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clay elasticity (or not)


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#1 Alina Albu

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Posted 09 November 2012 - 04:24 PM

I am trying to use a local stoneware that is not very plastic/elastic. What can I add to it in order to improve it? Many thanks.

#2 Marcia Selsor

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Posted 09 November 2012 - 05:50 PM

There is a commercial plasticizer available from some suppliers. Some people may suggest beer, yeast, urine, etc.
Aging may help. Adding some fine ball clay or a little bentonite.. Sometimes the way you process it helps. It you mix a slurry and let
it stiffen up..that promotes a plastic body.
Marcia

#3 Alina Albu

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Posted 10 November 2012 - 04:50 AM

There is a commercial plasticizer available from some suppliers. Some people may suggest beer, yeast, urine, etc.
Aging may help. Adding some fine ball clay or a little bentonite.. Sometimes the way you process it helps. It you mix a slurry and let
it stiffen up..that promotes a plastic body.
Marcia


Hi Marcia

Thank you for your reply.
I have already tried aging it - it improved a lot in terms of manageability. However, I still have problems pulling it up for taller forms. It keeps sliding back down, sort of thing.

I´ll try adding some bentonite or ball clay and see what happens. Don´t fancy adding urine(!). Now beer, that´s another story. "She throws with beer" could be my motto, "and sometimes she also adds it to the clay!" (sorry could not help myself:-)

Thanks again

#4 Marcia Selsor

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Posted 10 November 2012 - 08:24 AM

Beer stinks after a while. I think it is worse that moldy paper clay. Go easy on the percentage of bentonite. Try 2-5% and no more.
Ball clay can help and you can use more, 10% but test fire.
Marcia

#5 neilestrick

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Posted 10 November 2012 - 10:01 AM

What type of stoneware? White, grey, brown, smooth, groggy, etc?
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#6 Alina Albu

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Posted 12 November 2012 - 04:28 PM

What type of stoneware? White, grey, brown, smooth, groggy, etc?


Very smooth, and quite a dark grey. I buy it from the local supplier.

#7 neilestrick

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Posted 12 November 2012 - 04:51 PM

It's probably very high in ball clay, making it overly plastic. Some grog would help a lot.
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#8 Ben

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Posted 13 November 2012 - 08:46 AM

To test plasticity make a coil about the size of your finger and long as your hand. Try to wrap it around your finger. Does it break apart? does it show cracks? If it wraps around with few cracks it is very plastic.

Another test that would be helpful to you is a shrinkage test. Make some test bars about 6" x 1" x 1/2". Mark them with 5". dry them so they stay flat. I dry them on edge. measure the length of your marks when dry, after bisque and after high fire. Calculate the shrinkage percentage. (this is why I like 5" or 10 cm test lengths. makes the math easy.) For the 5" system just double the measurements and subtract ie: orig 5" end up 4.5" for a difference of 1/2" shrinkage at maturation. if we double all those 10"-9"=1" shrinkage. 1" is 10% of 10"...

Shrinkage in the low tens/teens is ok but when you get up past 14 it can become a problem and usually indicate fine particle size and increase plasticity.

If your clay is high plasticity ans shrinkage you may be able to counter that somewhat by the addition of grog (pre fired and crushed clay). You can buy it or you can make it from your clay if you have access to crushing and sieving equipment.
Grog additions would require further testing to determine an effective % addition of grog to the clay and to determine what that addition will do to glaze fit.

Where do you get your clay? I wish I had a local supply to experiment with but all I can find localy is earthenware.

Best wishes, Ben

#9 Alina Albu

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Posted 16 November 2012 - 07:37 AM

It's probably very high in ball clay, making it overly plastic. Some grog would help a lot.


Grog helped. Thank you.
Alina

#10 Alina Albu

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Posted 16 November 2012 - 07:40 AM

To test plasticity make a coil about the size of your finger and long as your hand. Try to wrap it around your finger. Does it break apart? does it show cracks? If it wraps around with few cracks it is very plastic.

Another test that would be helpful to you is a shrinkage test. Make some test bars about 6" x 1" x 1/2". Mark them with 5". dry them so they stay flat. I dry them on edge. measure the length of your marks when dry, after bisque and after high fire. Calculate the shrinkage percentage. (this is why I like 5" or 10 cm test lengths. makes the math easy.) For the 5" system just double the measurements and subtract ie: orig 5" end up 4.5" for a difference of 1/2" shrinkage at maturation. if we double all those 10"-9"=1" shrinkage. 1" is 10% of 10"...

Shrinkage in the low tens/teens is ok but when you get up past 14 it can become a problem and usually indicate fine particle size and increase plasticity.

If your clay is high plasticity ans shrinkage you may be able to counter that somewhat by the addition of grog (pre fired and crushed clay). You can buy it or you can make it from your clay if you have access to crushing and sieving equipment.
Grog additions would require further testing to determine an effective % addition of grog to the clay and to determine what that addition will do to glaze fit.

Where do you get your clay? I wish I had a local supply to experiment with but all I can find localy is earthenware.

Best wishes, Ben


Hi Ben

Maybe I was a bit loose with the term "local". It comes from North of Portugal - everything else here is aslo earthenware.
Alina




#11 neilestrick

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Posted 20 November 2012 - 06:16 PM

Earthenware white bodies are often not very workable. The problem is that they are not very high in clay content compared to a cone 6 or above stoneware body, and that they have very poor particle size distribution. That is, everything in it is a fine particle. For a body to be really workable, you need particles of various sizes. Grog, while rated at a specific mesh size, actually has particles of many sizes.
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#12 Alina Albu

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Posted 26 November 2012 - 04:10 AM

Earthenware white bodies are often not very workable. The problem is that they are not very high in clay content compared to a cone 6 or above stoneware body, and that they have very poor particle size distribution. That is, everything in it is a fine particle. For a body to be really workable, you need particles of various sizes. Grog, while rated at a specific mesh size, actually has particles of many sizes.


Hi Neil
Thanks for the info. It looks like what they call here "national stoneware" might actually be white earthenware(?). It fires at 1100-1200ºC.




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