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About ceramicfundamentalist

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  1. do you mean that razor sharp little bits of fired glaze popped off after the glaze firing? if so, that is a condition called shivering. it comes about because the clay body and fired glaze have different coefficients of expansion. as the pieces are cooling down in the kiln the clay shrinks more than the glaze and the glaze no longer fits on the clay. little shards of glaze pop off the pot to relieve the pressure. there is not much you can do about it except to alter the recipes of your glaze and/or clay (but it sounds like you are using pre-mixed clay and glaze) or to use a different clay or glaze. i have a similar problem with my white glaze and terra cotta body, but fortunately it is not a combination i use too often. if i can keep the glaze a bit thin then i seem to have a better chance of my piece coming out ok. also, your form might have something to do with it - shivering tends to occur at sharp edges on lips and handles. if you make those edges a little fuller and rounder it might have a positive effect.
  2. we had a "glaze" similar to this - it was called manganese lustre - when i went to school at nscad. it was dark, irredscent and textured - not to mention beautiful. i used it quite a lot on my sculptural pieces at the time, but even being the naive student i was then i sought out other glazes with similar properties that had less toxins in them. before i go any further just let me say that i am not disputing that the glaze is a hazard - it absolutely is. but, just for fun i will dispute your assertions that it is not in fact a glaze. you see, when i used this manganese lustre i used it like a glaze. i dipped my bisqueware into a big bucket of it and it came out looking like a glaze. i had to be extremely careful with the thickness, because anything over a certain thickness would run like the dickens. i've seen others use a pure manganese dioxide wash on bisqueware, and it doesn't get shiny or runny quite like the manganese lustre glaze. forgive my back-of-the-napkin calculation here (i don't have my glaze calculation software on this computer), but i think the formula for this glaze might be something like... MnO2 0.80 CuO 0.20 Al2O3 0.10 SiO2 0.20 ...if one assumes the manganese and copper can fall into the role of fluxes. this material certainly acts like a glaze, it gets shiny and melty, and i don't think those properties are a result of the material reducing to a metallic state because the glaze works just as well in an oxidation kiln. so it seems to fit the glaze/glass definitely of an amorphous solid. i would guess that the copper and/or manganese oxides are actually also fulfilling some of the role of the glass former in this glaze, not unlike, for instance, a glaze that might be made up from 20% silica and 80% boron oxide. in any case, a glaze that contains such low levels of silica in the formula couldn't possibly be durable (insoluble) enough for food surfaces, and the fact that the bulk of the glaze is made up of really toxic stuff ensures that plenty of those toxins will get into any food that comes into contact with it. that enough detail for you?
  3. i think that in a glaze like this, where it is made up of such large percentages of toxic materials, you would have to assume it is unsafe until you can convincingly demonstrate otherwise. you could get it tested professionally to see how much it does leach (it's definitely going to have some leaching), but as far as i know there are no industry standards for manganese leaching. in that case you'd have to compare the results with the standards for contamination in drinking water, or with the standards of the leaching of other metals in pottery that do exist (lead, cadmium). in either case its far from an exact science, so it would be nearly impossible to demonstrate that this glaze is safe. personally, i would never use it on food surfaces, because i don't think any test could convince me it's not a potential hazard, and i would think twice about even having it in my studio. in fact, i think it would be extremely irresponsible and dangerous to use this on a food surface.
  4. i mistrust any book that claims to be "complete". there are a lot of them out there in ceramics. why would an author ever purport to be able to distill something as complex as an entire artistic discipline down into 50 or 100 pages? those books are generally very superficial, and don't in any way do justice to the world of ceramics. anyways, my three must-have books are: the Hamer and Hamer Dictionary of Materials and Techniques Ceramic Science for the Potter by Lawrence Vall Cushing's Handbook
  5. What is it that makes bamboo brushes so much better for ceramic materials than regular brushes? Bamboo brushes work with slips, underglazes and glazes, but i've never been able to get a regular brush to work the same way. a regular brush just doesn't seem to hold the material, or it will "stick" as your drag the brush across the surface. is it the type of bristles used? is it the way the brush is constructed? and most importantly, does anyone know of a good source for brushes for use in ceramics?
  6. It could also be salts from the nepheline syenite or frit (both are somewhat soluble) collecting on the surface during evaporation (probably in conjunction with sulfur as JBaymore has noted). This is common in many clays, especially, i have noticed, porcelains that use nepheline syenite as a flux instead of feldspar. Do let us know how the clay comes out after firing - I'd be interested in trying it out myself.
  7. i guess i should also add that in my studio practice i dabble quite a bit with relational aesthetics and social practices, so it's not that odd that i'd be so interested in these group activities in my "off" time.
  8. i guess i have eclectic interests. I play a lot of sports, especially ultimate frisbee which i'd play every day if i could. i try to spend at least four weeks a year backpacking/bicycle touring/camping, usually in pretty extreme wilderness locations/circumstances, although this year i will get nowhere close to that. i have spent _a_lot_ of time volunteering in the contemporary art world (on gallery boards, juries, etc.). i've been a very active advocate for bicycling and other forms of sustainable transportation, i even worked full time for a year as an activist, but i've tailed off on that one a little bit lately. between art galleries, bicycle advocacy groups and sports organizations i've probably founded or co-founded about 5 non-profits in the past 8 years. and i have a huge soft spot for video games, especially classic 1980s era video games (this takes up more time than i'm prepared to admit here).
  9. i wouldn't bother washing my posts. i'm the only one firing my kilns right now, and though there is lots of student work in there i know everything that goes in and out. if you are in your own studio and you are in control of everything that goes in and out, i say don't worry about it (as long as you yourself are not doing anything too crazy). more than things sticking together a little bit i hate getting messy white specks all over my kilns and pots. of course it depends on what kind of firing you are doing. washing is essential in soda/salt, somewhat desirable in wood, but not really necessary in regular reduction or electric. and the higher the temperature the more i would encourage washing - i am currently firing at 04 and have never had the posts stick to the shelves. i should also note that i keep separate shelves for bisque and glaze firing. i never wash my bisque shelves, and they last forever with zero maintenance, and are never messy. i keep a good coat of kiln wash on my glaze shelves at all times.
  10. and two other classics that have more technical information, if you are so inclined: the potter's dictionary of materials and techniques, by frank and janet hamer ceramic science for the potter, by wg lawrence i'm curious why you say you're not ready to mix your own? that is exactly where you should start, and it's the easiest thing in the world. just get a few recipes from books or the net, try that a few times, and then start combining them, or use an existing recipe and swap out one material for another, or change the percentages a little, just to see what happens.
  11. I've been going through this little studio experiment of using some clay from my area, and I thought I'd share it with you since I'm enjoying it so much. I live on the west coast of Newfoundland which is one of the few areas of the island where there actually are decent deposits of clay. I had long heard that certain potters way back in the depths of time (the 1960s) had used the local clay in making their wares, and when I moved to this coast last summer I decided to go looking for it. Well summer turned into fall, and then to winter, and it's just been this past month since I've gotten around to doing it. My first trip out prospecting I hit pay dirt. I didn't even have good directions - a friend of mine who used this clay 40 years ago told me to just go across the bay and look around. There I found an eroding embankment above the beach, just 6km from my house in the middle of town. I brought a handful back the first trip, then a full bag on the second trip, and finally about 50kg on the third trip. It is actually a nice material straight out of the ground - if you bang it on a table a few times it loosens up and you can handbuild with less than 2 minutes of preparation. There are a few roots and pebbles in it, but the deeper down you dig the less you find of them, and there aren't that many to begin with. For use on the wheel I dried some out completely, then reslaked it, ran it through a sieve and set it out to dry. Without adding a single thing this clay turned into a fine material for throwing. It is a bit less plastic than what I'd expect from a commercial body, and I've had some trouble with cracking, which I think just require me to change the way I work with it. And though I was warned of scumming I've had no problems with that at all. I'm sure I could easily improve the working qualities by adding a few things - sand/grog, silica, fire clay - but part of why I like it so much is because it is perfectly fine just the way nature made it. It's a rich terra cotta colour, and very homogenous and fine grained. The shrinkage is 8.0% while drying, 8.2% at ^06, and 8.7% at ^04. The absorption is 16.5% at ^06, and 14.8% at ^06. I also plan to fire to ^02, but haven't gotten that far yet. Surprisingly, vessels made from this clay "leak" very little even when covered with a crazed glaze, despite the high absorption. My commercial white earthenware clay has a similar absorption, but leaks like a faucet. I'm not sure what accounts for the difference between the two, but I suspect it is because the commercial clay has silica or other fillers in it which connect the pores to make the clay more leaky. I've attached some pics below of pots made from this clay. I've also been experimenting with using it as a slip and in glazes, though with little success so far. The best thing to come of this material yet is probably the terra sigillata I made with it - a super shiny ruddy surface, that unfortunately, I haven't been able to capture in a photograph.
  12. and as a side note, wouldn't it be great if our fancy electronic kiln computers could compute the kiln's total energy usage per firing and display it in kilowatt*hours or joules?
  13. as opposed to special over night rates i have the complication of a "demand charge" on my electrical bill. i can't remember exactly how it works, but i know that i get charged extra if my peak usage exceeds a certain amount. this often happens when firing a large kiln while using other appliances during the day. i think it always makes sense to fire kilns overnight, or whenever your energy usage is the lowest, just to take advantage of the cheaper rates. i worked it out once, and found that firing during the day was twice as expensive as firing at night. if you want to know the exact cost of firing your kilns i would suggest you talk to your energy company directly and get the exact formula(s) they use.
  14. molecular weights: BaCO3 = 197.34 SrCO3 = 147.63 147.63/197.34 = .748 (~75%) so, you can multiply any amount of barium carb in a recipe by .748 to sub directly for strontium carb. for instance, in a recipe with 10% barium carb multiply by .748 to find that you need 7.48% strontium carb. of course this doesn't ensure the resulting glaze will look the same, but it is a start.
  15. thanks so much! i've been wanting to see this again for some time!
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