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Mark C.

Cone 6 vs Cone 10 clay strength

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Your "tests" are absolutely worthless, Mark, except that it helps you cling to your belief that cone 10 pottery is stronger than cone 6 pottery so that you can tell your customers such nonsense. The truth is that it is maturity and lack of impurities (including additions like sand and grog) that determine the strength of fired clay. You seem to be so attached to what you were taught in the 70's (or whenever) that you probably think heavy reduction firing makes clay even stronger--it doesn't, it makes it weaker.

 

Hell, I could grab a hammer and go into the back yard and break one of my cone 13 pots and then a cone 6 pot and make all kinds of stupid pronouncements about cone 6 pots being stronger and not needing a weatherman to confuse me with facts! The difference between cone 6 and cone 10 is only a little over 100 degrees. Some clays fired to maturity at cone 6 are stronger than some clays fired maturity at cone 10. Some clays fired to maturity at cone 10 are stronger than some clays fired to maturity at cone 6. It depends on the clay, not a 100 degree difference in temperature.

 

Thanks, Bcisketpotery, for posting Pete's test results. I especially like his finding that of the clays they tested scientifically a cone 04 clay was the strongest! Now, excuse me while I go take a hammer to a pot made of Lizella red so I can claim that it is the strongest clay in the world.

 

Jim

 

 

JIM;

I think your comments are mean spirited and do not do much to move the discussion ahead. Starting out by saying words like "absolutely worthless," and "stupid" just throw gas on the flames.I value Marks comments and actually consider him a friend even though I have never met the man.

Congatulations on firing to cone 13. I make pots on the oldest Brent CXC on the blog.Should we compare kiln sizes too?

I also started making pots in the 70's. Feel free to diss me as well.

I would like to see an apology to Mark on this forum.Soon.

Thank-you.

TJR.

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Your "tests" are absolutely worthless, Mark, except that it helps you cling to your belief that cone 10 pottery is stronger than cone 6 pottery so that you can tell your customers such nonsense. The truth is that it is maturity and lack of impurities (including additions like sand and grog) that determine the strength of fired clay. You seem to be so attached to what you were taught in the 70's (or whenever) that you probably think heavy reduction firing makes clay even stronger--it doesn't, it makes it weaker.

 

Hell, I could grab a hammer and go into the back yard and break one of my cone 13 pots and then a cone 6 pot and make all kinds of stupid pronouncements about cone 6 pots being stronger and not needing a weatherman to confuse me with facts! The difference between cone 6 and cone 10 is only a little over 100 degrees. Some clays fired to maturity at cone 6 are stronger than some clays fired maturity at cone 10. Some clays fired to maturity at cone 10 are stronger than some clays fired to maturity at cone 6. It depends on the clay, not a 100 degree difference in temperature.

 

Thanks, Bcisketpotery, for posting Pete's test results. I especially like his finding that of the clays they tested scientifically a cone 04 clay was the strongest! Now, excuse me while I go take a hammer to a pot made of Lizella red so I can claim that it is the strongest clay in the world.

 

Jim

 

 

JIM;

I think your comments are mean spirited and do not do much to move the discussion ahead. Starting out by saying words like "absolutely worthless," and "stupid" just throw gas on the flames.I value Marks comments and actually consider him a friend even though I have never met the man.

Congatulations on firing to cone 13. I make pots on the oldest Brent CXC on the blog.Should we compare kiln sizes too?

I also started making pots in the 70's. Feel free to diss me as well.

I would like to see an apology to Mark on this forum.Soon.

Thank-you.

TJR.

 

 

TJR, I like Mark, too, but I don't think calling an absolutely worthless "test" absolutely worthless is as much mean spirited as it is honest and direct. There's a misunderstanding about the references to the '70's. I potted in the 70's (then took 35 years off) so I remember how restricted it was in that you weren't a real potter unless you fired to cone 10 in reduction in a gas kiln. Right or wrong, Mark's attitude reminds me of that and that is the only reason for the reference to the '70's. Reference to cone 13 is simply pointing out that if you think pots fired to cone 10 are superior to pots fired to lower cones then pots fired to cone 13 would be even superior to cone 10 pots (nonsense, of course, but point made).

 

Jim

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I would like to continue the discussion about this issue , and be able to consider both anecdotal and research findings giving each weighted credibility,John pointed out some of the variables that would affect the fired strength of any clay. Observations may not be scientifically valid per se, but they can add to the summary knowledge that we have about materials and lead to hypotheses that can be tested in more scientific ways.

 

My research background is in the social sciences (soft science) and observational information is a valid way to direct much more scientific rigorous studies of a phenomenon.

 

Mark has presented what social scientists would call a case study and this cannot be generalized past his own experience, for him it is valid. It may raise many questions for others and that too is perfectly valid.

 

I work with a variety of clays and although I stick to c 10 clay for my functional work I would really like to know all I can about using clays at other temperatures and be able to evaluate some of the things like how glazes affect the tensile strength, etc and where to direct my energies in learning more.\\

 

thanks...please keep discussing. Kathy

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Observations may not be scientifically valid per se, but they can add to the summary knowledge that we have about materials and lead to hypotheses that can be tested in more scientific ways.

 

It is important to have such observational information lead to formulating a hypothesis which is then actually followed up with appropriate rigorous testing BEFORE conclusions are made. The handcraft pottery field is unfortunately FULL of people making final conclusions based on either insufficient or inaccurate data.

 

One of the core missions of Ceramic Arts Daily is education.

 

best,

 

.............john

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If I'm remembering correctly the Pinnell paper had some critics, perhaps one was Ron Roy. Like others (though not as educated as others on the subject), I think that the clay and the technique matter. I can see ^10 being stronger because of the clay/glaze interface being better. But on the other hand, cracks don't propagate as well through a crystalline body (04) as they do through a glassy body (10). Clay glaze compression vs. body thickness matters too, meaning that thin isn't the best way to make a usable product, unless a thin glaze is used. And as I've mentioned, I suspect that cross section along with fired unit volume vs. original unit volume is in there too.

 

I just can't see how a rigorous comparison can be done in any economical way. I think we can draw some generalizations from experience, but one potter's cone x mug may be stronger than another potter's mug, and one clay manufactures cone x clay may be stronger than another's. As was said, strong enough is strong enough. Use good clay, don't over reduce. Thin glazes are better if you throw thin ware. Generalizations like these work well for me.

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If I'm remembering correctly the Pinnell paper had some critics, perhaps one was Ron Roy. Like others (though not as educated as others on the subject), I think that the clay and the technique matter. I can see ^10 being stronger because of the clay/glaze interface being better. But on the other hand, cracks don't propagate as well through a crystalline body (04) as they do through a glassy body (10). Clay glaze compression vs. body thickness matters too, meaning that thin isn't the best way to make a usable product, unless a thin glaze is used. And as I've mentioned, I suspect that cross section along with fired unit volume vs. original unit volume is in there too.

 

I just can't see how a rigorous comparison can be done in any economical way. I think we can draw some generalizations from experience, but one potter's cone x mug may be stronger than another potter's mug, and one clay manufactures cone x clay may be stronger than another's. As was said, strong enough is strong enough. Use good clay, don't over reduce. Thin glazes are better if you throw thin ware. Generalizations like these work well for me.

 

 

I'd love to see what critics of the Pinnell "paper" said. I searched a little bit but couldn't find anything. It sure would be nice if you're going to make that statement to point us to those criticisms. By saying "I can see ^10 being stronger because of the clay/glaze interface being better," you're saying basically what Mark says that cone 10 pottery is stronger than ^6 potter which is not true. Once again, strength depends on the composition of a clay fired to maturity. Some cone 6 clays are stronger than some cone 10 clays and some cone 10 clays are stronger than some cone 6 clays. If clay has grog or sand in it it is weaker than the same clay without grog or sand. Why do you think a cone 10 clay and glaze has a better "interface" than a cone 6 or cone 3 or cone 04 clay and glaze? A cone 6 glaze can "fit" a cone 6 body as well as a cone 10 glaze can "fit" a cone 10 body. Obviously, a MOR test takes a bit more time and effort than banging two pots together, but one is a scientific test (and VERY easy and simple by scientific testing standards) and the other is a waste of time.

 

Jim

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more from the Clayart archives . . . google Pete Pinnell MOR test

 

 

adventures in earthenware - rr

 

updated tue 10 jan 12

small%20potters%20logo.gif

ronroy@CA.INTER.NET on mon 9 jan 12

 

 

Earthenware is not as strong as properly formulated and fired stoneware and porcelain - there is no question about that and that includes chipability as well. Pete admitted he did not resize all his samples - the earthenware bars were bigger than the stoneware and porcelain bars because earthenware does not shrink as much. If you want to do a scientific study on the subject you have to make sure the bars are the same size. I have cut hundreds of bars for my dilatometer - all kinds of clays and glazes - the saw goes through earthenware like butter compared to stoneware and porcelain. Pete should be congratulated for doing and sharing any research on clay and glazes - it is well needed - and his article has valuable information for all potters. It just happens to be inconclusive in that one area. All the literature on the subject points to earthenware being softer and weaker which is why he said "This will surprise you-it certainly did me."

 

 

 

RR

 

 

 

Quoting Lee :

 

 

 

This is absolutely untrue. Fired to a higher temp, Earthenware can be stronger than porcelain. I have found this true with my own work. I've always liked Raku much better than porcelain and even better than most stoneware.

 

 

 

I include Pete's entire essay on claybody strength below (and Kurt Wild's brief thank you to Pete.)

 

 

 

Claybody Strength

 

 

 

Pete Pinnell on thu 20 dec 01

 

 

 

 

 

For the final project in my Clay and Glaze class this semester, we mixed about 50 clay bodies for testing, including red and white earthenwares, stoneware, porcelain, and sculpture bodies. Besides other tests, we extruded numerous bars of each body and broke them to measure MOR (Modulus Of Rupture, which is a measure of the bending strength). There are other strength tests that can be done (chipping tests, for instance), but MOR is a quick and easy way to predict how well a body will hold up to the bumps of everyday use.

 

 

 

Out of all these tests, there were a number of interesting trends:

 

1. Any amount of grog weakens clay bodies, especially in sculpture bodies that are essentially underfired. Some of the sculpture clays were so weak at cone 04 that we couldn't measure them- the bars broke at initial contact before any stress was applied. Any texture in the clay tended to have the same result, though the texture from using 50 mesh fireclay seemed to have only a minimal effect. Really fine grogs- those less than 80 mesh- also had little effect.

 

 

 

2. Glaze made a huge difference in strength. Crazed glazes lowered results 50% or more from the strength of the same bar unglazed. Uncrazed glazes raised the strength of the bars from 50 to 100 %. I had read this before, and assumed that it was mostly related to the lack of surface flaws on a smooth glaze (cracks like to start at a flaw- take away the flaws and it more difficult for a crack to start). What I found interesting is that the amount of compression also mattered. We glazed the porcelain bars with three different versions of my Pete's Clear glaze, which ranged from mild compression for the original version to a very low expansion version that places the clay in a very high compression. Consistently, the higher compression versions produced higher MOR results.

 

 

 

3. Clays have to be fired to maturity to get good strength. Even firing porcelain bodies to cone 9 rather than 10 lowered strengths a good deal.

 

 

 

As an aside, I define maturity as the point at which a body achieves it's best strength and glaze fit, and no longer suffers from marked moisture expansion. Absorption, in my opinion, is not a good indicator except within one clay body group (such as high fire porcelain). Porcelains may need to have less than 1% absorption to avoid moisture expansion problems, while mature white earthenwares can have upwards of 20% absorption (which is why those cheap white tiles on our shower walls don't develop delayed crazing).

 

 

 

4. Smooth counts for more than glassy, which seems to contradict one bit of standard wisdom I've heard in the past.

 

 

 

5. Quartz seems to be a problem- at least in a minor way. Porcelain bodies that used a combination of pyrophyllite and quartz were stronger than those which used only quartz as a filler. It's a bit of a mixed bag, though, because glazes on pyrophyllite bodies tended to craze more.

 

 

 

What were the strongest clays? This will surprise you- it certainly did me. The strongest clays, consistently, were (drum roll, please) red earthenware clays fired to a full cone 04.

 

 

 

Yep, that's right. Plain old Redart based, smooth red earthenwares.

 

 

 

They were stronger than smooth, brown or gray stonewares, and even stronger (over all) than porcelain, which I had assumed would be best.

 

 

 

Yes, it was very important to fire them to a full cone 04: cone 06 didn't hack it. Surprisingly, taking them to cone 1 did not increase MOR, though they certainly were denser and felt more solid and chip resistant. Within red earthenwares, we got consistently higher strength from those using wollastonite as a secondary flux (5 to 10%), rather than talc. It seemed best to use red clay in amounts of 50 to 70%, and while Redart alone (for the red clay portion of the body) gave the best strength, we got much better workability (and only a tiny bit less strength) by using a mixture of red clays, such as Redart mixed with Ranger Red (from Texas) and Apache Red (from Colorado).

 

 

 

As with porcelain, the clay was made much stronger with glazes that fit, and higher compression glazes were strongest of all. Our all-time champion (for strength, NOT workability) was the following recipe, glazed with Linda Arbuckle Majolica and fired to a full cone 04.

 

 

 

Redart, 60%

 

KT 1-4 Ball Clay, 30%

 

Wollastonite, 10%

 

 

 

I thought you might find this interesting. I only teach a Clay and Glaze class one semester every three years, so while I plan to do some follow up tests (these tests raised as many questions as they answered), don't look for those results any time soon!

 

 

 

Pete Pinnell

 

University of Nebraska at Lincoln

 

 

 

 

 

Kurt Wild on thu 20 dec 01

 

 

 

A thank you to Pete Pinnell on his interesting post of 12/20 on "clay body strength".... worth reading!

 

 

 

Kurt

 

 

 

 

 

 

 

-- Lee Love in Minneapolis

 

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If I'm remembering correctly the Pinnell paper had some critics, perhaps one was Ron Roy. Like others (though not as educated as others on the subject), I think that the clay and the technique matter. I can see ^10 being stronger because of the clay/glaze interface being better. But on the other hand, cracks don't propagate as well through a crystalline body (04) as they do through a glassy body (10). Clay glaze compression vs. body thickness matters too, meaning that thin isn't the best way to make a usable product, unless a thin glaze is used. And as I've mentioned, I suspect that cross section along with fired unit volume vs. original unit volume is in there too.

 

I just can't see how a rigorous comparison can be done in any economical way. I think we can draw some generalizations from experience, but one potter's cone x mug may be stronger than another potter's mug, and one clay manufactures cone x clay may be stronger than another's. As was said, strong enough is strong enough. Use good clay, don't over reduce. Thin glazes are better if you throw thin ware. Generalizations like these work well for me.

 

 

I'd love to see what critics of the Pinnell "paper" said. I searched a little bit but couldn't find anything. It sure would be nice if you're going to make that statement to point us to those criticisms. By saying "I can see ^10 being stronger because of the clay/glaze interface being better," you're saying basically what Mark says that cone 10 pottery is stronger than ^6 potter which is not true. Once again, strength depends on the composition of a clay fired to maturity. Some cone 6 clays are stronger than some cone 10 clays and some cone 10 clays are stronger than some cone 6 clays. If clay has grog or sand in it it is weaker than the same clay without grog or sand. Why do you think a cone 10 clay and glaze has a better "interface" than a cone 6 or cone 3 or cone 04 clay and glaze? A cone 6 glaze can "fit" a cone 6 body as well as a cone 10 glaze can "fit" a cone 10 body. Obviously, a MOR test takes a bit more time and effort than banging two pots together, but one is a scientific test (and VERY easy and simple by scientific testing standards) and the other is a waste of time.

 

Jim

 

 

I read the paper a few months ago and was told by the master potter in the shop that there were critics, and I was thinking Ron Roy was one. I'll try and remember to ask who the critics were and where they spoke. I would like to note that I'm a Pete Pinnell fan.

 

Also, I gave reasons why both would be strong. The information I have states that a good clay/glaze interface (melt zone, or whatever it is called) combined with good coefficients of expansion and a good ratio of glaze thickness vs. wall thickness, make for a good pot. To my knowledge the interface gets better the hotter you fire. I can't say I know, I don't own a scanning-tunneling microscope. It seems logical though since I would expect a better interface between a vitrified body and a vitrified glaze, as opposed to a non-vitrified body and a vitrified glaze.

 

I think a few general understandings work quite well in creating a strong piece of pottery. And, I suspect doing an MOR in a lab isn't much more involved than "banging two pots together". I'm guessing all Pete Pinnell did was say, "Hey you, go calculate the MOR of that chuck of clay." In fact, given a 1942 Ford hubcap, a bobby pin, and some piezoelectric salts, I bet I could make a MOR measuring device in my garage, heheh.

 

Joel.

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If I'm remembering correctly the Pinnell paper had some critics, perhaps one was Ron Roy. Like others (though not as educated as others on the subject), I think that the clay and the technique matter. I can see ^10 being stronger because of the clay/glaze interface being better. But on the other hand, cracks don't propagate as well through a crystalline body (04) as they do through a glassy body (10). Clay glaze compression vs. body thickness matters too, meaning that thin isn't the best way to make a usable product, unless a thin glaze is used. And as I've mentioned, I suspect that cross section along with fired unit volume vs. original unit volume is in there too.

 

I just can't see how a rigorous comparison can be done in any economical way. I think we can draw some generalizations from experience, but one potter's cone x mug may be stronger than another potter's mug, and one clay manufactures cone x clay may be stronger than another's. As was said, strong enough is strong enough. Use good clay, don't over reduce. Thin glazes are better if you throw thin ware. Generalizations like these work well for me.

 

 

I'd love to see what critics of the Pinnell "paper" said. I searched a little bit but couldn't find anything. It sure would be nice if you're going to make that statement to point us to those criticisms. By saying "I can see ^10 being stronger because of the clay/glaze interface being better," you're saying basically what Mark says that cone 10 pottery is stronger than ^6 potter which is not true. Once again, strength depends on the composition of a clay fired to maturity. Some cone 6 clays are stronger than some cone 10 clays and some cone 10 clays are stronger than some cone 6 clays. If clay has grog or sand in it it is weaker than the same clay without grog or sand. Why do you think a cone 10 clay and glaze has a better "interface" than a cone 6 or cone 3 or cone 04 clay and glaze? A cone 6 glaze can "fit" a cone 6 body as well as a cone 10 glaze can "fit" a cone 10 body. Obviously, a MOR test takes a bit more time and effort than banging two pots together, but one is a scientific test (and VERY easy and simple by scientific testing standards) and the other is a waste of time.

 

Jim

 

 

I read the paper a few months ago and was told by the master potter in the shop that there were critics, and I was thinking Ron Roy was one. I'll try and remember to ask who the critics were and where they spoke. I would like to note that I'm a Pete Pinnell fan.

 

Also, I gave reasons why both would be strong. The information I have states that a good clay/glaze interface (melt zone, or whatever it is called) combined with good coefficients of expansion and a good ratio of glaze thickness vs. wall thickness, make for a good pot. To my knowledge the interface gets better the hotter you fire. I can't say I know, I don't own a scanning-tunneling microscope. It seems logical though since I would expect a better interface between a vitrified body and a vitrified glaze, as opposed to a non-vitrified body and a vitrified glaze.

 

I think a few general understandings work quite well in creating a strong piece of pottery. And, I suspect doing an MOR in a lab isn't much more involved than "banging two pots together". I'm guessing all Pete Pinnell did was say, "Hey you, go calculate the MOR of that chuck of clay." In fact, given a 1942 Ford hubcap, a bobby pin, and some piezoelectric salts, I bet I could make a MOR measuring device in my garage, heheh.

 

Joel.

 

 

Gee, Joel, I don't want to be accused again of being mean or rude so I won't say, "What a crock of crap!", but I sure am tempted to say that. One of the points made over and over in this discussion is that we're comparing clays fired to maturity, so your "... I would expect a better interface between a vitrified body and a vitrified glaze, as opposed to a non-vitrified body and a vitrified glaze" has nothing to do with comparing the strengths of clays fired to different maturing cones. Do you think all clays have to be fired to cone 10 to be vitrified? BTW, you don't need a scanning-tunneling microscope or a weatherman or a lab to do MOR tests, and while that test isn't perfect, it is really strange to see somebody (especially someone who thinks you need a lab with a scanning-tunneling microscope to do them) write that they think a MOR test "... isn't much more involved than 'banging two pots together.' " You say you're a fan of Pinnell but then you sarcastically dismiss his work with a silly bogus quote.

 

Maybe, you're right, Joel, "...a few general understandings" like cone 10 pots are stronger than cone 6 pots, the clay-glaze interface gets better the hotter you fire, a MOR test is no better than banging two pots together, air bubbles make pots blow up, blue pots are stronger than white pots, the earth is obviously flat are all we need.

 

Jim

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I suspect that if Pete were in on this conversation he would not dig in his heels and protest to it's accuracy but he might say ... Yeah, let's try another round of tests. I only know him as a workshop teacher and writer but I have observed that he is very curious and very open minded ...he is also of the bent to finding scientific answers rather than passing on anecdotal arguments.

With this is mind might I anecdotally say that all those low fired terra cotta pots from 6,000 years ago that I see exhibited in museums are cracked so how strong we're they, huh? :)

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more from the Clayart archives . . . google Pete Pinnell MOR test

 

 

adventures in earthenware - rr

 

updated tue 10 jan 12

small%20potters%20logo.gif

ronroy@CA.INTER.NET on mon 9 jan 12

 

 

Earthenware is not as strong as properly formulated and fired stoneware and porcelain - there is no question about that and that includes chipability as well. Pete admitted he did not resize all his samples - the earthenware bars were bigger than the stoneware and porcelain bars because earthenware does not shrink as much. If you want to do a scientific study on the subject you have to make sure the bars are the same size. I have cut hundreds of bars for my dilatometer - all kinds of clays and glazes - the saw goes through earthenware like butter compared to stoneware and porcelain. Pete should be congratulated for doing and sharing any research on clay and glazes - it is well needed - and his article has valuable information for all potters. It just happens to be inconclusive in that one area. All the literature on the subject points to earthenware being softer and weaker which is why he said "This will surprise you-it certainly did me."

 

 

 

RR

 

 

 

Quoting Lee :

 

 

 

This is absolutely untrue. Fired to a higher temp, Earthenware can be stronger than porcelain. I have found this true with my own work. I've always liked Raku much better than porcelain and even better than most stoneware.

 

 

 

I include Pete's entire essay on claybody strength below (and Kurt Wild's brief thank you to Pete.)

 

 

 

Claybody Strength

 

 

 

Pete Pinnell on thu 20 dec 01

 

 

 

 

 

For the final project in my Clay and Glaze class this semester, we mixed about 50 clay bodies for testing, including red and white earthenwares, stoneware, porcelain, and sculpture bodies. Besides other tests, we extruded numerous bars of each body and broke them to measure MOR (Modulus Of Rupture, which is a measure of the bending strength). There are other strength tests that can be done (chipping tests, for instance), but MOR is a quick and easy way to predict how well a body will hold up to the bumps of everyday use.

 

 

 

Out of all these tests, there were a number of interesting trends:

 

1. Any amount of grog weakens clay bodies, especially in sculpture bodies that are essentially underfired. Some of the sculpture clays were so weak at cone 04 that we couldn't measure them- the bars broke at initial contact before any stress was applied. Any texture in the clay tended to have the same result, though the texture from using 50 mesh fireclay seemed to have only a minimal effect. Really fine grogs- those less than 80 mesh- also had little effect.

 

 

 

2. Glaze made a huge difference in strength. Crazed glazes lowered results 50% or more from the strength of the same bar unglazed. Uncrazed glazes raised the strength of the bars from 50 to 100 %. I had read this before, and assumed that it was mostly related to the lack of surface flaws on a smooth glaze (cracks like to start at a flaw- take away the flaws and it more difficult for a crack to start). What I found interesting is that the amount of compression also mattered. We glazed the porcelain bars with three different versions of my Pete's Clear glaze, which ranged from mild compression for the original version to a very low expansion version that places the clay in a very high compression. Consistently, the higher compression versions produced higher MOR results.

 

 

 

3. Clays have to be fired to maturity to get good strength. Even firing porcelain bodies to cone 9 rather than 10 lowered strengths a good deal.

 

 

 

As an aside, I define maturity as the point at which a body achieves it's best strength and glaze fit, and no longer suffers from marked moisture expansion. Absorption, in my opinion, is not a good indicator except within one clay body group (such as high fire porcelain). Porcelains may need to have less than 1% absorption to avoid moisture expansion problems, while mature white earthenwares can have upwards of 20% absorption (which is why those cheap white tiles on our shower walls don't develop delayed crazing).

 

 

 

4. Smooth counts for more than glassy, which seems to contradict one bit of standard wisdom I've heard in the past.

 

 

 

5. Quartz seems to be a problem- at least in a minor way. Porcelain bodies that used a combination of pyrophyllite and quartz were stronger than those which used only quartz as a filler. It's a bit of a mixed bag, though, because glazes on pyrophyllite bodies tended to craze more.

 

 

 

What were the strongest clays? This will surprise you- it certainly did me. The strongest clays, consistently, were (drum roll, please) red earthenware clays fired to a full cone 04.

 

 

 

Yep, that's right. Plain old Redart based, smooth red earthenwares.

 

 

 

They were stronger than smooth, brown or gray stonewares, and even stronger (over all) than porcelain, which I had assumed would be best.

 

 

 

Yes, it was very important to fire them to a full cone 04: cone 06 didn't hack it. Surprisingly, taking them to cone 1 did not increase MOR, though they certainly were denser and felt more solid and chip resistant. Within red earthenwares, we got consistently higher strength from those using wollastonite as a secondary flux (5 to 10%), rather than talc. It seemed best to use red clay in amounts of 50 to 70%, and while Redart alone (for the red clay portion of the body) gave the best strength, we got much better workability (and only a tiny bit less strength) by using a mixture of red clays, such as Redart mixed with Ranger Red (from Texas) and Apache Red (from Colorado).

 

 

 

As with porcelain, the clay was made much stronger with glazes that fit, and higher compression glazes were strongest of all. Our all-time champion (for strength, NOT workability) was the following recipe, glazed with Linda Arbuckle Majolica and fired to a full cone 04.

 

 

 

Redart, 60%

 

KT 1-4 Ball Clay, 30%

 

Wollastonite, 10%

 

 

 

I thought you might find this interesting. I only teach a Clay and Glaze class one semester every three years, so while I plan to do some follow up tests (these tests raised as many questions as they answered), don't look for those results any time soon!

 

 

 

Pete Pinnell

 

University of Nebraska at Lincoln

 

 

 

 

 

Kurt Wild on thu 20 dec 01

 

 

 

A thank you to Pete Pinnell on his interesting post of 12/20 on "clay body strength".... worth reading!

 

 

 

Kurt

 

 

 

 

 

 

 

-- Lee Love in Minneapolis

 

 

 

This must be what Yedrow was referring to in his post. Obviously, Ron is only disputing the "earthenware is strongest" result, not the overall conclusion that it is composition not temperature that determines strength.

 

Jim

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I suspect that if Pete were in on this conversation he would not dig in his heels and protest to it's accuracy but he might say ... Yeah, let's try another round of tests. I only know him as a workshop teacher and writer but I have observed that he is very curious and very open minded ...he is also of the bent to finding scientific answers rather than passing on anecdotal arguments.

With this is mind might I anecdotally say that all those low fired terra cotta pots from 6,000 years ago that I see exhibited in museums are cracked so how strong we're they, huh? :)

 

 

I think you're absolutely right about Pete. I wish he taught his clay and glazes class more often so he'd do more tests. One thing (among many) that I like about Pete and Steven Hill is that even though both of them have been potting and teaching for decades, both are willing to embrace something that contradicts something they've done, believed, or taught for most of their professional lives. I remember an article Pete did a while back pointing out that until he actually tested it, he believed that air bubbles caused pots to explode and went on to blame that fallacy on lazy teachers who find it easier to blame an exploded pot on the student not wedging enough than on the teacher not waiting for a student's thick, heavy pot to dry completely. And Steven Hill suddenly, after most of his life as a potter believing that you have to fire to cone 10 reduction to get beautiful, subtle, complex glazes, turns on a dime and rejects all that when he finds he can get as good, if not better, results firing to cone 6 in an electric kiln.

 

Jim

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If I'm remembering correctly the Pinnell paper had some critics, perhaps one was Ron Roy. Like others (though not as educated as others on the subject), I think that the clay and the technique matter. I can see ^10 being stronger because of the clay/glaze interface being better. But on the other hand, cracks don't propagate as well through a crystalline body (04) as they do through a glassy body (10). Clay glaze compression vs. body thickness matters too, meaning that thin isn't the best way to make a usable product, unless a thin glaze is used. And as I've mentioned, I suspect that cross section along with fired unit volume vs. original unit volume is in there too.

 

I just can't see how a rigorous comparison can be done in any economical way. I think we can draw some generalizations from experience, but one potter's cone x mug may be stronger than another potter's mug, and one clay manufactures cone x clay may be stronger than another's. As was said, strong enough is strong enough. Use good clay, don't over reduce. Thin glazes are better if you throw thin ware. Generalizations like these work well for me.

 

 

I'd love to see what critics of the Pinnell "paper" said. I searched a little bit but couldn't find anything. It sure would be nice if you're going to make that statement to point us to those criticisms. By saying "I can see ^10 being stronger because of the clay/glaze interface being better," you're saying basically what Mark says that cone 10 pottery is stronger than ^6 potter which is not true. Once again, strength depends on the composition of a clay fired to maturity. Some cone 6 clays are stronger than some cone 10 clays and some cone 10 clays are stronger than some cone 6 clays. If clay has grog or sand in it it is weaker than the same clay without grog or sand. Why do you think a cone 10 clay and glaze has a better "interface" than a cone 6 or cone 3 or cone 04 clay and glaze? A cone 6 glaze can "fit" a cone 6 body as well as a cone 10 glaze can "fit" a cone 10 body. Obviously, a MOR test takes a bit more time and effort than banging two pots together, but one is a scientific test (and VERY easy and simple by scientific testing standards) and the other is a waste of time.

 

Jim

 

 

I read the paper a few months ago and was told by the master potter in the shop that there were critics, and I was thinking Ron Roy was one. I'll try and remember to ask who the critics were and where they spoke. I would like to note that I'm a Pete Pinnell fan.

 

Also, I gave reasons why both would be strong. The information I have states that a good clay/glaze interface (melt zone, or whatever it is called) combined with good coefficients of expansion and a good ratio of glaze thickness vs. wall thickness, make for a good pot. To my knowledge the interface gets better the hotter you fire. I can't say I know, I don't own a scanning-tunneling microscope. It seems logical though since I would expect a better interface between a vitrified body and a vitrified glaze, as opposed to a non-vitrified body and a vitrified glaze.

 

I think a few general understandings work quite well in creating a strong piece of pottery. And, I suspect doing an MOR in a lab isn't much more involved than "banging two pots together". I'm guessing all Pete Pinnell did was say, "Hey you, go calculate the MOR of that chuck of clay." In fact, given a 1942 Ford hubcap, a bobby pin, and some piezoelectric salts, I bet I could make a MOR measuring device in my garage, heheh.

 

Joel.

 

 

Gee, Joel, I don't want to be accused again of being mean or rude so I won't say, "What a crock of crap!", but I sure am tempted to say that. One of the points made over and over in this discussion is that we're comparing clays fired to maturity, so your "... I would expect a better interface between a vitrified body and a vitrified glaze, as opposed to a non-vitrified body and a vitrified glaze" has nothing to do with comparing the strengths of clays fired to different maturing cones. Do you think all clays have to be fired to cone 10 to be vitrified? BTW, you don't need a scanning-tunneling microscope or a weatherman or a lab to do MOR tests, and while that test isn't perfect, it is really strange to see somebody (especially someone who thinks you need a lab with a scanning-tunneling microscope to do them) write that they think a MOR test "... isn't much more involved than 'banging two pots together.' " You say you're a fan of Pinnell but then you sarcastically dismiss his work with a silly bogus quote.

 

Maybe, you're right, Joel, "...a few general understandings" like cone 10 pots are stronger than cone 6 pots, the clay-glaze interface gets better the hotter you fire, a MOR test is no better than banging two pots together, air bubbles make pots blow up, blue pots are stronger than white pots, the earth is obviously flat are all we need.

 

Jim

 

 

Don't worry about being mean to me, I get that all the time, lol.

 

What does bother me is the appearance that you aren't actually reading the post you are commenting on. I made a joke about the ease of doing an MOR, but examining clay/glaze interface is different. There are nifty things happening there. I think a paper came was introduced in the last NCECA that pointed out an exchange of calcia and alumina between glaze and body respectively. Lots of stuff is happening and a magnifying glass isn't going to detect much of it.

 

 

I'm probably wrong about earthenware clays. I had assumed they were, by definition, non-vitrified.

 

Oh, and I suspect that all you need to do a sufficient MOR test are some blocks of varying size and known weight (scrap), two fulcrums, and an articulated lever. Measure weights under the lever, then put your sample across the fulcrums and measure. You don't really need to compare it to modulus, you just need to know which takes more weight to rupture the piece.

 

Even then you aren't going to get the whole story, and as Roy said, you aren't talking about exactly the same thing since the unit volume will change when the pieces are fired; Hence my original presumptions = the cross section was the strength, not the material.

 

And, as I was saying, crack propagation also plays a role since an (presumably crystalline) earthenware body doesn't allow cracks to propagate as efficiently as an amorphic body does. Also, it seems I read somewhere that a good interface also acts to block crack propagation. The rupture of the object will be accompanied by cracking.

 

Keep in mind that MOR is testing tensile strength on the side opposite the force applied (along with compressive strength on the side of the force). In a vessel these qualities will likely be different, as opposed to a block of clay.

 

Joel.

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Joel,

 

This is why in one of my other posts I said something about "strength" was a rather general term and what exactly are we really talking about here?

 

A material can be strong in compression, but weak in tension.... like glass and glazes.

 

As I said before this is NOT a simple subject. The variables are legion.

 

And yse... the corrs section for an actual MOR test is VERY important. 1 cm x 1cm............... surface machined to this in industry.

 

best,

 

.........john

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Chris's response summed up that day well for me

(The best you can say for your experiment is that on this particular day, with these particular bowls, held by a non-ambidextrous male, struck in a more or less forceful way ... One broke before the other did. )

Thanks TRJ for the kind words but thats not going to happen-this is not my 1st go round with this person

After getting beat up on this post I'm gun shy to say more so I'll add a positive lesson I have learned long ago

You cannot shake hands with a fist-I learned that back in the 50s

So for now back to making mugs

Mark

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