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douglas

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Everything posted by douglas

  1. One other possible cause is too much glaze or using a runny glaze on the inside of the pot. Maybe when you are pouring the glaze out it is taking a while and the inside has a thicker than normal application? If the glaze is too thick at the bottom of the pot, and if the glaze doesn't shrink at the same rate as your clay body during firing, the thick glaze pooled at the foot is creating a stress crack and the sudden temperature change is causing it to fracture along the spot where the thick glaze ends and the clay wall begins.
  2. Plastic bats seem to cause more cracking than plaster or masonite bats. If I have to use plastic, I wire it off immediately after throwing, and flip it onto it's rim when the rim is strong enough to support itself. If the base is too soft to flip and will sag, then I transfer the plate to a board with newspaper so the base can move easier as it dries and contracts. With plaster or masonite bats, I think they wick away enough moisture from the bottom of the plate to avoid the stress from uneven drying on the rim and the foot, and I don't have to be as careful.
  3. If nothing comes out when you flip the mold over, then you might not be filling the mold up completely. If the walls are hollow that would explain why you are getting two halves instead of a solid bowl. You need to screen or thoroughly mix the slip to remove the lumps. You may also need more water in your slip if it is not able to run all the way into the mold before it dries at the base, blocking you from adding enough slip to fill the mold.
  4. If you have access to a ball mill, try running the glaze through that before using - that might break up the particle clumps.
  5. Glaze fit means the glaze shrinks at a rate close enough to the clay body shrinkage, that it will not crack or shiver off the clay. Just because your glaze and clay are firing to the same cone, does not mean the glaze will fit the clay. Apologies if you know this, but it seemed like you are equating firing temperatures with whether the glaze should fit in your descriptions. The root of the problem you are experiencing is you are buying off-the-shelf products. Since you don't control the ingredients you don't have control over whether they play well together in the kiln. Learning glaze chemistry is an option, mixing your own, and adjusting to fit your clay body, but that takes most people a lot of time to work out. If you want to be able to just buy glazes and clay, then you should ask your vendors to recommend clay bodies or families of glazes that should work well. If your glaze vendor's response to using earthenware was don't use earthenware, then you either need to follow their advice and choose a different clay body, or keep your clay body and find a different glaze.
  6. Coleman porcelain has warped quite a bit in wood fires for me (cone 10-12 with lots of ash). You might want to load fewer porcelain pieces since your space is limited.
  7. Some general advice is to do a good amount of research before diving in with any ceramic project. The lusters you want to use are toxic to work with. If you use precautions it is not a problem, but if you don't it can lead to health problems. I don't know if luster glazes are safe after firing with prolonged skin contact. It's worth checking out before you make these. Lusters are not glazes. They are metal and binders that melt and adhere to glaze and I am guessing they would not hold up to daily wear and tear on jewelry. If this is just for you and friends that may not be a big deal, but if you sell them you want to test the durability so you don't get angry customers.
  8. My guess is you are letting them dry on plastic or some other smooth surface. The rim might have adhered to the surface (lots of moisture trapped under plastic could make a small amount of slip where the rim touched the surface). When the foot and insides of the platter shrank, the rim stayed put and caused the cracking. One fix for this is to use paper or foam under the pot while it dries so the clay can easily move on the surface as it shrinks.
  9. Use a cheese cutter wire with adjustable roller. Hold your thumb on the roller while you cut or glue it so that it won't roll. Hold the wire against the rim of the pot, and adjust the depth of the wire to the roller to 1/3 or 1/2 the width of the clay wall. Then when you cut the roller will prevent your from cutting too deep (assuming consistent clay wall thickness).
  10. It might be possible to dip the thin delicate bisque in an engobe to build up the clay thickness and re-bisque the piece.
  11. Sounds like your supplier may have sent you a bad mix of glazes. It sounds like they send you a premixed bag, so I would contact them and see if any of their materials may have changed. It does not sound like a user error on your part, but a problem with the glaze ingredients.
  12. I am not a chemist, but from what I have read and observed, I think the risk of copper poisoning from ceramics use is overstated. If you drink a liter a day of water that has up to 1.3 mg in it, and that is considered safe, any copper leaching out of a mug would be far less than this. I doubt a mug would have 1.3 mg of copper in the whole glaze application, let alone a lethal dose. People have been drinking vodka mules out of 100% copper cups, and we have not seen the new stories about these deadly cocktails killing consumers. That is because in order to get copper poisoning in this fashion, you would drink yourself to death long before the copper got you.
  13. You handled this well by moving on, this allowed the learning and creativity to continue. If you took the bait to engage with her pettiness it would have taken away from the other students who were there to learn and create. My advice is refuse to let her in the group again. She has nothing to contribute to your class and if she makes other accusations or complaints, it takes your time away from students that appreciate the efforts you are making to teach them. I am sure you can find a better student to sit in her seat. If she complains or asks why, say "you called me a liar, you didn't apologize, I refuse to teach someone who treats others the way you do."
  14. Another approach to aging the handle is to take a walk outside with some sand paper and an x-acto knife and scratch and sand away some of the under glaze. Use fine sand paper at the end to get rid of any jagged spots. You can still add a wash to this afterwards if it needs it.
  15. Joseph, I just ordered a bag of chicken scratch after seeing your results. I have some wood fires coming up, but I usually fire reduction gas. Would the granite pop off and possibly hurt glazed pots around it at cone 10, or was it pretty stable? I don't want to risk hurting the other pots while experimenting.
  16. Since you are making the original model out of wood, you might be better off using a hump mold and slabs vs. trying to master slip casting. I know you are not proficient in clay, but using a slab and hump mold is pretty easy even for beginners. There are plenty of youtube tutorials you can watch, but here is a quick read to get the idea. This way you are only paying for clay, glaze, and firing fees. You would probably need to use several large slabs since rolling a huge slab like that might be difficult. https://ceramicartsnetwork.org/daily/pottery-making-techniques/handbuilding-techniques/how-to-make-a-platter-using-a-slump-mold/
  17. You should probably bail. However it also sounds like there are customers, and people willing to volunteer. So maybe if the owner is losing money, it is because of their incompetence and not that the location and market are a losing proposition. Maybe if you and the other volunteers group together you can offer to take the business to a non-profit status. If they are losing money every month, and the equipment is as bad as it sounds, they would probably do better walking away from the rent payment than hang on a few more months before shutting the doors.
  18. Just spitballing here, as I have not tried this. If you used very soapy water for your slip, and used an electric mixer to foam/create bubbles, this might create air pockets which would be additional insulation. Or mixing in fine sawdust might play a similar role. Then dip the greenware in a thin slip to smooth out any surface irregularities.
  19. I have re-fired on medium speed without incident, but it is safer to use the slow speed. We work in a community studio so re-fires happen at the same speed as regular firing, which takes around 11 hours, so our kiln is not heating up as quickly as yours is.
  20. If you are new, I would stick to smaller forms like you mentioned. You are more likely to get lots of small pieces in than larger ones. You may want to bring one or two large pieces just in case it is ok with the kiln lead. I sprinkle wood ash on my glazes if it is a short firing. That way even if you don't get much ash buildup, you will get some movement in the glaze. In general Shinos are good choices for glazes. Most woodfires get between cone10 and cone 14 so make sure your glazes are cone ten. Most people do a liner glaze on the pot interior. You can leave the outside bare, or dip the rim and maybe the top third of the pot in glaze so when the glaze drips it won't go all the way down to the kiln shelf. Use cone 10 clay. Ask your supplier what they recommend for wood. I've used Highwater Phoenix clay and liked it, but most people use stonewares with iron in them (Phoenix is a white stoneware. Delicate textures and carvings may get covered by ash, so you may want to use bold textures and carvings that can stand up to the ash deposits if you get them. Good luck and post photos when you are done.
  21. Is anyone else freaked out with the casual way they are recommending cadmium stains? They caveat that any glazes for food use should be tested, but many/most potters just use recipes and don't send glazes out for testing. Maybe I am overreacting, I am not a chemist, but since even trace amounts can cause cancer or be fatal, this seems like a poor ingredient to recommend to independent studios. https://en.wikipedia.org/wiki/Cadmium_poisoning
  22. It might be worth doing some small batches with new iron oxide, firing alongside the glaze using the old, and see if you get different results. I did not think iron oxide changes, but maybe it continues to oxidize over time, changing the color.
  23. Make your own saggar out of your reclaim clay -- that way you know what temperature it can reach, and you can make it the right size
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