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Hi, I have this problem of cracking or clay splitting when throwing in the wheel, pls find the attached picture and also, after firing at cone 08, the clay has a lot of surface cracks and sometimes structural cracks pls find the attached pic, pls advise why this is happening and how to avoid this in future. Thanks.
I am making myself a simple form so I can log what I am doing with my initial kiln, clay, and glaze tests. I have columns for the ID of the piece (with dimensions & a sketch or pic), the clay body, the target cone, 1st or 2nd fire, the glazes for each fire, length of time or other firing details and Issues/Notes. I am using commercial clay and glazes "as is". Am I missing anything crucial? Thanks in advance. LeeU
There has been some interest in how to use your own dug clays, so I thought I'd share what I've been up to and maybe help a few people on their own journey to using materials claimed themselves. Today I was processing some clay I harvested this week and last year, and I thought i'd share what I do. To begin your search, I would advise you track down a publication from your State, provincial, or regional geological office. In my case, this is "The Clay and Shale resources of Ontario" (link here: OFR5134 - Clay and shale resources of Ontario - Geology ... ) an excellent publication that analyzes the composition of clay and shale from around the province, fires them, and characterizes other features, such as the geology of the areas where they found the clay etc. Your area likely has something similar. When you've located a promising sample, before anything else, determine if it's worth your while even to consider it. Test its plasticity. (All links from this point on are to my gallery) Edit: To actually perform this test, wedge the clay in your hands out of the ground with some water to a good working consistency. Roll into a snake, and bend over itself and twist. http://community.ceramicartsdaily.org/gallery/image/4146-img-0472/ If you can't get your clay to do that, move on. It might be a good glaze, you might have a good additive for a clay body, but you're not going to be able to use it out of the ground. If your plasticity test is good, awesome, dig up a bucket full. In my case, I know this clay is good, so I picked up several hundred pounds of the stuff. Here's a small sample. http://community.ceramicartsdaily.org/gallery/image/4147-img-0473/ Next, cover it water. Cover it, and let it fester in a warm place for as long as you can stand. For this particular batch, I'm going to leave it for two weeks. It's what I can afford to leave it for. http://community.ceramicartsdaily.org/gallery/image/4149-img-0475/ For the batch below, I left it to sit for a year. The longer the better, I've found. I don't have a real explanation but, I think it has to do with a change in chemistry. Throws better. The water is cloudy because I stirred it up to feel how it was working. http://community.ceramicartsdaily.org/gallery/image/4148-img-0474/ Next, take a quantity and REALLY stir it, until it's like a milk shake. Mix it as thoroughly as possible. You'll notice that there's clumps of fibre, rocks, and other stuff in the mix. The goal is to wash this debris of the clay its holding onto and get as much of the clay into solution as possible. http://community.ceramicartsdaily.org/gallery/image/4150-img-0476/ Secure a window screen over a second bucket. In my case, the trimming scrap bucket for the same clay. You may find you need a finer mesh screen to get your clay to work for you. Experiment. Window screening works for me because there's literally nothing in my clay but roots and moss. You'll find that as you pour, the screen will get clogged with debris. Scrub this debris against the screen, squeeze it in your hands (the clay will escape, the most of the debris will stay in your hand), and return it to your first bucket. http://community.ceramicartsdaily.org/gallery/image/4152-img-0478/ Give the bucket a thorough stirring to "wash" the clay off the debris. At the end, you'll be left with a little cow pat of a bit of clay, debris, and maybe stones. http://community.ceramicartsdaily.org/gallery/image/4153-img-0480/ Once filtered through a window screen, it will pass through a 40 mesh sieve without any debris on the screen. You may find you have a different situation. However, you may want some debris in your clay--it adds character. John Baymore has discussed this and the results can be pleasing. After you've filtered your clay, let it settle out, siphon off the excess water and dry, pug, wedge, and otherwise process as you would reclaim. It's now time to test it. Form it into bars, measure the bars when wet, then dry, and fire them at different temperatures, if they survive drying. How does it look at cone 04, 1, 5, and 10? Did it spall because of lime? Find a new source if so, but remember that some pottery styles (esp. in Mexico) embrace lime pops. My particular clay melts at cone 4 and slumps at around cone 1.5. Measure your bars after firing to each cone. I've got a pretty heavy shrink from wet to dry and dry to mature. This is something I've got to work toward fixing. Another thing to remember is to break the bars to check for carbon coring. This is a real pain with wild clays as there is usually MUCH more organic matter than what you'd buy ready processed. If you've got a dark interior to your clay, adjust your firing schedule to include a good long soak (sometimes over an hour) in oxidation from when the pots begin to get colour up to about cone 04. If you decide your clay needs grog, a cost effective way of making it is to allow a portion of your clay to dry, pulverize, screen to a desirable size, and fire in a bowl to bisque temps. I have not found that crushing failed bisqueware is a cost effective way of making grog without a ball mill. I hope you guys found this useful. I thought I'd share what I do. Here's a pot I threw a few weeks ago with clay processed this way. I'm drying it as slowly as possible. I'll be firing it either this week coming or next. Not sure it will survive, as i'm still debating if I need grog or not, but it threw nicely all the same. http://community.ceramicartsdaily.org/gallery/image/4154-img-0444/