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Tyler Miller

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  1. 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.
  2. 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.
  3. 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.
  4. Not as is. Is this a raw, as dug, product? The iron content will make a cosmetically unacceptable tile.
  5. 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.
  6. 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.
  7. 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?
  8. 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.
  9. 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.
  10. Bill, I see the confusion you’re having now. You’ve equated the possibiity of fast firing with the reality of cooling rates being irrelevant, without taking into account the effect of unevenness of cooling. Fast firings are indeed possible, but under specific conditions that demand a very even heating and cooling of a kiln. This is accomplished by using a kiln of low thermal mass, but very well insulated (ie they’re made of rcf wools etc). Also, they tend to use clay bodies specifically formulated to fast firings—that is, bodies with very little free silica. The issue isn’t the cooling rate, but the evenness of cooling. Cristobalite inversion cashes out in the real world as a dramatic change in the dimensions of a pot, and it occurs over a narrow range. So, open the kiln at the wrong point and remove the stabiliIng thermal mass of the lid, and you could have a pot with one side above and one side below the temp of inversion, then PONG! That red stoneware platter is now two pieces. It is a real and regular defect in ceramics—either with clay bodies with too much free silica, clay bodies with cristobalite deliberately added (they used to do this with earthenwares to get better glaze fit by forcing compression), or with kilns opened too early. Hamer and Hamer discuss this in the Potter’s Dictionary (page 120 in their discussion on dunting), and it’s also a problem in industry. Check this article out on cooling dunts from Ceramic Industry: https://www.ceramicindustry.com/articles/87399-drying-firing-solutions-cooling-dunts ) I find it cool that the industrial practice of firing high for the initial firing creates issues for cristobalite dunting in the subsequent re-heat. So, I’d suggest doing some reading about firing defects and their causes. I stand by my original statement that cristobalite inversion is the last significant structural change in a clay body that could cause failures in a firing cycle. Peace.
  11. Dunno what crystobalite is, but cristobalite is common in non vitreous stoneware bodies and forms above 1100C (2000 F) which is well within the the temp range of a cone 6 firing. In studio pottery it’s more about how much flux is in a body. You are right, though, even cooling is the key thing. Cristobalite inversion is just the last significant cooling change in a clay body and going through it unevenly is asking for risk.
  12. As long as you’re below cristobalite inversion, you’re golden.
  13. @Michael Sorum Before we get too technical, How exactly are you s t r e t c h i n g the sides of your pots? This word jumped out at me.
  14. The material you are looking for is concrete. Cheap, castable, available in every country on Earth, and sets without firing.
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