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Everything posted by curt

  1. 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).
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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..
  7. 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?
  8. 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...
  9. 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!
  10. 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...
  11. 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.
  12. 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 bisque cones which we might be tempted to reuse? Will have to save some underfired 04 0r 05 cones cones next time the situation arises!
  13. 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 / thermochemical change process may be significantly advanced at the micro structural level before there is any change evident in the physical shape of the cone as seen with the naked eye. The point is this: If a cone is pre-calcined, then the melting process is pre-advanced. All else equal this used cone will (could?) fall much earlier than a brand new cone right beside it. Finally, add to this the fact that most of a cone’s heatwork takes place in the first part of its bending (see Orton’s own pictures regarding this) and I think the conclusion must be that re-using previously fired cones could give very misleading information about the heatwork actually experienced by pots and glazes (ie, overestimate it).
  14. 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 mostly not plastic. For plasticity and workability you need legitimate clay-sized particles - eg, ball clay or stoneware clay - in the tens of microns particle size. Think maximum 200 mesh or finer and you will be starting to get there. OR, you can try to crowbar in some plasticity with small additions of bentonite if you are willing to make this adjustment. But even that has its limits.
  15. 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) will probably not be too high compared to the price of the product itself . Oh, and better get some very clean silica while you are at it. But there is some good news! You can just skip the ball clay altogether - not necessary. Expensive, yes, but oh my do these make nice porcelain!.... Limoge wishes it was this white. If all this seems too hard just cave for some Southern Ice. You will actually be pretty close. Finally, better even than NZ Halloysite would probably be Dragonite Halloysite from the US (Utah I think). Even cleaner! But as a highly refined cosmetic and scientific grade product, probably even more expensive than the NZ stuff - IF you could even get it... would be interested to know if anyone has used it.
  16. 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.
  17. 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....
  18. 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.
  19. 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.
  20. Why is it that you don’t want to heat up the object?
  21. Is the complete deletion of individual posts in a thread now allowed on these forums? (Not talking about editing an existing post). If there is a policy on deletion could someone please direct me to it?
  22. Every bit is good! Look forward to seeing the results.
  23. Or maybe there was iron chunks in the glaze itself. This looks very similar to a glaze called Basic Ash from John Britts High Fire book. Except that MY ash has rust particles from my firebox in it, so when I mix it, it gives a similar effect to above. The tip-off (for me) was the inconsistent size of the iron spots, suggesting that the iron contamination is coming from the glaze itself... Maybe.
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