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Sputty

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Everything posted by Sputty

  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.
  16. Sputty

    Wide Range firing clay bodies

    Gives 500 million European 'people in general' a problem, doesn't it? 'Cos everything here is sold with a possible firing range. Perhaps we'll all just give up. Or, perhaps we'll all just say that, sure, each clay will have a theoretical sweet spot, but around that sweet spot is a relatively wide latitude which - for practical purposes - provides leeway which can be exploited to no detriment whatsoever. That's because you misinterpreted what was said. Look again at the context.
  17. Sputty

    Wide Range firing clay bodies

    Well, not quite 04 to 10 (1060-1300 deg C, 1940-2380 deg F), but a range is usually given that is broader than that given in the US, it's true. Is that because clay manufacturers in the US assume that their customers don't want the hassle of thinking, and want everything laid out for them just so, or is it because clay manufacturers in Europe assume that their customers don't give a cuss, and recognise that there are so many other variables that one more won't matter? Dunno. Either way, I've actually noticed no practical problem with this at all, to be honest, although I do understand from a nerdy point of view that a clay has an absolute optimum point of firing. But if that point is at the crest of a bell curve, then there's a range either side that gives perfectly acceptable results, in the real world. The stoneware clay I use is given as having a range between 1180-1280 deg C (cone 4 to cone 9). I use it at cone 6, and it's tight, strong, and with less than 3% absorption as far as my primitive devices can tell. Certainly never had a problem with it, and neither have my customers. The earthenware I use is given as having a range between 970-1050 deg C (cone 107 - cone 04). Same story as above, minus the absorption bit. Now, you could argue that there is a precise point in the given ranges of those two clays that gives an optimum - what? Strength? Whatever, it's not as important as is made out, in my opinion, in a purely practical sense. Somewhere in the middle will do, and will be absolutely fine for 99% of uses. I think what becomes 'common acceptance' is whatever works well for a given purpose.
  18. Sputty

    Wide Range firing clay bodies

    That may be true, although I could argue endlessly about the sharpness of the dividing line. How is it relevant to using an engobe? Or are you referring to my lead glaze aside? No, it didn't. Sorry. I must be a bit dense today, which would not be unusual. Whilst I genuinely appreciate your technical take on things, I also appreciate an empirical approach that builds a corpus of enduring knowledge for everyday work. Some people, when they make bread, obsess and fuss endlessly about hydration levels to the nearest tenth of a percent (I exaggerate, but only slightly), the relative importance and ratios of gliadin and glutenin, and so on. Others just go ahead and make perfect bread, regardless. Both camps are interesting, and justified in their viewpoints, but neither has an exclusive claim to the high ground (and I doubt you'd try to stake that anyway). Most of us sit somewhere in the middle, I suspect, and just want to make some pots (or bread) before we die. Back to @Polydeuces original post - Gimme stuff that works and cut the willy waving! ...I think is the translation. I've tried to do that, based on my own experience of what actually obtains in the real world.
  19. Sputty

    Wide Range firing clay bodies

    In extremis, that is of course true. But within the limits of everyday practice, I disagree. Well, I'd suggest you tell Robin Hopper, but he's taking a long rest, so you'll get no reply. In the meantime, I'll rely on his (and to a lesser extent, my) extensive - practical - experience of using the slip across a wide range of temps. In what way 'won't it work' at some point, in any real-world situation? Bearing in mind the purpose of an engobe? Why do you class that particular slip as cone 10, rather than cone 6 (let alone cone 04, where I have used it with great success)?
  20. Sputty

    Engobe Questions

    Why? I've used it at cone 04, and it was perfect. No issues whatsoever. Took to the pot beautifully, fired beautifully, took glaze beautifully. Utterly reliable whenever and wherever I've used it, at any temp I've tried. It gave an astonishing 'old English' cream under a lead glaze, and 'stained' with copper carbonate, an impossibly gorgeous green under the same. (Yes, I know, lead and copper don't mix, at least on functional ware. But the colour was phenomenal - superb for garden pots.) It is (or was) his slip for doing pretty much anything with (including mocha): SLIP-SLIDIN'- AWAY!
  21. Sputty

    Engobe Questions

    The engobe recipe itself (Florence's engobe de base) is for earthenware temps, yes. But the information about colourants is relevant at other temps, and the reason I linked to it. The engobe recipe I gave above is better, in my opinion. The Frit she uses can be used as a glaze on its own, virtually, at cone 04.
  22. Sputty

    Engobe Questions

    Not sure I understand your sudden fibrillation... I've used this slip from cone 04 to cone 8, with no problems whatsoever. Hopper used it all the way up to cone 12, equally with no problems. What am I missing?
  23. Sputty

    Engobe Questions

    Specifically with respect to colouring engobes, here are two interesting links: Engobes Faïence Petite histoire de mes expériences sur les engobes They're both in French, but both should be easy enough to figure out even if you have none of the language. Any problems with understanding what's being said, ask me. For a 'true' black stain, it's actually easier to purchase a Mason or something than it is to mess making your own. Various mixtures of Manganese, Iron, Cobalt, and Chrome can be tried, but in all honesty just buy it. For the sake of interest, have a look at this chart: Mason Stain Content Reference Near the bottom of the first table are a set of blacks, and what they contain (although not the ratios of the elements). In general, it sounds like you want to (say) cover your pot with engobe(s), perhaps sgraffito through it/them, biscuit, then glaze with a clear glaze. Sounds like a plan to me. In my slippy days, I used to use the following as an all-purpose engobe, courtesy of Robin Hopper: Ball Clay - 75 China Clay - 10 Silica - 10 Potash Feldspar - 5 Good for any cone. Get a Ball Clay low in iron, and you have a good base for adding stains. Mix (and sieve through 100's mesh) to a creamy consistency.
  24. Sputty

    Engobe Questions

    There was a recent thread about engobes which threatened to descend into unresolvable argument. Or perhaps it actually did... Anyway, it's worth a read, if only so you can come back and define your terms more clearly: are these engobes? From my perspective, answers are: 1) - Leatherhard. Thick enough to cover. Like a luscious chocolate enrobing. 2) - The engobe is just a slip which opaquely covers the clay underneath. Once fired, it will thus be the texture of fired clay - generally smooth, and 'tight'. Any gloss will be imparted by a glaze afterwards. 3) - Stains or oxides in the engobe will give you colours. You might have to use quite high ratios of stains, which will get expensive. There you go. That's from a European perspective. Specifically a French perspective. Someone will come along shortly and tell you something different - that an engobe is a vitrified slip, or a half-glaze, or some-such. At the end of the day, it doesn't much matter - what matters is what you are trying to achieve. Which is?
  25. That's why paperclay is the answer. Tens of thousands of tiny wicks to the surface make for fast, even drying. I would happily use my porcelain paperclay to make things many inches thick, without worrying at all. Yes, I do fire cautiously, but I'm not paranoid about it, just a little more careful than I might be with thinly potted stuff.
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