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About curt

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  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 lot?) of additional fluxing power to the base glaze and hence it cannot be used without them?
  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 wheel would have this option but worth investigating while you are figuring this out.
  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. 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..
  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-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?
  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 bisque temps. And one thing we know about ceramic melt processes generally is that once one material starts melting, it tends to pull other materials into the melt more quickly, accelerating the melt process and the whole thing snowballs from there. This is one reason (beyond the eutectics themselves) why the more fluxes you have the more likely you are to get a good melt. I can see how your 250 F rule of thumb is quite useful - and reasonable! - in the applications you are mentioning such as teaching concepts, automated control, overall heatwork evaluations, pizza cooking ( :-) ) etc, it is a useful approximation... And no doubt that last 250 degrees is the most potent part of the heatwork process not least because, at that point, pretty much everything has been pulled into the molten material of the melt. Only its own stubborn refractoriness - in spite of now multiple eutectics now going on around it - will save a given material it from being assimilated into the Borg, I mean the melt. The twist in this particular thread is that by re-using cones we are accumulating heatwork over multiple heating episodes, rather than seeing the cones in the context of a one-shot, continuous-straight-throught-to-the-end process (which is the other 99.99% of the cases). Since time - at some level and to some degree - does matter to heatwork, then if the melt process at the micro-structural level of the materials is getting going early (depending on materials, eutectics, etc.) , this would lead to a situation where every time it is used a cone is storing up heatwork, starting in many cases at surprisingly low temps. And the more times we use them the resident heatwork ratchets up. Wouldn't it be handy to have an Orton research technician wade in about now?? One other loosely related observation I will add is that I was always taught that bone-dry clay bodies do not shrink when fired to bisque. After testing quite a number of clay bodies of all sorts I now believe this is incorrect (note to those of you who nest things when bisque firing). Assuming this is true, why is there shrinkage from the dry to bisque stage? Is it shrinkage due to pore and molecular water loss? Is it shrinkage due to some kind of sintering loss? Or is it to some degree shrinkage due to the beginning of heatwork amongst the constituent materials? Just asking as I do not have a good explanation. I can hear the voodoo drums starting up in the background now so will stop here...
  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 no doubt they are not intended by the manufacturer to be reused Lol. My point earlier was that it is highly likely that the materials in a cone are undergoing heatwork well before the cone starts to deform. More specifically, I believe that the “relevant heatwork zone” (for our purposes here) is far greater than just the last 250 F. Boric Oxide, (which was used in Seger cones up to cone 6, and is still used in some Orton cones?) starts melting at 300 Celsius, and is fully fused at 700 C. The potash-alumina-silica eutectic (eg, as found in a frit) softens (is no longer a solid and begins to move) at 870 C. Just because the cone is still straight does not that mean dramatic changes are not taking place inside of it! And, as you point out, the fact that time and temperature ARE to a small degree substitutes in heatwork, somehow confirms that, while we may never be able to soak a cone 6 down at cone 3 temps unless you hold it for an incredibly long time, some kind of heatwork is still taking place on that cone. Lastly, I would say that all these observations should also generally hold true for the bisque-level comes. In particular, if they have boron, I would suspect that there is much more going on in the cone than just sintering (which is more of a clay particle issue?), which means that they are also accumulating heatwork effects well before the last 250 degrees. (Even if they aren’t bent at all) Rockhopper, to borrow from an older thread on this topic, cones are just mile markers along the firing highway, which have no relationship to the particular kind of car you are driving. Hence they may or may not reflect the heatwork your pots may have received, only your pots themselves will know this. Also, the stop-start process of reusing a cone may add an extra dimension of variability to the whole heatwork process (eg it is easier to re-melt a glass than remelt raw materials) which we really haven’t discussed here but which is probably also relevant for your pots. Kind of an interesting idea though.
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