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Originally published in February 2019 issue of Ceramics Monthly, pages 66,67, & 68. http://www.ceramicsmonthly.org . Copyright, The American Ceramic Society. Reprinted with permission."

techno File section: Clay Body Shopping

 

 

 

Nerd

Edited by glazenerd
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I want a porcelain that fires hard as nails is translucent and is super cheap ((300$)a ton) only needs 1,000 degrees to mature (for low firing cost)Has great throwing legs will not crack and has zero platelet memory hand builds great throws great fires SUPER white.I have few other requirements but this is the short list. Any suggestions on a where to get this body???

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I am limited to my clay right now as I don't want to travel to get some. I am trying out some Babu porcelain from laguna. I prefer Coleman but not available here. I have wanted to try the Babu and was advised it is very similar in chemical composition to Nara which I have tried and liked.

There are so many choices out there. Still searching.

Thanks Nerd for that summation. Good way to look at clays.

 

Marcia

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Great Write Up, Thanks.

 

I'll admit I'm confused about clay ball.  Some of the things I read says its a bad thing to have in a clay body while others say its important to have.

 

I know the answer my question will start with "It Depends" and possibly get 30 different answers from 10 different potters. So here goes anyway.

 

When or why is clay ball a good or bad thing?

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I want a porcelain that fires hard as nails is translucent and is super cheap ((300$)a ton) only needs 1,000 degrees to mature (for low firing cost)Has great throwing legs will not crack and has zero platelet memory hand builds great throws great fires SUPER white.I have few other requirements but this is the short list. Any suggestions on a where to get this body???

Somewhere on the moons of Jupiter? <_< 

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I continue to have a good relationship with Standard, as it is close, Pittsburgh. I usually talk to them on the phone, or in person, tell them what I am looking for, discuss the options, and they make a recommendation usually with some samples to try for future reference. This has kept me a customer, and also an advocate. I think that getting a good relationship with a supplier is a great way to make certain you have good options. However, local suppliers sometimes do not have the type of variety you may need, and sometimes there is not a supplier that is . . . . local.

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I want a porcelain that fires hard as nails is translucent and is super cheap ((300$)a ton) only needs 1,000 degrees to mature (for low firing cost)Has great throwing legs will not crack and has zero platelet memory hand builds great throws great fires SUPER white.I have few other requirements but this is the short list. Any suggestions on a where to get this body???

 

Some songs came to mind:

 

This is planet earth

Somewhere over the rainbow

Imagine

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I want a porcelain that fires hard as nails is translucent and is super cheap ((300$)a ton) only needs 1,000 degrees to mature (for low firing cost)Has great throwing legs will not crack and has zero platelet memory hand builds great throws great fires SUPER white.I have few other requirements but this is the short list. Any suggestions on a where to get this body???

 

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only needs 1,000 degrees to mature

Perhaps a joke; but actually technology is using nano-particle kaolins, feldspars, in combination with chemical solvents to produce porcelain coatings that vitrify at 800C. I would suspect that the cost is closer to $3000 per ton. Those circuit boards in your phone, computers, and other electronic gadgets have a nice coating of zinc-silicate (crystalline glaze) that are applied with 800C steam shots. You can get what you want, but you do have to pay for it- dearly.

 

Nerd

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RonSa asked- When or why is clay ball a good or bad thing?

All stoneware bodies have some ball clay in them; some have a lot. Ball clays serve two purposes; primarily to impart plasticity, and secondly to lower absorption values. To help you visualize how it works: kaolins look like this O, and ball clays look like this O, or this O, or this O.  The smaller the particles get, the more plastic they become. So ball clays are a good thing: the primary component for making clay plastic. Plasticizers are even smaller than this O, but they can add .08 up to 0.15 ¢ a lb of clay. So you rarely see plasticizers used in clay bodies with the exception of high end porcelains. Plasticizers equal = macaloid and V-gum T.

 

So when stoneware bodies start with large particle fire clays like this O, then they have to add percentages of intermediate clays like this O, and then finer ball clays like this O, and this in order to fill in the voids (PSD)and to impart plasticity. This use of ball clays is mandatory in stoneware bodies in order to keep the absorption rates low. High absorption rates also equal weeping. If a body will absorb water, that also means it will leak water.

 

The problem with ball clays is their purity levels: IE- carbon content. Kaolins (porcelain) are considered clean clays because they have carbon content of less than 50 parts per million: PPM. Ball clays can have 100, and well over 1000 PPM of carbons. So I will visualize that: a clean kaolin, or kaolinitic ball clay looks like this O, and high carbon ball clays look like this O. So just by the color, it is easy to imagine how it effects the color of the clay. The color is really not the problem, it is what the carbons do in firing. Most carbons in ball clay are sulfates and sulfides: lignite coal dust is a primary example. Sulfates will burn off in a firing, sulfides will not. Sulfides will off gas as well, and as they do they build up an impermeable barrier just below the face of the clay. When that occurs, the off gassing from the feldspars get trapped below it, and bloating occurs. They can also cause a muddy appearance in glazes, and if large enough: the appearance of fine pepper like particles on the face of the clay.

 

The gold standard for porcelain is grolleg kaolin, feldspar, silica and plasticizers: which produce a high white, highly translucent body. The problem being this standard runs between 0.75 to 0.90 ¢ a lb. We potters like our clays to be cheap, which requires makers to use cheaper kaolins and ball clays to meet those price points. The intermediate porcelain price points still use cleaner clays, but not nearly as clean as pure porcelain bodies: which is reflected in the loss of translucency. The cheaper versions of porcelain use very cheap ball clays: which also means they are much higher in carbons. You can tell by the color; usually a medium grey color due to higher carbon ball clays. When high carbon ball clays are used in porcelain: translucency is completely lost. The usual bright white ends up being off white, to a very light grey cast. It also effects the brightness of the glaze, because enough carbons are leached from the clay body to cause color shifts in the glaze.

 

These general principles also apply to clay additions made to glazes. If you use a higher carbon ball clay in a glaze; it will reduce its brightness, and give a muddied appearance if enough is used. The reason I prefer New Zealand kaolin in glazes over EPK, OM4, or others. There is a noticeable difference in glaze finish and appearance when higher purity clays are used in glazes.

 

Nerd

 

Sorry about the spacing issues; tried several times to correct it.

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Most Excellent, thank you Nerd

 

Don't worry about the spacing, it makes it easier to read. Which I need to do a few times to absorb all the info you presented.

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I am limited to my clay right now as I don't want to travel to get some. I am trying out some Babu porcelain from laguna. I prefer Coleman but not available here. I have wanted to try the Babu and was advised it is very similar in chemical composition to Nara which I have tried and liked.

There are so many choices out there. Still searching.

Thanks Nerd for that summation. Good way to look at clays.

 

Marcia

 

Babu is great stuff.

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