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curt

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Everything posted by curt

  1. If you are recycling clay it could be contaminants getting picked up in your clay body from somewhere in your process, which may explain why some pots have it and some pots don’t. Have you tried firing with brand new clay right out of the bag. Not thrown, not even wedged, just straight out of the bag and in to the kiln?
  2. Also, you may want to get their specification of “dust” before loading up the trailer. My experience is that what you and I call dust may not be what they call dust...
  3. Agree it looks underfired just from the picture. That satiny-matte-like finish with a surface that is not quite smooth looks like other underfired work I have seen. Also the opaqueish whiteness (as opposed to transparency/clearness) suggests that there are underfired ingredients in the glaze which have not combined in to the melt. also, eyeballing the ratio of outright silica to alumina in this glaze - before even including the kaolin - already firmly suggests matte glaze to me. Add in the kaolin and it would be even more matte I think. Perhaps the stains were meant to add some (a lo
  4. Yes now that is a cooling dunt. Very sharp edged, sharp enough to cut you if you run your finger over it with a bit of pressure. The earlier crack does not have anything like this. The glaze knew the crack was there from the very beginning and simply pulled away from it throughout the firing, just like it was pulling away from ridges elsewhere on the pot (since it is pretty clearly a breaking glaze).
  5. What Neil said. Glaze pulled away from crack just like where it breaks elsewhere on the pot says this crack was there pre-glaze-firing. Likely from Bisque either fired too soft or Bisque cooled too quick, or both. Have seen similar cracks in our studio. Or as Neil suggests possibly there from drying stress for the types of reasons Babs quoted above.
  6. I got a brand new Shimpo VL Whisper from the US a couple years ago, and directly on the circuitboard there is an option to change voltage from 110 to 240 by unsoldering a connection and resoldering it into another connection nearby. Then you don’t need a voltage adapter of your own. I did this and it worked fine. Just need the physical plug adapter, but make sure it is grounded! For a while mine was not and I can tell you that is not good. Shimpo had instructions on how to do the soldering change which were able to be downloaded from their website if I recall. Not sure if your whe
  7. No foot at all really, Babs. The outside line of the pot was just straight down the wall (right past the level of where the bottom was inside) with a slight turn in at the very bottom outside to create a small shadow line. Amazing how it almost sheared off all the way around, just slightly below the level of the inside bottom.
  8. Had exactly this happen with a 15” round casserole dish I made out of a nice porcelain. First use in the oven the whole bottom cracked off just around the foot ring so neatly that I was able to save the (slip decorated) bottom and turn it into a nice cheese plate. Fine, tight clay bodies with small particle size and lots of glass in them when fired, do not like thermal shock is what I learned from that. So I would bet that the clay body is the issue in that use.
  9. Don’t have a specific kiln in mind for this yet. Just exploring what may be possible for the moment. Also keen to think it through as much as possible before actually exposing any kiln to this kind of treatment! So far it seems straightforward. Kiln will cool as fast as you want. Low thermal mass of fibre enables quick cooling, and presumably the kiln frame can take it. Open the door, blast the kiln vent, use whatever method you want to cool it down fast. I take your point about uneven cooling but the idea is that simple forms can accomodate differing temperatures....hopefully.
  10. How fast is it possible to cool a kiln from stoneware temperatures? Are there limitations as far as the kiln is concerned? Maximum cooling speed? Pretty sure this would be very hard on kiln bricks, shelves, props, etc., so let’s assume I am firing a FIBRE kiln with sacrificial kiln furniture (or none at all) and ware that is impervious to thermal stress. What is to stop me firing to top temperatures and then shutting down and then, say, running the kiln vent full blast for several hours? I have read industry does this kind of thing regularly, with incredibly fast “cool-to-coo
  11. Yes thought it might be Flir. Was looking at them a while back. They seem to be the most widely available in many different models. Good to get a real user review! I imagine I will be able to point it at myself and get some idea of whether or not I should even bother venturing into the studio...
  12. Amazing Bill and great pics! What kind of camera, software, cost, etc if you can? Seems like a new toy I need for my studio for sure!
  13. Bill yes agreed interesting discussion, plenty to explore here. Completely agree with this, and of course the whole notion of a eutectic (groups of materials which melt together at a lower temperature than any one of them alone) is a well established tool that every pot we make depends critically on! Boron is interesting outlier here because - all alone and on its own - it starts melting at 300 C and is completely fused by 700 C (see Digitalfire on Boric Oxide). Basically this means that in any clay or glaze containing boron, the melt process is actually starting way before bisqu
  14. Bill do you (or anyone else) have a table of the chemistry for Orton cones they could share? I only have an old one for Seger cones, and although they are similar I know that there are slight variations. I also read on Orton’s website that there is other stuff in Orton cones (eg organic binders), and possibly many other additions beyond the basic ones(eg, quite possibly some Frits?), so they are likely a bit more complex than just the standard raw glaze materials we use. Point being that all these ingredients are designed to ensure that Orton cones always fall when they should. And
  15. Bill, Given melting temps and eutectics of the materials in the cones, I suspect heatwork is starting well before the last 250 degrees (Fahrenheit?) of firing, particularly in the case of mid fire or stoneware temps. Do you have a reference on this? However, if you are right, cones that have not gotten to within 250 degrees should be identical to brand new cones (say, in a failed firing where the kiln was shut down well before reaching top temp)? I guess this would be a very testable proposition... Did get me thinking about whether or not these same results would hold for bi
  16. I have reused cones a few times, including next to brand new ones, but can’t really tell you whether it works reliably or not. It seems roughly good enough, judging from glaze results on pots, etc.. But knowledge of the melt process tells me that even though a cone is still perfectly straight after firing, chemical change is nevertheless occurring inside the materials of the cone. Boron for instance is/was used in Seger cones up to and including cone 6. Now, boron starts melting at a very low temperature, and thereby begins to pull other materials into the melt. This melting / t
  17. Some silt in your clay body is tolerable - but probably not more than 10% by weight. Think of it like grog. It is not really going to melt in the firing, and will form a large particle aggregate which acts like gravel does in concrete. but, I think as Bill said you want way finer size if you are looking for actual clay particles in the classic sense. Silt - even fine silt - does not behave like clay and is almost impossible to work with. It just falls apart when you try to work it. So it cannot make up the majority of your clay body. Even most Kaolins at 100 or 200 microns are
  18. There is a price for everything, and at the right price everything is available... NZ Halloysite can certainly be had from Walker’s Ceramics in Australia - if you are willing to pay the shipping I suppose. Of course you want to make your porcelain from it not just due its (very) low iron content, but also for its (very) low titanium content, because it is the both of them together which conspire to make otherwise white clay grey and yucky (not just iron alone). Veegum T can also be had, and international shipping on a 100 gram container (which is probably all you can afford) wil
  19. Porcelain becomes pyroplastic (“melty and saggy”) at high temps. Nature of the beast. The more cantilevered a form is the more likely it is to sag. You can try firing a bit lower - say 10 or 15 degrees (but your glaze may not like this) - or making your forms a bit thicker so they stand up better. Or try another porcelain that is less fluxed.
  20. Glad to hear you are having some success! do you want the plates to flatten out or not? Not quite clear if that is a good thing or a bad thing....
  21. This is a very interesting and worthwhile discussion, since we have all probably spent many hours agonizing over how to get a better clear glaze recipe. (I know I have). In my own testing, particularly with currie tiles, I have seen a few things: 1.The most consistent and predictable crazing is because the simple overall amount of flux in a glaze is simply too high compared to the amount of silica and alumina in that glaze. This happens particularly in the bottom left, or "C" corner of a typical currie tile, like clockwork. The implication is that a crazing glaze is quite likely si
  22. Agree with Min and Neil. That kind of cracking (right across the middle of a wide flattish form from side to side) is the classic glaze-on-one-side-only problem having to do with the glaze having a higher CTE than the clay and “pulling” the clay on the glazed side putting the clay body under stress until it gives up and cracks to relieve the stress. The thinner the clay object and the thicker the glaze layer the more likely it is to happen.
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