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Tyler Miller

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  1. I seem to have stepped into something unintentional. I said two things in my initial comment. 1) There is a legal aspect of this that should be discussed with the proper experts 2) There is a ceramic aspect, which, given the conditions of the OP, is terracotta. I also said flameware bodies are superior in certain applications. The reasoning for my comment was the cost of research and development of a flameware body for personal use is disproportionate to its benefit. I would argue that the proper development of a flameware body for market is cost prohibitive for reasons related to the legal aspects (in Canada, I wouldn't try it). I then came to the defence of terracotta as a non-toxic material. The cited website's claims and studies are absurd, but so is the worry of toxicity of fired terracotta. My reason for saying this is that it's not a known issue. Firstly, if there were lead or cadmium in a meaningful form in a commercial terracotta body we'd know about it, especially the Californians. MSDS would contain it. So it's not a mainstream issue. Secondly, if you were to dig for the clay, you're doing a lot of testing,if bringing that to market, and the clay content is going to come to light anyway. Thirdly, there's no history of terracotta causing poisoning (FDA calls this generally recognized as safe). I'll fully concede the drawbacks. It is porous, until properly sealed. But so is cast iron, which will rust and leach and cause all kinds of issues if not properly cared for. It will break, and probably when fully of liquid and hot. I would never sell a terracotta body on any scale, and that's where a proper flameware body comes in. I'm of the opinion that studio potters shouldn't mess with flameware commercially because it's too much work and too much trouble to come back at you when things go bad. I apologize. I've misrepresented myself. I think we're in more agreement than initially thought, the devil is just in the details.
  2. Bill, I took the time to find the context of the article you posted there. I'm not sure it's saying what you're thinking it does? Full article you excerpted is here: https://maxwellsci.com/print/rjees/v3-541-545.pdf Health Canada's maximum acceptable concentration for arsenic in drinking water is "10 ppm" this can be expressed as 10 μg/g (see link here https://www.healthlinkbc.ca/healthlinkbc-files/arsenic-drinking-water). 10 micrograms per gram is the maximum concentration allowed to be contained in drinking water. The article expresses that the average As content of the earth's crust is 2 mg/kg of soil (2 ppm), with a range of 1-50 mg/kg possible (pages 541-542 of the article). In the context of this article's discussion on clay soils containing arsenic and mercury. 4/11 samples were within acceptable limits for drinking water, 3/11 are more or less at the MAC, and those that weren't are within 8.47 μg/g (ppm) of being so. And that's unfired soil samples and a limit set for drinking water--likely a very conservative limit for soil for clay. There is no mention of a firing schedule, temperatures reached, etc. This article, to mind, establishes that arsenic poisoning from clay from the developing world isn't an issue if the clay isn't contaminated. The devil's in the details, in this case, knowing that 10 ppm of As is an acceptable standard of drinking water and that μg/g of something is equal to its ppm. I would not put too much weight in the authors' knowledge of ceramics however, because they advise "the earthenware products should be well fired to reduce Hg levels to the lowest minimum since Hg has a tendency to evaporate at high temperatures." Arsenic also does this, as previously mentioned, and the mercury level is remarkably low (I'm sure your broad experience will agree). This might suggest that the implications of this data may not be entirely clear to the authors. If, as you say, As is at these levels pretty much everywhere. We're golden. No issues. Arsenic is within acceptable levels in some cases to drink the soil. Not saying anyone should drink soil, but what Health Canada says and the Ghanan scientific community have to say is interesting. I would like to see the many cases of heavy metal poisoning from eating clay. Soil ingestion is usually cited as a potential cause of lead poisoning, but it's also not something I've heard of happening in real life (maybe it was a thing when gas and paint were still leaded?). I feel like poisoning via soil poisoning is more likely to come from bacteria or parasites than the discussed metals and arsenic. Cadmium in rice: I don't know a lot about this, but the sense I get is that it's a concern in the sense that Cd is indeed an issue with rice, there have been historic incidents of contaminated grain, but actually characterizing the risk has been difficult. It seems like more of an economic issue (rejecting grain that exceeds maximum allowable levels). Articles that gave me this sense: Canadian discussion of Cd in rice. In over 55% of samples Cd was undetectable. https://www.inspection.gc.ca/food-safety-for-industry/chemical-residues-microbiology/chemical-residues/cadmium/eng/1369229974936/1369230037986 2008 Hong Kong discussion on this issue https://www.cfs.gov.hk/english/multimedia/multimedia_pub/multimedia_pub_fsf_26_01.html 2017 Article discussing cadmium uptake. https://www.sciencedirect.com/science/article/pii/S0269749116321376 Interestingly, Arsenic might be more of an issue with rice than Cd: https://www.fda.gov/files/food/published/Arsenic-in-Rice-and-Rice-Products-Risk-Assessment-Report-PDF.pdf As for terracotta being practical, useful, special or ideal, as a student of world cuisine I'd strongly disagree. Things heat and caramelize differently in earthenware. A well-seasoned tajine or cazuela is magical, a seasoning unto itself. Clay pot rice (I think called smashed flying rice) from Vietnam is one of the best things ever. Beans taste better when cooked in unglazed clay and La Grande Confrérie du Cassoulet have strong opinions on what kind of vessel their prized dish is baked in (glazed earthenware . This is a point of taste, no one ever need cook on terracotta and life is complete without it, but to me, it's definitely special, like cast iron or tinned copper ware. All the best guys. Edit: In the interest of balanced discussion, I've found this article on Fish consumption advisories I'd bookmarked (in particular, Ontario's, which I use) which includes much more stringent consumption guidelines (and a critique of their adequacy). I suspect the reason for these guidelines being more stringent is that the chemicals found in fish are much more bioavailable than in drinking water. An example of how this is significant is the potential for a dammed waterway to release inorganic mercury from the soil into bioavailable things like methyl mercury by means of bacteria action. Google Site C Dam in BC for a current case study of this effect. Article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5381969/ Relevant Table: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5381969/table/t1/?report=objectonly And a much loved resource (don't eat salmon from Erie or Ontario): https://www.ontario.ca/environment-and-energy/eating-ontario-fish
  3. The website is health washing. Re: lead cadmium and barium (and arsenic! mentioned on the website). As long as your clay isn't coming from an industrial waste dump, it's a non issue? Cadmium that doesn't come from ceramics really only comes from industrial waste (google "itai-itai disease"). I'm pretty sure arsenic salts as a general rule sublimate out before earthenware matures (google that, though). I'm also pretty sure that any chemically significant amount of lead in a clay body (significant enough to cause toxicity issues) would have undesirable effects as a body flux on the clay in question. But that's giving too much credit to the issue. Before you have a solution you have to have a problem. I have never once heard of someone being poisoned by means of a terracotta vessel. Risks of lead glazing, cadmium pigments, chrome pigments, copper, cobalt, silica, barium etc etc all of these are characterized well enough in the literature to establish some kind of risk assessment. But where are the clay poisoning cases? People eat clay (Geophagia - sometimes in areas of poverty and famine by pregnant women to obtain vital minerals in the absence of other sources), people rub it all over their bodies, use it to stop bleeding (google Coolclot). Like, I'm sure someone's made a pot with mine tailings and that didn't work out for the end user, but it's literally not an issue. Just maybe no anti-scumming barium?
  4. There are two issues at work here. One ceramic, one legal. The legal aspect, is jurisdictionally related and is best left to the OP to figure out in their jurisdiction. Legal advice from legal experts, ceramics advice from ceramists. Mmkay? Good. The second question is re: non toxic cookware body. Terracotta, as has been mentioned, has been considered the standard cookware the world over for millenia across all human cultures. Firing low (like cone 017), sealing with oil, and using a heat diffuser on the stovetop are all you need. There are other flameware bodies (which are better suited to certain applications), but if it’s for personal use, not commercial, or if it’s for a market that you’ve done your due diligence on, terracotta is the best option. Flameware clay bodies require a lot of R and D to make work, especially of glazed.
  5. https://en.m.wikipedia.org/wiki/Green_liquor It’s prolly this. All the necessaries are in the ash and iron is more likely a metallic contaminant than copper—occurring in an abundance orders of magnitude greater than Cu. This paper maybe helpful: https://www.nrcresearchpress.com/doi/pdfplus/10.1139/v78-462 (edited to make functional link) Kind regards, TM
  6. You’re missing flux. While I wouldn’t put too much weight on this comparison, the rice straw ash probably has more in common with the feldspar than it does with washed mixed hardwood ash.
  7. Wood ash is generally a source of flux, unwashed woodash a mixed of soluble potash and soda ash, washed woodash is mostly calcium carbonate. There is generally very little silica (with exceptions). Rice straw ash is high in silica (over 50% I think), like bamboo ash and other grasses—wheat straw ash can be silica rich too. Rice hull ash is almost entirely silica.
  8. Neil, I realize you addressed your comment to glazenerd. That said, if the sample isn’t ball clay or terracotta, the analysis coild be kaolinitic soil or something like it—something not too common in North America, but very common in tropical countries. Kaolin, organic matter, and maybe some silica sand, which tends to hang with kaolinite. 18% LOI—assuming complete analysis.
  9. Not as is. Is this a raw, as dug, product? The iron content will make a cosmetically unacceptable tile.
  10. Try to trim like the first three examples (badly drawn—my hands are a little cooked today), so that you’ve got the same thickness throughout the same pot. The foot ring shouldn’t create any thick spots where it joins the rest of the pot. The two X’d out show examples of pot bottoms that are likely to cause problems, either in drying, firing, or as weak points in use. They’re supposed to represent a bowl with an outer contour that doesn’t match the inner (bottom bowl) and a bowl with an extra thick ring around the base (middle right), which will cause a whole constellation of problems for all the reasons others have listed above. Uneven drying stress, uneven firing and cooling stress (dunting), etc Likely extraneous, but sometimes visual examples help learning. When you trim, check your work by cutting a few in half.
  11. Bill, I’m sorry, I’m afraid I’ve given you the wrong impression. To clarify, cristobalite inversion is mostly done by 200C (392 F), and is traditionally considered to happened between 210 and 250 C (410-480 F) which is hotter than the above quoted 300 F (150ish C) by Neil et al. Sorry again for the confusion. I just wanted to frame a temperature in terms of what’s going on in the clay body/kiln, since that’s what matters. A temperature is just an arbitrary number until the mechanics of why it matters are there—and i feel like the OP deserves that kind of answer—at what temp and why. Sorry, I wasn’t clear with what the temp actually was—a poor assumption on my part.
  12. I sortof thought it fell under the category of pyroplastic deformation, but creep might be an interesting way to look at it? I don’t know how useful those chapters will be in answering that, honestly. The reason I recommended it is that it talks about what the author calls “scatter of mechanical properties,” which is essential to understanding the behaviour of ceramic, in addition to the chapters “Fracture mechanics” and “Determination of Strength.” There are a lot of books that seem to be called “Mechanical properties of Ceramics,” but one with a more general chaper on deformation (as well as fracture): Pelleg, J.: Mechanical Properties of Ceramics, Springer, 2014 It’s geared toward an undergrad audience, though?
  13. Solid bibliography, LT! Looks like there’s a newer edition of the book I recommended in there. I’ll confess I probably got the original citation from a course biblio like that. A habit I’ve gotten into for research is to assemble a file to cut and paste citations into for later ref. Journal articles cite books or other articles I can’t read at the time, but feel might be useful, so i copy and paste the citations into a file. You end up getting a solid bibliography on a topic pretty quick. Sometimes a mailing list is a help for new stuff, if you can get on a publication review mailing list. I don’t know of ceramics one (maybe someone here does), but the Bryn Mawr Classical Review is forwarded to a file folder in my google account that now works as a searchable database for all forthcoming work. For those looking to buy these books, I recommend something like abebooks. Academic books are stupid expensive and hard to find, and for studio purposes the most current edition doesn’t matter. Universities are great for getting around academic journal paywalls and you can usually access them from whole if you have an alumni password.
  14. This book might be useful in this discussion: Munz, D., Fett, T.: Ceramics, Mechanical Properties, Failure Behaviour, Materials Selection, Springer, 1999. From the dust jacket: Ceramic materials are widely used as components in a great variety of applications. They are attractive due to their good high temperature strength, high wear resistance, good corrosion restistance and other special physical properties. Their major drawback is their brittleness and the large scatter of mechanical properties. This book describes failure phenomena in ceramic materials under mechanical loading, methods for determining the material properties, and the principles that one should apply when selecting a material. The fracture-mechanical and statistical principles and their use in describing the scatter of strength and lifetime are also covered. Special chapters are devoted to creep behaviour, multiaxial failure criteria and thermal shock behaviour.
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