone has some low COE (5.0 - 6.0 range ideally) glaze recipes they're willing to share I would be grateful as well.
  2. Quick update. I stopped by Clay Planet again today and they were super helpful. They're going to test out the glacia, and they hooked me up with a bag of Pier to try out. We'll see how it goes!
  3. Wow I didn't expect so much helpful advice so quickly. Thanks everyone. I didn't realize how much clay some of the production-oriented potters here go through, so I should probably clarify... It's only about 50 lbs in total of dry and slip. It's not a huge financial loss if I have to get rid of it. I also have very little reclaim that may have some plaster contamination anyway. I talked to Clay Planet about this. I measured ~3% +/- 0.5% absorption. They mentioned it hadn't been tested in a while, and suggested adding some feldspar. That was about it though. I'm a bit concerned about trying to fix it because it might be a fair bit of testing work, and probably only relevant for this batch. I'm also not terribly excited about the prospect of fixing future batches, and not too thrilled with the Glacia in general. It's nice enough for throwing, but it's a bit dirty and flabby for slipcasting. I think I might just throw it in storage and switch to a new formulation. That being said, what would everyone suggest for a new porcelain? Is it reasonable to formulate my own, or is that a world of frustration? Has anyone used either Pier or Miller 550? Is it unreasonable to avoid Pier because of this experience with the Glacia, just because it's a Clay Planet clay? No hard feelings to them, I just don't want to waste too much more time on clays that aren't performing.
  4. Hm, interesting thought, but I don't think that's the problem. They don't provide instructions but I deflocculate based on measured viscosity and specific gravity, not just a fixed recipe, so I have a pretty well-controlled process. Plus I bisque everything so all the casting water should be long gone. I guess maybe the particles could be too far apart or something? No idea if that's possible. Maybe I'll mix some up some tests without deflocculant until it's just wet enough to wedge and roll out to see if that makes it better. > Not sure, just a shot in the dark For sure, it's a strange problem to have with a commercial clay.
  5. Hi all! I just started firing on my own after a while at the local community centre. I'm doing some slipcast cups as a first experiment using Clay Planet's dry Glacia porcelain. However, I've put two test glaze loads through the kiln and the results have been pretty disappointing. The clay feels unvitrified. The texture is closer to bisque than finished ware, and it seems to soak up water slightly. All of the glazes I tried crazed in the bottoms of the cups. Leaving water in one of the crazed cups overnight, I can see it creeping up inside the glaze, soaking the porcelain. This seems pretty odd to me. The first firing was a solid ^10 (I used the standard guide/target/guard cones), and the work is very thin, <2 mm, so it shouldn't take long to come up to temperature. The second firing I added a soak for 30 minutes, and ^11 drooped significantly, but the clay feels the same. A loose piece of broken bisque I set the cones on also broke very easily. The 3 glazes I used were Tony Hansen/Digitalfire's G1947U, the transparent used at the community centre, and a modified version of that sourcing CaO from Wollastonite instead of Whiting. I've never had trouble with the community centre transparent fitting the pugged Glacia I use there, and G1947U should be even lower expansion, but all crazed. For the second firing, I tried to correct the glazes, but even super low expansion borderline unmelted glazes performed very badly. All in all, I'm having very bad luck with the dry glacia. Here is where the questions start: 1) Has anyone seen this before, especially with this clay? Any high-level ideas for what I could be doing wrong? I don't really want to brute force it by just firing super hot/long, especially because I made it well into ^10. 2) Is it worth trying to fix the Glacia, probably by adding feldspar? I have tons of bisque, slip, and dry clay of the Glacia left, so this is somewhat attractive. It would be a bit of a pain though and I would probably have to throw out the bisque. 3) I'm kinda soured on this whole deal, so maybe it's worth just switching clays? The options are probably dry Clay Planet Pier or pugged Laguna Miller 550. I kinda want to avoid another Clay Planet formulation after the Glacia issues, but I also don't want to make slip out of the pugged Miller. 4) Maybe I should just formulate my own porcelain? On one hand, I'm pretty comfortable with the chemistry, mixing, and testing. On the other hand, it would be nice to finish some work and not just test things forever. Thanks everyone for reading and any advice will be greatly appreciated.
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