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curt

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  1. I would be looking at the calcium borate frit. What chemistry? Could be a low melter making trouble with the other glaze ingredients...
  2. 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?
  3. 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...
  4. 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 lot?) of additional fluxing power to the base glaze and hence it cannot be used without them?
  5. 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).
  6. 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.
  7. 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 wheel would have this option but worth investigating while you are figuring this out.
  8. 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.
  9. 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.
  10. 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. Neil is your test kiln brick lined, and if so have you noticed any degradation from fast cooling? (In it, or any other kilns you have examined?) So that brings us to the ware. Tiles are what I am thinking of, and possibly kiln shelves - or pieces of kiln shelves cut up - that would hopefully take the thermal stress. (or that you wouldn’t mind terribly losing if they cracked.) would start with standard stoneware bodies and if they didn’t survive look at formulating hardier body recipes. As you said Neil, raku bodies are different: grog-ier and more open, and not intended to go to full vitrification. If quartz inversion is the danger zone, then maybe look at low silica, high alumina body recipes to reduce this risk? the dry pressed tiles are interesting, will investigate further, ie how much pressure to form, etc..
  11. 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-cool” firing times. And we know that raku firings are basically this. Trying to see if it is possible to fire a kiln from 5pm start to midnight cone 10 then shut down cool it to be able to open and unload it by, say, 7am the next morning. Dealing with kiln issues first, worry about what kind of ware later... Thoughts?
  12. 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 simply an over-fluxed glaze. So the time-tested remedy of adding more silica or clay - or both - makes sense. If you don't know what a currie tile is search these forums... However... 2. Simply adding more clay and silica in the same ratio quickly becomes impractical, mainly because glazes with too much silica and clay compared to fluxes simply become too viscous and do not melt or heal over well, mostly because silica and alumina are very refractory and stiffen a glaze, something Neil was alluding to above. The extreme version of this problem, ie when there is far too much clay and silica in the glaze (or possibly the clay - see below), is crawling. This effect is also very evident, particularly on the upper portions of a typical currie tile. One approach, if you have too much silica and clay and not enough flux), is simply to apply more heat, ie, fire higher. And I think this is something that and and should be considered, in addition to changing chemistry, particularly if alkaline earth fluxes such as calcium and magnesium are available, Their melting power is good, but it takes more heat to crack it. Oh, and as long as you clay body will take it. Anyway, if you had to add something to adjust the chemistry, my tests suggest that glazes are much more tolerant of more silica than of more clay. 3. Once you are beyond the issues outlined in 1 and 2 above (ie, there is not too much, nor too little clay and silica in the glaze), the clay body becomes a very important factor. Stoneware glazes all produce a glaze-clay body interface layer between the glaze and clay body when fired, where the glaze and the clay body "intermingle" at high temperatures, creating a new , third layer, which has chemistry of its own, distinct from that of either the clay body or the glaze themselves, but more like some intermediate version of the two. My take is that when glazes are thinly applied (as we like to do with clear glazes), this interface layer has a heavy influence on glaze appearance and behaviour, including crazing. This interface effect appears most prominent in (relatively) heavily fluxed porcelain clay bodies, where the flux at the surface of the clay body appears to be augmenting the flux in the glaze as it intermingles, resulting in a somewhat over-fluxed final glaze chemistry. The fine particle size of clay body materials in a porcelain body (and also of the materials in the glaze) is likely also playing a role in the glaze melt, as is the fact that we are usually trying to fire porcelain to porosity of near zero. In any case, the thought is that all else equal, if you have a very fluxy, fine-particled clay body that you intend to fire to near-glass state, you can probably get away with even a bit more clay or silica addition than with, say, a coarse, groggy sculpture body (not that you would be putting clear glaze on such a body anyway?). So, as Min suggested, looks like I am ending up in the same place as others here, but perhaps for somewhat different reasons.
  13. Every bit is good! Look forward to seeing the results.
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