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Alina Albu

clay elasticity (or not)

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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

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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

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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

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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

 

 

 

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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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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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