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curt last won the day on June 17

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About curt

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  1. Search this forum for threads on Ian Currie test tiles. I think this is very similar to what Dave is talking about. It is a good way to examine the effects of systematic changes in silica and alumina on a glaze.
  2. I was in Berlin a couple of years ago and found a small studio in Kreuzberg around the corner from my apartment making hand-thrown lamps out of Limoges. I spoke to the woman who ran it and she gave me contacts for a couple of other potters in Berlin but I ran out of time. I will have a look for her card. Very interested in what you discover as I am not sure if we have many German potters on the forums?
  3. LT thanks for this amazing reading list. I have spent some time with Shelby, very good value. Will trawl through the rest in days ahead. The only thing that bothers me a little about trying to apply lessons from the glass guys is that they do not try to make their glass batches on top of a ceramic mass underneath which they are trying to mature at the same time! To your point about igneous petrology, I found a paper recently on magma recently that seemed like they could have been talking about glaze! A silicate melt which heats and cools and gets affected by all sorts of alkalis, metals, etc. along the way. Surprising parallels.
  4. Agreed. It was not so much a conclusion, but rather more of an idea about a path for further exploration to help solve your mystery. My take on glaze eutectics is that the more oxides you have, the more complex eutectics have the opportunity to form, and the lower the temperature at which everything melts. Thinking more about it, I agree with you that the boron could be the catalyst. Boron starts melting at around 840 C and is a glass by 950 C, so (depending on how much boron you use) this could be getting the overall melt going way earlier than typical midfire temperatures. I wonder if this also means that oil spots could appear in a reducing environment if the temperature at twhich the melt really got going was lower than when the iron products in question started to decompose?
  5. Pieter, something I posted in a similar thread of yours last October: Phase diagrams for FeO-SiO2-Al2O3 show a eutectic as low as 1088C. This occurs with (approximately) 12 Alumina, 40 Silica and 48 Iron, an entirely possible combination. This is FeO but I will look for Fe2O3 phase diagram as well. The main message is that iron decomposition could be taking place at lower temps?
  6. Just had a look at Currie's Stoneware Glazes. There is a great discussion on oil spot glazes in the chapter on iron glazes, particularly on pages 166 and 167. One of the sources he refers to describes oil spotting as the result of bubbles of oxygen being released as red iron oxide decomposes to magnetite. Reduction firings cause this decomposition to happen too early, before the glaze has actually melted, so no bubbles form, which is why oxidation is generally necessary for oil spots. He also says there that the thicker the glaze, the larger the spots. Have you noticed anything like that? Also had a quick look at your epsilon-iron article, some cool close-up photos there of the oil-spot crystals. Viscosity and surface tension are closely related (see Hamer for a good discussion on surface tension), so Pieter no doubt both are at work in your tiles. But I am not sure we should expect that bubbles will disappear in a high surface tension glaze any more than crawling will. Once glaze has separated - either across a valley in crawling or across the circular void of a burst bubble - surface tension is not strong enough to smooth it over, particularly if the glaze is as you say rather viscous. And the bubbles in your arcs seem fairly large. I would again advocate for using a "6 oclock" mark on your currie tiles. Once you have applied the glaze to each cell, go back and score through the (still slightly soft) glaze with a needle tool in a short single straight line down to the surface of the bisqued clay body underneath. You can see this in several of the currie tiles in my gallery (occasionally I forget to put them in doh!). This mark gives additional information on which cells are melting (or not), how well, and which ones melting well enough to heal over mark. Sometimes this is not exactly obvious looking at the glaze surface. And if you can be bothered to go back and do Curries second-layer "striding man" (on pretty much all the tiles in his Revealing Glazes book) that also gives additional information about melting. I really like those multicolored spots you are getting. And your post has re-kindled my interest in going to back to look at the oilspots I got recently.
  7. One other thing: your story of the iron/alumina solubility, and the fact that the white stoneware tile clay body likely has much more alumina than the earthenware one, may explain why the overall cell character of the stoneware tile is darker iron look.
  8. Interesting Pieter, thanks for posting. I have looked at these tiles a few times since you posted, first trying to understand exactly what was going on and then thinking about what it meant. So alumina is held constant across any given row (eg, alumina is the same on a UMF or mole% basis in cells 1 - 5) but decreases from top to bottom. Meanwhile, iron is held constant in, for example, cells 1, 6, 11, 16, 21, 25, and 31, but then increased in the next column to the right. So is it fair to say the increasing iron across a row is proportionally decreasing silica and other fluxes? Also, can you tell us approximately where the original glaze is on your tile? I am not clear on how these tiles have been fired. You say the glaze is cone 4, but applied to an earthenware tile (cone 03?) and then to a stoneware (cone 10?) tile. Have the tiles been fired to these temperatures? And if so was the glaze designed/expected to work at these temps? Or were all tiles fired to cone 4 regardless of clay maturity? Can you clarify? I think this glaze has serious surface tension issues. the main tipoff is in cell 5, particularly in the earthenware tile. See how the glaze has pulled away from the corners and sides of the cell and mounded in the middle, even though it appears to have originally wetted those parts of the cell when applied? This is apparent in surrounding cells as well, and in the upper right zone of both tiles. Simlarly, I think surface tension issues are why the bubbles have failed to heal in both tiles. None of this is surprising given that alumina is the material with the highest surface tension available to us. I suspect that the arc of bubbles is higher up and to the right on the earthenware tile because the iron content of the clay (and/or possibly other ingredients in the clay body) is adding additional flux to the glaze. Hence the size of the well-melted zone is much larger on the earthenware tile, and the number of cells with good iron spotting in them is much higher in the earthenware tile. It is also interesting to me that the size of the iron-spots you are getting is very consistent across the tile. If you look at the earthenware tile, the spots are plain to see in many cells across the tile, but my take is that (coloration aside) those spots are largely the same size and consistency from cell to cell. If you look at my avatar to the left, or at this series of tile shots in my gallery, the size and character of the spots I was getting varied much more from cell to cell. What the consistency of your iron spots suggests to me that iron component of your glaze, while necessary for spotting, may not be the main glaze ingredient driving the spotting. Not sure about this. Finally, I also looked at the glazy.org site and your glaze pics there. Interesting to see that LESS iron produced a visibly runnier glaze. Have I got that right? Could additional iron be making the glaze more viscous? This would somehow seem to be agreeing with the evidence from your tiles as well. This seems counterintuitive to me because I see iron as a flux, and more flux should make the glaze runnier. But maybe that is true in reduction but not oxidation... Hmmm...
  9. I think Marcia is on the right track. The likely way we even out heat in the kiln at the end of the firing is to a) slowly work damper in a little at a time (to start to bottle up the heat in the main kiln chamber rather than letting it escape freely up the flu) while at the same time backing off fuel at the primaries slowly to prevent reduction, but not so much that the temperature begins to fall. Assuming you are trying to achieve a soak here. Or at least that would be the procedure we use to clean up the kiln atmosphere in the last hour of what would normally be a reduction firing. In your case, if you are in oxidation the whole way before this, then your damper may start this phase a lot more open than if it was a reduction firing. But this is just a matter of degree, not procedure, I think. As you may suspect, getting the adjustments, timing and balance right takes some practice.
  10. Trouble Glazing Locally Harvested Clay

    Here is one way to approach the problem of native clay: http://community.ceramicartsdaily.org/topic/14935-native-clay-odd-smell/?p=111579
  11. Firing costs are a big deal. If I had to pay for propane to fire my kilns, I would have to charge at least twice as much for clay to cover the firing costs, which would definitely affect my business. Costs matter. I get that. This is true for every business. However, if margins are so tight that firing costs begin to dictate what kind of ceramics we can and cannot do, I think that is a compromise too far. Not saying it has never happened, but if it does maybe more than just firing costs needs a rethink? Similarly, if we allow economic considerations to dictate what students in ceramics schools should and should not be taught, I think this misses the point of an education. Being a student is exactly the time to be shown and experiment with everything (including the finer points of running a ceramics business). Ultimately students need to see and contemplate the full breadth of what is on offer to make the right choices for themselves. Finally, there still seems to be plenty of ceramics of all firing types - stoneware, midfire, raku, woodfired, gas, etc.. being sold as fine art, at least where I am at. I don't think costs - firing or otherwise - is the primary driver for most of these fine artists (nor is money the only compensation).. They just do what they have always done, and a few are successful and can quite their days jobs, stop depending on their partners, etc.. Very few are hard-edged business people as far as I can tell.
  12. Hi Joseph, is this like a little hand-help open propellor gizmo that you stick down into the cup? Kind of like a hand blender but without the cover over the blades? Presuming you can just rinse this after each use in a small container of clean water nearby?
  13. I very much doubt that the future of this medium will turn on firing costs. And if it does I will hang my head in despair...
  14. Hi Nerd, Sounds like a great title. Looking forward to the book coming out. There was originally a post up here just after my question above, from you, responding to my question? Do you know what happened to that post? Did you take it down or has it just disappeared somehow?
  15. Sheer Thinning

    Not sure what Bralizian Luan is, but it sounds like serious stuff! Shear thinning is a much more specific phenomenon than just pushing water around by force. It describes temporary changes to the microstructure of the whole material, not just the water in it. Think house paint and ketchup (hat tip Wikipedia) and how they work when you touch them. Thick, then suddenly much thinner when you move it, then a second later thick again. Clay bodies don't work this way (at least mine don't). Point is, I don't think seeing this as shear thinning will move you in the right direction to understanding what is going on. Just my two cents.