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Sputty

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  1. Oh, but the warmth, depth and personality of earthenware - the soul of the stuff! It's humanity itself (well, the best bits, anyway), like terracotta in the garden. Anyway, drifting ever sideways, this is interesting: Strength, Toughness and Thermal Shock Resistance of Ancient Ceramics and Their Influence on Technological Choice - (PDF) ...where 'strength' is defined as ability to withstand stress fractures, and 'toughness' is defined as ability to withstand impact, (and 'temper' is a deliberate inclusion of other materials in the clay, such as sand or grog). Both of which conclusions go against what has been said in this thread so far by everyone - me, (and by extension Pinnell and Arbuckle) included. By the lights of this research, high-fire (and low inclusion) ceramic pottery should have the better resistance to stress fractures (as opposed to Pinnell's findings), and low-fire (and high inclusion) pottery should have the better resistance to impact fractures (and hence be less 'chippable' <---- my extrapolation) (as opposed to Mark C's experiences, and I suspect most people's expectations). Assuming I've understood even a tenth of it, of course, which as assumptions go... It's just possible that things have moved on in 2000 years. Not quite the modern industrial practices I was looking for, but relevant in a sort of way.
  2. Sputty

    ^04 for dinnerware?

    I think that's right - the test is a Modulus of Rupture test. But it does seem that this is the method used in industry, as far as I can ascertain. I know very little about this, and will go off and look further. But it would be interesting if anyone else has any ideas about how tests might be done, and how one might realistically begin to measure physical durability of pottery. I can certainly see that what is effectively a 'flex it 'til it breaks' test *might* not seem adequate to test for, say, chippability - but what test would? As I say, I know next to nothing about the way industry tests these things, but if anyone else can chip in (ho ho), it would be interesting. In the meanwhile, I'll have a look to see what I can find when I have a moment. It's all interesting stuff.
  3. Sputty

    ^04 for dinnerware?

    On the other hand, Arbuckle and Pinnell make some very good points here: earthenware (longer rant); pete pinnell on clay body strength I particularly like Arbuckle's strong defence of earthenware. If only I could rant so cogently. TL;DR - properly formulated, properly glazed, properly fired earthenware is (if anything) stronger than stoneware.
  4. Sputty

    ^04 for dinnerware?

    True. I only mentioned it because it seemed rather likely that one of the reasons OP had previously come across for believing earthenware might be unsafe would be the use of lead in low-fire glazes. It never hurts to cover that particular point. It's all knowledge, after all, and all helps to paint the broad landscape of possibility. What is needed is a good handbook for earthenware/low-fire. There are so many for high-fire, and many for mid-fire. But I don't know of a single (modern) one that exists for low-fire, except for niche areas like raku, or pit-fire, or somesuch. I'm thinking of something like the 'Ceramics Handbook' series, which generally cover enough ground to get an interested beginner going. I blame Leach for being utterly dismissive of earthenware, and setting the direction of western ceramics for generations. ( <---- It's possible to spot an over-generalisation here, if you squint.)
  5. Sputty

    ^04 for dinnerware?

    ...and your experience here may be similar! In principle, there is absolutely nothing to stop you making earthenware which is both safe to use, and which is reliable. Many, many potters do so, although the majority of these appear to be outside the US. In Europe, it is commonplace. Random example: Josie Walter Josie makes functional ware, fired to cone 03, using a lead glaze. and very lovely it is, too. She is but one of many, and I could provide a list longer than you'd bother to read of others doing the same. HOWEVER: There is one potential problem with earthenware, and that is the glaze. Lead glazes are now supposedly the big, bad boy on the block. A badly formulated lead glaze will certainly leech lead, and that's not good. The good news is twofold: first, lead glazes can be well-formulated, and be safe. Second, there are plenty of earthenware glazes that are lead-free, and this above all suspicion (caveat: there are other baddies, but then that's not just restricted to earthenware). It would be recommended that - certainly for someone inexperienced - the latter course would be best. People will also tell you that earthenware is porous, glazes will craze, bacteria will breed, plot, and kill you in the micro-crevices thus formed. I've personally always found the argument to be utterly spurious. I can't find a single incidence in the literature of anyone actually being harmed in this way, and given the widespread use of functional earthenware, I wouldn't worry unduly about that. So: avoid poisoning yourself (effectively, use a well-formulated glaze, probably lead-free if you are of a nervous disposition), and all will be well.
  6. Hello - I suppose much depends on the volume and weight of your mom's exhibition pieces. I'm not sure I'd ever give anything fragile to an airline and expect it to arrive in one piece, so you may just end up driving! If the event is this - Argilla - then it looks like Finland were the guest country in 2012. There's an exhibitor list (PDF) available - perhaps you could contact one or more of those who took part then, and ask how they did it? There seem to have been about 16 from Finland (see page 7 of the exhibitor list). Looks like a great opportunity, though. Hope it works out!
  7. Sputty

    Wide Range firing clay bodies

    It's interesting. When I first touched clay seriously in the mid-1980's, we used a clay called Moira, from Burton-on-Trent in Staffordshire, England. It was beautifully plastic, very nice to throw, and fired to a pleasant creamy buff. This was a clay that was essentially dug out of the ground, slurried, screened, and then filter-pressed into usable cakes. That's it, no other processing. As is fairly common, the clay seam sat next to a coal seam, and we'd occasionally get specks of coal in the clay. As these specks became more frequent, and bigger, we guessed that the seam was running out. Sure enough, production became erratic, and stopped soon after. Fast forward a few decades, and the availability of what might be called semi-synthetic bodies is widespread. They're a long way from any coal seam. Now, you might say something has been lost along the way - the old clay had 'life', 'character', 'complexity' - and I'd probably agree. But there are gains, too, not least the opportunity to tailor the characteristics of a body in a far more controlled manner. And of course, there are still plenty of the more 'natural' clays to be had if you want.
  8. Sputty

    Wide Range firing clay bodies

    Mostly white, cream, or buff, although a couple are brown/red, or flecked with stuff. So in general, not much iron.
  9. Sputty

    Wide Range firing clay bodies

    I think the reference to cone 04 to cone 10 was in relation to an engobe rather than a body. And I'm still happy with the assertion that as such, it's fine, especially having used the particular recipe extensively at the lower end of that range, and (less extensively at the mid of that range). Hopper, whose recipe it is/was, stated that he'd used it much higher. I have no reason to disbelieve him, and indeed every reason to just accept what he said. An engobe has much less 'to do' than a body, and thus can be viewed in a more flexible way. That's my take, and empirically, it seems to work.
  10. Sputty

    Wide Range firing clay bodies

    Heh! Well, that's what I was told today, after I asked questions of manufacturers and suppliers. Exactly as you describe - mature across the whole range. Perfectly logical - if the clay has an absorption rate of less than 1% at the lowest temp in the given range, it's only going to stay the same (or improve) as the temperature increases. The trick then is to get the clay to behave itself (not bloat, etc.) as it goes up the scale. That's the achievement here. You have to accept that these are manufactured clays - they're not just dug up from some seam or other, cleaned up, ready for use. But that then gives the possibility for all sorts of trickery to extend firing parameters, etc. As I said above, this is a well established and highly successful range of bodies, used across Europe, but particularly in the UK. I first came across them 10 or 12 years ago, and was suitably impressed with the couple I tried.
  11. Sputty

    Wide Range firing clay bodies

    But the clay in the example above has an absorption rate (or whatever you'd like to call it) of less than 1% at its lowest recommended temperature. That's not going to weep, is it? I understood than anything less than 2 or 3 % is considered pretty good. Bear in mind that these clays (and there is really a very wide range of them) have been developed only over the last 15-20 years to a specific end - flexibility of firing. They are designed to work in that way, and do. It's possible that this is a European thing only, and that developmental process hasn't happened in the US.
  12. Sputty

    Wide Range firing clay bodies

    Apparently not. See the answer I was given directly above the post directly above this post. (Eh?)
  13. Sputty

    Wide Range firing clay bodies

    I couldn't agree more. Let's hope your suggestion is widely adopted.
  14. Sputty

    Wide Range firing clay bodies

    OK - so I've had one reply to my enquiries so far. And it's fairly interesting, I think. I asked about a (randomly selected) clay that is being marketed as a clay which matures between 1180-1300 °C (2150-2370 °F) - a fairly large span of cones, I think you'll agree, perhaps cone 4 to cone 10. I am told that the clay has been developed to have less than 1% absorption at 1180°C. It can then be fired between the lower temp to the maximum before the clay would start to bloat, which in this case is some 1300°C. This effectively means that it's possible to get a clay which won't seep, across a large firing range. A temperature/cone is chosen within that range that suits glazing requirements. It's a generalised explanation rather than a particularly technical one, but it'll do me. This clay is one of an extensive range, hugely popular in the UK and across Western Europe, and the same explanation applies to its sister products. Now, maturity of a clay may be more than just whether it has a low absorption or not, but that does seem to be the major concern when discussing the issue. (@Pres et al.) Unless physical strength is severely compromised by firing at the extreme low or high end of the range, and I have absolutely no reason to believe that to be the case in any meaningful way, then I think it really can be claimed that the clay has a wide maturing range, at least as far as the practical concerns of the everyday potter are concerned, and that it is perfectly legitimate to market it as such. Should I hear back from the second manufacturer, I'll report further.
  15. Sputty

    Wide Range firing clay bodies

    To return to the (evolved) topic, I've emailed a couple of European manufacturers for their take on the matter of describing clays as having a wide-maturing range, rather than simply indicating a theoretical (and narrow) optimum. It will be interesting to see what they have to say.
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