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Adventures in earthenware - PP

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  • Lee
    Pete said it was okay to share this private email. I just removed personal notes. This, and the next post will help clarify what RR brought to our attention
    Message 1 of 18 , Jan 10 9:39 PM
      Pete said it was okay to share this private email. I just removed
      personal notes. This, and the next post will help clarify what RR
      brought to our attention about Pete's thoughts about his clay body
      research.

      Hi Lee,

      I have to make this quick- I have a meeting to go to. It's fascinating
      to me that something I wrote in 2001 is still being passed around.
      Those who imply that earthenware doesn't work for functional pots
      (because it's seen as weaker than stoneware, etc.) are missing the
      point about why we make and use handmade pots in the first place: we
      do so for aesthetic reasons, and not purely practical ones. If
      durability were the only criteria for picking a material then I'd be
      working in injection-molded plastic. EVERY time we make a pot out of
      clay and make it by hand we are making a STRONG statement that
      aesthetic experience is important enough to justify compromising pure
      practicality. This topic seems to come up often enough that it makes
      me think that I ought to write a column.

      Bottom line: humans have made and used earthenware vessels for
      millennia and have fired them (depending upon the clay, the technology
      and the desired results) to anywhere from cone 08 to cone 5 or 6. Like
      ALL ceramic materials, earthenware has strengths and weaknesses and
      savvy makers will use materials appropriately. We could easily argue
      that porcelain is a lousy material for pots because it transmits too
      much heat though the walls. So, for instance, in the case of a teabowl
      this quality increases the risk of burning our customers hands and
      porcelain teapots allow the tea to cool too quickly. Throw the bum
      out! Obviously we are willing to take these qualities into account
      when we make pots out of porcelain. As a parallel, a lot of very nice
      (and highly valued) clothes are made out of silk or wool, which aren't
      the most durable of materials in some respects- you sure can't throw
      them in the washer and dryer. If functionality were all that mattered
      then I'd be sitting here wearing polyester instead of the cotton (and
      wrinkled) kakis or the wool sweater I'm currently wearing. I'd argue
      that my cotton and wool clothing are quite functional, despite the
      shortcomings inherent to both materials.

      As for strength, earthenware has plenty for pots if it's used
      appropriately. Remember, sophisticated makers tend to make sensitive
      decisions based on the qualities of the finished material. As I've
      argued in many private emails on this topic, what does it matter if a
      given stoneware body is 50% stronger in MOR if the earthenware pot is
      100% thicker? Many who work in earthenware tend to make thicker pots
      because the qualities of the materials invite this.

      One more issue: industrial research indicates that porous bodies tend
      to chip before breaking and vitreous bodies tend to break before
      chipping (these studies usually compared vitreous and so-called
      "semi-vitrous" ware). This makes perfects sense if you understand the
      ability of pores to interrupt cracking. Earthenware, stoneware and
      porcelain each tend to fail in different ways. I own a lot of old
      earthenware pots and many are chipped. My old stoneware and porcelain
      pots have usually gone to the landfill in two or more pieces.

      I work at all temperatures and love all clays, precisely because each
      body and temperature has different qualities. We need to learn to
      celebrate these varied qualities and take advantage of them while
      avoiding the shortcomings that each clay or glaze possesses. I hope
      this begins to address the issue.


      Peter Pinnell
      Interim Chair
      Hixson-Lied Professor
      Department of Art and Art History
      120 Richards Hall
      University of Nebraska
      Lincoln, NE 68588-0114
      (402) 472-5522


      --
       Lee Love in Minneapolis
      http://mingeisota.blogspot.com/

       "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
      land of eternal youth is behind the house, a beautiful land fluent
      within itself." -- John O'Donohue
    • Lee
      This post directly addresses RR s questions about Pete s protocol. Enjoy! Hi Lee, Feel free to share my comments. As for not adjusting the figures to take
      Message 2 of 18 , Jan 10 9:41 PM
        This post directly addresses RR's questions about Pete's protocol. Enjoy!

        Hi Lee,

        Feel free to share my comments. As for not adjusting the figures to
        take shrinkage into account, I didn't in 2001 but did in the fall of
        2010 when I last taught the class (I never had the time to publish
        that data- it's still in a big folder on my computer at school). In
        the real world there was little difference on a 5/16" bar (the  wet
        diameter of the bars we made)- the increased shrinkage was difficult
        to measure with a micrometer and altered the final computed strength
        very little. In fact, the differences were well within the margin of
        error we got with different bars of the same clay bodies. In other
        words, it made no difference.

        Even if it had resulted in markedly different readings, I'll still
        argue that the method we used is more useful for the real world of
        pottery. We used the same die to extrude all the bars, fired each to
        their maturing temperature and then simply recorded the amount of
        force (weight in sand) that it took to break that bar when suspended
        across a given span. My interest is in knowing which clay makes the
        strongest bar. If it takes 20 lbss of sand to break one bar and 18
        lbs. to break another, then the first clay is stronger. In other
        words, if I make two pots from two different clay bodies and throw
        them with exactly the same wall thickness, then which will be the
        stronger pot? It's a simple real-world test in which the results are
        directly applicable to pottery-making by hand.

        The alternative is to compute the Modulus of Rupture (MOR) which
        provides a comparable strength number (Pounds per square inch, for
        instance). We added the MOR formula to an Excel spread sheet so MOR
        was automatically figured when the diameter, span and breaking force
        were entered into the sheet. MOR is a very useful figure for comparing
        similar clay bodies (porcelain to porcelain) or comparing the results
        of our own bodies with known published data. If we want an accurate
        MOR value for our clays then naturally we have to accurately measure
        the diameter of the fired bars and use that actual figure in the
        formula- which we did. This method adjusts for shrinkage and
        (ironically) results in a higher "strength" for clay bodies that
        shrink more- there's an obvious numerical advantage to figuring the
        MOR with a smaller diameter bar. So- when, in the real world, is it an
        advantage for your clay to shrink more? I completely understand the
        value of figuring an MOR, but for my purposes I'd actually prefer to
        have clay bodies that are strong and shrink less. The method we used
        for the test is easily duplicated by any potter using stuff that
        everyone has in the studio, and no math skills are necessary other
        than being able to read pounds off a scale. And the results are easy
        to see- stronger clays take more weight to break.

        BTW, I learned one two thing this time. If you want to do one thing to
        increase the strength of any fired clay, simply run it through a
        de-airing pugmill. Off the top of my head I've forgotten how much, but
        every clay was much stronger when it had been pugged. We can all put
        forward our own theories why, but the difference was marked with every
        clay.

        All interesting stuff! Now if I only had time to make pots.

        Pete

        Peter Pinnell
        Interim Chair
        Hixson-Lied Professor
        Department of Art and Art History
        120 Richards Hall
        University of Nebraska
        Lincoln, NE 68588-0114



        --
         Lee Love in Minneapolis
        http://mingeisota.blogspot.com/

         "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
        land of eternal youth is behind the house, a beautiful land fluent
        within itself." -- John O'Donohue
      • Margaret Flaherty
        A good example of why it s always rewarding to read what Pinnell has to say....
        Message 3 of 18 , Jan 11 5:49 AM
          A good example of why it's always rewarding to read what Pinnell has to
          say....

          On Wed, Jan 11, 2012 at 12:41 AM, Lee <cwiddershins@...> wrote:

          > This post directly addresses RR's questions about Pete's protocol. Enjoy!
          >
          > Hi Lee,
          >
          > Feel free to share my comments. As for not adjusting the figures to
          > take shrinkage into account, I didn't in 2001 but did in the fall of
          > 2010 when I last taught the class (I never had the time to publish
          > that data- it's still in a big folder on my computer at school). In
          > the real world there was little difference on a 5/16" bar (the wet
          > diameter of the bars we made)- the increased shrinkage was difficult
          > to measure with a micrometer and altered the final computed strength
          > very little. In fact, the differences were well within the margin of
          > error we got with different bars of the same clay bodies. In other
          > words, it made no difference.
          >
          > Even if it had resulted in markedly different readings, I'll still
          > argue that the method we used is more useful for the real world of
          > pottery. We used the same die to extrude all the bars, fired each to
          > their maturing temperature and then simply recorded the amount of
          > force (weight in sand) that it took to break that bar when suspended
          > across a given span. My interest is in knowing which clay makes the
          > strongest bar. If it takes 20 lbss of sand to break one bar and 18
          > lbs. to break another, then the first clay is stronger. In other
          > words, if I make two pots from two different clay bodies and throw
          > them with exactly the same wall thickness, then which will be the
          > stronger pot? It's a simple real-world test in which the results are
          > directly applicable to pottery-making by hand.
          >
          > The alternative is to compute the Modulus of Rupture (MOR) which
          > provides a comparable strength number (Pounds per square inch, for
          > instance). We added the MOR formula to an Excel spread sheet so MOR
          > was automatically figured when the diameter, span and breaking force
          > were entered into the sheet. MOR is a very useful figure for comparing
          > similar clay bodies (porcelain to porcelain) or comparing the results
          > of our own bodies with known published data. If we want an accurate
          > MOR value for our clays then naturally we have to accurately measure
          > the diameter of the fired bars and use that actual figure in the
          > formula- which we did. This method adjusts for shrinkage and
          > (ironically) results in a higher "strength" for clay bodies that
          > shrink more- there's an obvious numerical advantage to figuring the
          > MOR with a smaller diameter bar. So- when, in the real world, is it an
          > advantage for your clay to shrink more? I completely understand the
          > value of figuring an MOR, but for my purposes I'd actually prefer to
          > have clay bodies that are strong and shrink less. The method we used
          > for the test is easily duplicated by any potter using stuff that
          > everyone has in the studio, and no math skills are necessary other
          > than being able to read pounds off a scale. And the results are easy
          > to see- stronger clays take more weight to break.
          >
          > BTW, I learned one two thing this time. If you want to do one thing to
          > increase the strength of any fired clay, simply run it through a
          > de-airing pugmill. Off the top of my head I've forgotten how much, but
          > every clay was much stronger when it had been pugged. We can all put
          > forward our own theories why, but the difference was marked with every
          > clay.
          >
          > All interesting stuff! Now if I only had time to make pots.
          >
          > Pete
          >
          > Peter Pinnell
          > Interim Chair
          > Hixson-Lied Professor
          > Department of Art and Art History
          > 120 Richards Hall
          > University of Nebraska
          > Lincoln, NE 68588-0114
          >
          >
          >
          > --
          > Lee Love in Minneapolis
          > http://mingeisota.blogspot.com/
          >
          > "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
          > land of eternal youth is behind the house, a beautiful land fluent
          > within itself." -- John O'Donohue
          >
        • Lee
          On Wed, Jan 11, 2012 at 7:49 AM, Margaret Flaherty ... Anybody who has workshopped with Pete comes away knowing that he is not only brilliant, he is generous
          Message 4 of 18 , Jan 11 7:28 AM
            On Wed, Jan 11, 2012 at 7:49 AM, Margaret Flaherty
            <mflahertyster@...> wrote:
            > A good example of why it's always rewarding to read what Pinnell has to
            > say....

            Anybody who has workshopped with Pete comes away knowing that he is
            not only brilliant, he is generous with his knowledge.

            --
             Lee Love in Minneapolis
            http://mingeisota.blogspot.com/

             "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
            land of eternal youth is behind the house, a beautiful land fluent
            within itself." -- John O'Donohue
          • Lee
            Tony wondered if pugging clay would help it perform better in the microwave. It probably does, because the clay become denser. I wanted to make sure folks
            Message 5 of 18 , Jan 11 8:19 AM
              Tony wondered if pugging clay would help it perform better in the
              microwave. It probably does, because the clay become denser. I
              wanted to make sure folks saw the last part of my second forward from
              Pete. Pugging increases the strength of your clay.
              I will start pugging my earthenware too, either cleaning the
              pug between clays or getting another just for terracotta.


              ---------- Forwarded message ----------
              From: Lee <cwiddershins@...>

              BTW, I learned one two thing this time. If you want to do one thing to
              increase the strength of any fired clay, simply run it through a
              de-airing pugmill. Off the top of my head I've forgotten how much, but
              every clay was much stronger when it had been pugged. We can all put
              forward our own theories why, but the difference was marked with every
              clay.


              --
               Lee Love in Minneapolis
              http://mingeisota.blogspot.com/

               "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
              land of eternal youth is behind the house, a beautiful land fluent
              within itself." -- John O'Donohue
            • May Luk
              Hi Lee, or Tony & Clayart; What is perform better ? This needs to be clarified so that we are comparing similar experience. Comparing clay bars is good for an
              Message 6 of 18 , Jan 11 10:47 AM
                Hi Lee, or Tony & Clayart;

                What is "perform better"? This needs to be clarified so that we are
                comparing similar experience.

                Comparing clay bars is good for an academic exercise and to further
                our understanding. In the end of the day, how do we apply this info to
                use? We don't eat off clay bars.

                We also need to understand human behavior and consider the end-users.
                We make wares to have somebody in mind, hopefully paying customers.

                Wares come in different size and shapes, and temperature of food
                varies. From heating lunch at day job for several years, I have never
                have "good" experience (ie, the bowls are very hot and difficult to
                carry to the serving table) with any types of clay and wares ( cone 6
                stoneware, Chinese high-fire porcelain, US cone 6 porcelain, British
                high-fire porcelain & stoneware, ceramics from mass market, hotel
                wares) I always have to put a plate underneath in order to carry it to
                my desk. (Maybe I should get myself a serving tray!)

                If I buy ceramics for the microwave, I would like it to have:
                1- handles on both sides for carrying from the oven to the table.
                2- Oversize single loop handle
                3- Flare out thick lips, not straight up & down wall
                4- Generous sizing
                5- white liner glaze

                My 'better experience" to only fill the bowl 1/3 to half full for microwaving

                Best Regards
                May.


                On Wed, Jan 11, 2012 at 11:19 AM, Lee <cwiddershins@...> wrote:
                > Tony wondered if pugging clay would help it perform better in the
                > microwave.    It probably does, because the clay become denser.   I
                > wanted to make sure folks saw the last part of my second forward from
                > Pete.    Pugging increases the strength of your clay.
                >          I will start pugging my earthenware too, either cleaning the
                > pug between clays or getting another just for terracotta.
                >
                >
                > ---------- Forwarded message ----------
                > From: Lee <cwiddershins@...>
                >
                > BTW, I learned one two thing this time. If you want to do one thing to
                > increase the strength of any fired clay, simply run it through a
                > de-airing pugmill. Off the top of my head I've forgotten how much, but
                > every clay was much stronger when it had been pugged. We can all put
                > forward our own theories why, but the difference was marked with every
                > clay.
                >
                >
                > --
                >  Lee Love in Minneapolis
                > http://mingeisota.blogspot.com/
                >
                >  "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
                > land of eternal youth is behind the house, a beautiful land fluent
                > within itself." -- John O'Donohue



                --
                http://www.artspan.org/artist/mayluk
                http://www.ceramicsbrooklyn.com/
              • Lee
                ... A large range of our ceramics are not eaten off. With Grandma s pennies not having copper in them any more, I have been using a copper liner in my
                Message 7 of 18 , Jan 11 1:58 PM
                  On Wed, Jan 11, 2012 at 12:47 PM, May Luk <claywedgie@...> wrote:

                  > Comparing clay bars is good for an academic exercise and to further
                  > our understanding. In the end of the day, how do we apply this info to
                  > use? We don't eat off clay bars.

                  A large range of our ceramics are not eaten off. With
                  "Grandma's pennies" not having copper in them any more, I have been
                  using a copper liner in my vases.

                  > If I buy ceramics for the microwave, I would like it to have:
                  > 1- handles on both sides for carrying from the oven to the table.
                  > 2- Oversize single loop handle
                  > 3- Flare out thick lips, not straight up & down wall
                  > 4- Generous sizing
                  > 5- white liner glaze

                  I am supposing number 0 is "Not ugly." As Pete mentioned, we
                  can't forget aesthetics.

                  --
                   Lee Love in Minneapolis
                  http://mingeisota.blogspot.com/

                   "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
                  land of eternal youth is behind the house, a beautiful land fluent
                  within itself." -- John O'Donohue
                • May Luk
                  Hi Lee; I thought we were talking about microwave and crazing. Then what is perform better ? Roll it down the hill and see which arrives to the bottom first?
                  Message 8 of 18 , Jan 11 3:35 PM
                    Hi Lee;

                    I thought we were talking about microwave and crazing. Then what is
                    "perform better"?

                    Roll it down the hill and see which arrives to the bottom first?

                    "-)

                    It sounds nice talking about aesthetics on paper. (Teachers, Ugh!)
                    Just don't burn my hands and have me drop the bowl on the floor and I
                    have to clean it up.

                    I don't have anything against Pete. I live in the real world and I'm
                    not in school anymore. that's all. Thanks for the lecture,
                    nevertheless.

                    Best Regards
                    May


                    On Wed, Jan 11, 2012 at 4:58 PM, Lee <cwiddershins@...> wrote:
                    >
                    >
                    >    A large range of our ceramics are not eaten off.      With
                    > "Grandma's pennies" not having copper in them any  more, I have been
                    > using a copper liner in my vases.
                    >
                    >> If I buy ceramics for the microwave, I would like it to have:
                    >> 1- handles on both sides for carrying from the oven to the table.
                    >> 2- Oversize single loop handle
                    >> 3- Flare out thick lips, not straight up & down wall
                    >> 4- Generous sizing
                    >> 5- white liner glaze
                    >
                    >   I am supposing number 0 is "Not ugly."   As Pete mentioned, we
                    > can't forget aesthetics.
                    >
                    > --
                    >  Lee Love in Minneapolis
                    > http://mingeisota.blogspot.com/
                    >




                    --
                    http://www.artspan.org/artist/mayluk
                    http://www.ceramicsbrooklyn.com/
                  • Lee
                    This is one of the first clay pieces I made, in terracotta. It is life sized. I made it in my first clay class with Curt Hoard at the UofMn in 1990. It is
                    Message 9 of 18 , Jan 11 10:10 PM
                      This is one of the first clay pieces I made, in terracotta. It is life sized.
                      I made it in my first clay class with Curt Hoard at the UofMn in 1990.
                      It is titled "Homage To A Hawaiian Shirt." It is a copy of my favorite
                      Hawaiian Shirt that disintegrated. This was also the year Paul
                      Wellstone was elected. I had this show in the Minnesota State Fair as
                      a part of the Northern Clay Show. An Aide to Paul Wellstone asked if I
                      would consider allowing this to be put in Wellstone's new D.C. office.
                      I said "SURE!" The Aide said that Paul wanted to have Minnesota art in
                      his office. The funding for this project never came through, sorry to
                      say. Much to my disappointment, his piece "disappeared" while I was
                      living in Japan. I am happy I have a photo of it!

                      http://bit.ly/wcSZBU

                      or

                      https://fbcdn-sphotos-a.akamaihd.net/hphotos-ak-snc7/396282_10150486687397057_550727056_9129789_275267688_n.jpg

                      --
                       Lee Love in Minneapolis
                      http://mingeisota.blogspot.com/

                       "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
                      land of eternal youth is behind the house, a beautiful land fluent
                      within itself." -- John O'Donohue
                    • Gerholdclay
                      I wonder if the apparatus that Pete is talking about to test clay strength has been published anywhere? Is this a standardized test or just one Pete has
                      Message 10 of 18 , Jan 12 4:49 AM
                        I wonder if the apparatus that Pete is talking about to test clay strength has been published anywhere? Is this a standardized test or just one Pete has developed?


                        This also sounds like a test that could be adapted to test strengh of clay joins possibly resolving the long standing question of the best way to make stong joins.

                        Paul

                        Sent from my iPad

                        On Jan 11, 2012, at 12:41 AM, Lee <cwiddershins@...> wrote:

                        > This post directly addresses RR's questions about Pete's protocol. Enjoy!
                        >
                        > Hi Lee,
                        >
                        > Feel free to share my comments. As for not adjusting the figures to
                        > take shrinkage into account, I didn't in 2001 but did in the fall of
                        > 2010 when I last taught the class (I never had the time to publish
                        > that data- it's still in a big folder on my computer at school). In
                        > the real world there was little difference on a 5/16" bar (the wet
                        > diameter of the bars we made)- the increased shrinkage was difficult
                        > to measure with a micrometer and altered the final computed strength
                        > very little. In fact, the differences were well within the margin of
                        > error we got with different bars of the same clay bodies. In other
                        > words, it made no difference.
                        >
                        > Even if it had resulted in markedly different readings, I'll still
                        > argue that the method we used is more useful for the real world of
                        > pottery. We used the same die to extrude all the bars, fired each to
                        > their maturing temperature and then simply recorded the amount of
                        > force (weight in sand) that it took to break that bar when suspended
                        > across a given span. My interest is in knowing which clay makes the
                        > strongest bar. If it takes 20 lbss of sand to break one bar and 18
                        > lbs. to break another, then the first clay is stronger. In other
                        > words, if I make two pots from two different clay bodies and throw
                        > them with exactly the same wall thickness, then which will be the
                        > stronger pot? It's a simple real-world test in which the results are
                        > directly applicable to pottery-making by hand.
                        >
                        > The alternative is to compute the Modulus of Rupture (MOR) which
                        > provides a comparable strength number (Pounds per square inch, for
                        > instance). We added the MOR formula to an Excel spread sheet so MOR
                        > was automatically figured when the diameter, span and breaking force
                        > were entered into the sheet. MOR is a very useful figure for comparing
                        > similar clay bodies (porcelain to porcelain) or comparing the results
                        > of our own bodies with known published data. If we want an accurate
                        > MOR value for our clays then naturally we have to accurately measure
                        > the diameter of the fired bars and use that actual figure in the
                        > formula- which we did. This method adjusts for shrinkage and
                        > (ironically) results in a higher "strength" for clay bodies that
                        > shrink more- there's an obvious numerical advantage to figuring the
                        > MOR with a smaller diameter bar. So- when, in the real world, is it an
                        > advantage for your clay to shrink more? I completely understand the
                        > value of figuring an MOR, but for my purposes I'd actually prefer to
                        > have clay bodies that are strong and shrink less. The method we used
                        > for the test is easily duplicated by any potter using stuff that
                        > everyone has in the studio, and no math skills are necessary other
                        > than being able to read pounds off a scale. And the results are easy
                        > to see- stronger clays take more weight to break.
                        >
                        > BTW, I learned one two thing this time. If you want to do one thing to
                        > increase the strength of any fired clay, simply run it through a
                        > de-airing pugmill. Off the top of my head I've forgotten how much, but
                        > every clay was much stronger when it had been pugged. We can all put
                        > forward our own theories why, but the difference was marked with every
                        > clay.
                        >
                        > All interesting stuff! Now if I only had time to make pots.
                        >
                        > Pete
                        >
                        > Peter Pinnell
                        > Interim Chair
                        > Hixson-Lied Professor
                        > Department of Art and Art History
                        > 120 Richards Hall
                        > University of Nebraska
                        > Lincoln, NE 68588-0114
                        >
                        >
                        >
                        > --
                        > Lee Love in Minneapolis
                        > http://mingeisota.blogspot.com/
                        >
                        > "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
                        > land of eternal youth is behind the house, a beautiful land fluent
                        > within itself." -- John O'Donohue
                      • ivor and olive lewis
                        May Luc asks us the following question.. What is perform better ? This needs to be clarified so that we are comparing similar experience. Comparing clay
                        Message 11 of 18 , Jan 12 7:49 PM
                          May Luc asks us the following question..
                          "What is "perform better"? This needs to be clarified so that we are
                          comparing similar experience.
                          Comparing clay bars is good for an academic exercise and to further our
                          understanding. In the end of the day, how do we apply this info to use? We
                          don't eat off clay bars."



                          Perhaps this is one issue that divides Artists from The Rest. May is right
                          to think that we need a higher degree of objectivity in the way information
                          is presented. Without objective qualification and quantification statements
                          such as "perform better" are subjective and bring no observations or
                          measurements to the workshop or studio that can be used to give credence to
                          the material we are using. Trust and confidence come from precision of
                          representations of what we believe to be valid facts.

                          Regards,

                          Ivor Lewis,
                          REDHILL,
                          South Australia
                        • Soj
                          I find this odd, as I routinely microwave all my stoneware, whether I threw it or not, and have never had a bowl get so hot I couldn t handle it. The only ware
                          Message 12 of 18 , Jan 13 2:54 AM
                            I find this odd, as I routinely microwave all my stoneware, whether I threw
                            it or not, and have never had a bowl get so hot I couldn't handle it.

                            The only ware I've ever had this happen with was some older (like 40 or 50
                            yrs old or possibly older) commercial dishware. I can't remember the maker.
                            But this stuff got hot enough to cook on just from the heat of the plate.
                            It had belonged to my stepmother. I don't know what was in the clay that
                            had been used to make the stuff but this stuff got volcano hot. I ended up
                            throwing it all away as I could not, in good conscience, even donate it due
                            to the risk of burning some unsuspecting child down the road, given the way
                            we all heat things up in microwaves these days.

                            I wonder what could be in the clay body that could cause this, as I have
                            never experienced it with any other substance in the microwave. Does
                            anybody have any guesses? I would like to actively avoid any clay body that
                            would behave this way...

                            On Wed, 11 Jan 2012 13:47:18 -0500, May Luk <claywedgie@...> wrote:

                            > ... I have never
                            >have "good" experience (ie, the bowls are very hot and difficult to
                            >carry to the serving table) with any types of clay and wares ( cone 6
                            >stoneware, Chinese high-fire porcelain, US cone 6 porcelain, British
                            >high-fire porcelain & stoneware, ceramics from mass market, hotel
                            >wares) I always have to put a plate underneath in order to carry it to
                            >my desk.
                          • John Hesselberth
                            ... Hi Soj, Water. Absorbed water. Microwave ovens are designed to very efficiently heat water. Apparently your stoneware is well enough vitrified that it
                            Message 13 of 18 , Jan 13 6:27 AM
                              On Jan 13, 2012, at 5:54 AM, Soj wrote:

                              >
                              > I wonder what could be in the clay body that could cause this, as I have
                              > never experienced it with any other substance in the microwave. Does
                              > anybody have any guesses? I would like to actively avoid any clay body that
                              > would behave this way...


                              Hi Soj,

                              Water. Absorbed water. Microwave ovens are designed to very efficiently heat water.

                              Apparently your stoneware is well enough vitrified that it doesn't do this; however you even have to be careful with some stoneware bodies. If that are not formulated to be down to 2-3% absorption at their glaze firing temperature they can have the problem too. Earthenware, of course, normally has absorption levels of 6-10% when fired to the normal 06-04 range.

                              Some people believe high iron glazes also cause the problem. I have never been able to duplicate their results with glazes containing up to 12% RIO if the body was 2% or lower absorption. But there may be some conditions under which this happens.

                              Regards.

                              John

                              John Hesselberth
                              www.frogpondpottery.com

                              "Man is a tool-using animal....without tools he is nothing, with tools he is all" .... Thomas Carlyle
                            • marci Boskie's Mama =^..^=
                              ... (snip) ... COOL piece, Lee. What is the coloring on it, other than the terra cotta? Slip and mason stains? marci the chinapainter
                              Message 14 of 18 , Jan 13 7:35 AM
                                At 11:00 PM 1/12/2012, you wrote:
                                >From: Lee <cwiddershins@...>
                                >Subject: Re: Adventures in earthenware - PP
                                >
                                >This is one of the first clay pieces I made, in terracotta. It is life siz=
                                >ed.
                                >I made it in my first clay class with Curt Hoard at the UofMn in 1990.
                                >It is titled "Homage To A Hawaiian Shirt." It is a copy of my favorite
                                >Hawaiian Shirt that disintegrated.

                                (snip)
                                ----
                                COOL piece, Lee. What is the coloring on it, other than the terra
                                cotta? Slip and mason stains?
                                marci the chinapainter
                              • Edouard Bastarache
                                John, Earthenware, of course, normally has absorption levels of 6-10% when fired to the normal 06-04 range. Some doing earthenware accept a higher
                                Message 15 of 18 , Jan 13 8:44 AM
                                  John,

                                  "Earthenware, of course, normally has absorption levels of 6-10% when fired
                                  to the normal 06-04 range."

                                  Some doing earthenware accept a higher absorption,like 14 % at the Gien
                                  Factory in France and they fire to cone 1. Naturally the pieces are
                                  completely covered by glazes.

                                  Gis,

                                  Edouard Bastarache
                                  Spertesperantisto

                                  Sorel-Tracy
                                  Quebec

                                  http://www.flickr.com/photos/30058682@N00/
                                  http://edouardbastarache.blogspot.com/
                                  http://smart2000.pagesperso-orange.fr/bloggs_edouard.htm
                                  http://www.facebook.com/edouard.bastarache







                                  ----- Original Message -----
                                  From: "John Hesselberth" <jjhesselberth@...>
                                  To: <Clayart@...>
                                  Sent: Friday, January 13, 2012 9:27 AM
                                  Subject: Re: Adventures in earthenware - PP


                                  On Jan 13, 2012, at 5:54 AM, Soj wrote:

                                  >
                                  > I wonder what could be in the clay body that could cause this, as I have
                                  > never experienced it with any other substance in the microwave. Does
                                  > anybody have any guesses? I would like to actively avoid any clay body
                                  > that
                                  > would behave this way...


                                  Hi Soj,

                                  Water. Absorbed water. Microwave ovens are designed to very efficiently heat
                                  water.

                                  Apparently your stoneware is well enough vitrified that it doesn't do this;
                                  however you even have to be careful with some stoneware bodies. If that are
                                  not formulated to be down to 2-3% absorption at their glaze firing
                                  temperature they can have the problem too. Earthenware, of course, normally
                                  has absorption levels of 6-10% when fired to the normal 06-04 range.

                                  Some people believe high iron glazes also cause the problem. I have never
                                  been able to duplicate their results with glazes containing up to 12% RIO if
                                  the body was 2% or lower absorption. But there may be some conditions under
                                  which this happens.

                                  Regards.

                                  John

                                  John Hesselberth
                                  www.frogpondpottery.com

                                  "Man is a tool-using animal....without tools he is nothing, with tools he is
                                  all" .... Thomas Carlyle
                                • Taylor Hendrix
                                  Many, commercially produced plates are not made with stoneware or are not fired to the optimum vitrification of the clay if it is stoneware. This we know from
                                  Message 16 of 18 , Jan 13 9:26 AM
                                    Many, commercially produced plates are not made with stoneware or are
                                    not fired to the optimum vitrification of the clay if it is stoneware.
                                    This we know from reading and from common experiences. Several of my
                                    plates purchased in Europe as seconds heat up a bit in the microwave
                                    and show crazing probably from dishwasher usage. Our Polish pottery
                                    plates come to mind.

                                    Taylor, in Rockport TX
                                    wirerabbit1 on Skype (-0600 UTC)
                                    http://wirerabbit.blogspot.com
                                    http://wirerabbitpots.blogspot.com
                                    http://www.flickr.com/photos/wirerabbit/
                                    https://youtube.com/thewirerabbit



                                    On Fri, Jan 13, 2012 at 8:27 AM, John Hesselberth
                                    <jjhesselberth@...> wrote:
                                    > On Jan 13, 2012, at 5:54 AM, Soj wrote:
                                    >
                                    >>
                                    >> I wonder what could be in the clay body that could cause this, as I have
                                    >> never experienced it with any other substance in the microwave.  Does
                                    >> anybody have any guesses?  I would like to actively avoid any clay body that
                                    >> would behave this way...
                                    >
                                    >
                                    > Hi Soj,
                                    >
                                    > Water. Absorbed water. Microwave ovens are designed to very efficiently heat water.
                                    >
                                    ...
                                  • Eric Koenig
                                    I haven t been able to find any specifics about which particular clay bodies heat up like that; but after searching around a little, I ve found a technical
                                    Message 17 of 18 , Jan 13 9:34 PM
                                      I haven't been able to find any specifics about which particular clay bodies
                                      heat up like that; but after searching around a little, I've found a technical
                                      term for it that might help somebody to track it down:

                                      thermal runaway.

                                      Apparently, it happens in some ceramics and glasses (note that anything that
                                      fuses and cools before it has a chance to form crystals is a glass). From the
                                      little I skimmed from various articles, there is some kind of heat feedback loop
                                      instead of the heat properly dissipating, which is something I've never heard
                                      of. It can actually cause some of these materials to melt under high exposure
                                      to microwaves.

                                      Eric
                                    • Lee
                                      On Fri, Jan 13, 2012 at 9:35 AM, marci Boskie s Mama =^..^= ... Marci, I didn t think anybody would look at it. ;^) I used commercial engobe, underglaze and
                                      Message 18 of 18 , Jan 14 9:14 PM
                                        On Fri, Jan 13, 2012 at 9:35 AM, marci Boskie's Mama =^..^=
                                        <marci@...> wrote:


                                        >  COOL piece, Lee. What is the coloring on it, other than the terra
                                        > cotta? Slip and mason stains?

                                        Marci,
                                        I didn't think anybody would look at it. ;^)
                                        I used commercial engobe, underglaze and overglaze, and
                                        the class clear, firing multiple times. It wouldn't fit in the
                                        "common" electric kilns in the big room. Linda Sikora (She is at
                                        Alfred teaching now) took me under her wing and fired it in the grad
                                        students' electric that had an extended ring.

                                        The shirt went missing while I was in Japan, but I have two
                                        other pieces I made with it in my first clay class.

                                        Hey, tonight, got accepted to a gallery I applied to a
                                        couple years ago. My plan is to have new work there after my month
                                        in Japan and trip to Korean after the spring:
                                        http://thegrandhand.com/

                                        See Hawaiian Shirt at these links:

                                        http://bit.ly/wcSZBU

                                        or

                                        https://fbcdn-sphotos-a.akamaihd.net/hphotos-ak-snc7/396282_10150486687397057_550727056_9129789_275267688_n.jpg

                                        --
                                         Lee Love in Minneapolis
                                        http://mingeisota.blogspot.com/

                                         "Ta tIr na n-óg ar chul an tI—tIr dlainn trina chéile"—that is, "The
                                        land of eternal youth is behind the house, a beautiful land fluent
                                        within itself." -- John O'Donohue
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