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About curt

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  1. This topic is intended to continue the discussion in another recent thread “New to clay, need help with firing temperatures”, which had digressed somewhat from the topic raised by the OP. Bill Klieb asked what mechanisms make clay less dense and begin to bloat when severely overfired (say, 3 or more cones higher than recommended). The reason clay bloats when severely overfired is due to the off-gassing of decomposing clay body materials and/or the re-gassing (coming out of solution) of previously dissolved gas bubbles, as temperatures increase. Thousands of small air pockets make bubbles which start “inflating” the now glassy clay body like air in a balloon, or blowing in to very thick honey with a straw. Chemically and microstructurally, this phenomenon was discussed somewhat in another thread in November 2015 entitled “Bubble, bubble, toil and trouble,” started by Highwater, on page two of that thread.. It is the same kind of off-gassing that initially results in small bloats on the clay body surface when a body is mildly overfired (see some pictures in a thread called “Pictures of bloating close up”). Just much more exaggerated. Anyone who has broken open some severely over-fired (ie melted mess) earthenware will notice that it looks like a honeycomb candy bar inside. For those that have no idea what honeycomb candy is, think of heavily leavened bread. Or a sponge. Lots and lots of small round holes/“pores”, separated by thin glassy walls. Of course the individual roundish pores in an overfired (very glassy) clay body are very different from the “pores” in an under-fired clay body, which are more like small twisty cave-like passages between large chunky particles. I agree with Min and Bill that when seriously over-fired, clay density may become less dense compared to the density at the proper maturing temperature, because every cubic centimetre of clay will contain a lot of these little air bubbles suspended in a very glassy mass, and hence would weigh less than a cubic centimetre of solid glass (using weight as a proxy for density) Clay this overfired does sound clunky when tinked due to this porous structure, and I know what you are talking about and have seen it myself.
  2. When a clay is overfired, I would say it becomes “overvitrified”, meaning a greater percentage of the claybody ingredients have been pulled in to the glaze melt (ie, turned in to glass) than originally intended by the claybody designer. Under these conditions the clay body becomes more dense, not less. Porosity goes down even further, approaching zero. Some clay body ingredients which are well-behaved at the designated firing temperature may begin to off-gas at overly high temperatures. With the now very glassy body melt in full swing, filling every available pore, these new bubbles cannot escape from inside the body, hence small bloats begin the occur, which can be seen on the surface of the clay body. Since the glassy part of the clay body has low viscosity (is very fluid) at those higher-than-intended temperatures, and as more and more glass is being created inside the body as heat work continues, the the whole clay body becomes “structurally compromised”. The ratio of glassy phase to solid phase (ie, remaining unmelted identifiable particles of original clay body ingredients, such as grains of silica) in the clay body becomes so great that the pot begins to succumb to the forces of gravity. The shape of the pot begins to change on its own. Making problems you thought you had masked or repaired re-appear (, thick/thin throwing problems). Rims of wide bowls lose the perfect roundness they had off right the wheel. Carefully applied handles on thinly thrown mugs begin to sag down, changing the shape of the mug rim. Slight imperfections in the shape of the kiln shelf underneath the pot begin to change its shape as the pot melts down to conform to the kiln shelf unevenness. Keep going like this and eventually it is all a molten mass stuck to the kiln shelf like those cone 10 earthenware disasters we always see pictures of. Devitrification (ie, “de-glassing”) is actually glaze ingredients re-crystallising (rather than remaining in an amorphous glassy state) after having been melted at some point. Often this happens from slow cooling (deliberate or accidental), but is a fairly complex topic on its own. What kinds of crystals form is highly dependent on the nature and amounts of the original clay body ingredients and what intermediate molecular forms they take. Usually we think of glazes devtrifying (eg, clear glazes getting cloudy) rather than clay bodys, which routinely have lots of crystal phase within and amongst the glassy phase. Eg, Mullite and Cristobalite are both commonly found crystal structures in clay bodies.
  3. I seem to recall that clayworks and walkers may have carried a few dry powder clays in the past, but only their more expensive ones, and possiblymcasting slip. Check their websites. But I agree with Liam that premixed clay is the way to go. Unless you have wet mixing tanks and a filter press handy, you will be hard-pressed to get consistency, plasticity, etc on par without a lot of small batch wedging. Casting slip is a bit different since it stays wet, but for throwing clay just leave it to the pros and buy premixed.
  4. curt

    Understanding COE

    I think if you add 10% flux (molar %) to a standard cone 10 clay body you are going to be getting some serious bloating, warping/sagging and other evidence of overfiring. That amount of flux is excessive at the relatively high firing temperatures of Cone 10, and IMO might only really (possibly) be suitable for a midfire or lowfire body. But I don’t fire at those temps so will let others comment on that. Take C Banks’ example clay recipe with 70% Silica and 24% alumina. That leaves 6% for flux, which is much closer to the flux levels for the collection of cone 10 stoneware bodies I am familiar with. In my experience 8% molar is just about the maximum flux a cone 10 stoneware body can handle. Minimizing free silica (what nerd is calling “ejected”) is important for reducing the chances of cristobalite (which is just a specific form of silica). But overfluxing is not an ideal way to achieve this. Better to minimise the free silica in the first place by not adding more silica to the clay body than is needed (ie, can reasonably be absorbed in to the melt), and making sure the particle size of that silica is not too small (because smaller particles get fluxed into the melt more easily). if you are not going with a (presumably pre-tested) commercial clay body, or a tried and tested recipe, than as usual the answer is...test, test, test!
  5. curt

    Understanding COE

    Nothing magic about 4:1. Many different ratios will work, including 3:1. Depends very much on what you need your clay body to do performance-wise. Much more important in my view is the ratio of silica+alumina to flux, particularly for functional bodies. And what kind of fluxes you are using - all fluxes are not created equal in terms of melting/vitrification power. And particle size of the constituent raw materials is also right up there in terms of importance. And...and... There is no one size fits all.
  6. Cracks o’plenty can come from firing. See Hamer and Hamer (web searchable) on Cracks for an exhaustive review. Bisque cracks can come due to firing too low or cooling too quickly, among others (read: trying to cut corners by firing too fast...)
  7. curt

    Clumpy glaze

    As said earlier, smells like some unhelpful interaction between the Gerstley Borate, the Epsom salts, the relatively high clay content, and possibly even the chemistry of the water you are using. All that said, I would focus on the Gerstley Borate and the Epsom salts. Are you dissolving the Epsom salts in warm water before adding them? If not, could it be that chunks of epsom salt are providing the “seed” for the clumps? Maybe try dissolving the Epsom salt first and seeing if that helps. if not, maybe mix up a small batch with no Epsom salts and try adding a small incremental amount at a time and stirring, and observe gelling behaviour. If nothing happens try the same thing with the Gerstley Borate, i.e., mix the glaze without it, then add it in in small incremental amounts and see what happens.
  8. curt

    Ash glazes

    Further to Neil’s point, 100 kilos of wood makes about 1 to 1 1/2 kilos of ash. Totally clean ash is OK, but a lot of what makes ash sexy is all that unknown stuff that comes along with it out of your fireplace and everyone else’s. However, when you source your ash this way every batch is unique, and never to be repeated. Let the dilemmas begin....
  9. Pres I like your question and understand that my comments here are offered in the spirit of provocative analysis rather than attempting to invalidate the issue. Maybe I don’t need to say this, but this topic may be emotive for some. Theft is a pretty strong term. Almost makes it seem like there was something private that you have taken away from the owner without permission. I do not view cultural symbols, icons or art in this way. They are in fact public, meant to be seen, acknowledged and interpreted by others. Further, “cultural theft” may almost be a contradiction in terms. Culture cannot be owned. On the contrary, it is a shared construct. Its manifestations are an invitation from insiders to outsiders to engage and participate. A culture’s ability to survive and thrive depends critically on its ability to be communicated and understood - and potentially adopted, or adapted - by those coming to it for the first time. Those treating culture like a secret birthright that only the high priests can discuss are missing the point. Success is where everyone is discussing it, learning it, sharing it. However, since art is a primary vehicle for communcating culture, using imagery or symbols from a culture other than your own in your artwork, possibly out of (cultural) context, is risky business. If misused, or possibly even when appropriately used, it could be misinterpreted, or seem like a cliche’, or possibly offend those who (legitimately) identify with those symbols as part of their own personal value system. A bit akin to driving without a license, or sufficient training or experience - you probably just shouldn’t be out there. Objects may be closer than they appear.
  10. curt

    Seeking Advice

    I would be very relaxed about this student interaction. Progress is an individual matter, and relative to the individual themselves. We all progress in our own ways, and according to our own capacities. More advanced students helping less advanced students is positive, unless this is for a serious grade or something. Ultimately, fellow travellers helping each other on the journey is a beautiful thing. Treasure it like gold.
  11. curt


    Most likely a bad batch of mugs, maybe a problem in the casting process? Judging by its location and intensity, that contamination in the pictures looks like impurities in the slip casting clay or the molds. It is too concentrated and deep and localised to be contamination you are introducing in handling at your end, or from your kiln environment. They are specks, not smudges, so unlikely to be handling contamination. Also, the slight fuzziness around the spots and the way they are breaking (at least on the mug handles) through the glaze surface suggests to me that they are a significant contamination (some kind of metallic based crud) coming from the surface of the clay itself, which is poking up through the glaze. Or possibly, but less likely in my view, contamination in the glaze itself (but the pattern of specking is too particular to be from the glaze in my opinion.) are you glazing them yourself of do they come pre-glazed? If unglazed, I might try washing the mugs carefully and examining with a magnifying glass around the rims and handles. Also maybe if unglazed try firing one without glaze and see if it happens.
  12. curt

    What kind of clay am I digging up

    Test all three levels. Form marble sized balls of each. Make sure they are bone dry and dry them in the oven (slowly) for an hour or so at 105 degrees Celsius to be sure they really dry. Just fire marble sized balls of each in some kind of container in the kiln to the hottest temp you guy. Note if they melt or not
  13. curt

    What kind of clay am I digging up

    Have you tried firing some in your furnace?
  14. curt

    Why is glaze blue

    Phosphorous in the bone ash may also be contributing to the blue. The blue looks far too uniform to be cobalt flashing to me

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