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Pat Newman

Correcting Porcelain Recipe

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Hi, I'm trying to produce my own porcelain. I began with the most simple recipe

 

15% Caolin

15% white clay

60% feldspat

10% flint

It's to cone 6 1.230° Celsius

 

It stays really white but not to plastic, not flexible, it cracks when I try to bend into a plaster mold, and if I add more water, it turns into mud. What can I do to correct and have a more plastic porcelain so I can have a very thin wall.

Thanks,

Pat

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Try adding Bentonite or Vgum to the mix. Start with about 2% bentonite and increase it until it works for you. it takes a long time to develop a clay body that works well. Porcelain takes longer. I am on year 32 I will let you know when I get the one I like.

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Guest JBaymore

The usual "starting point" for a cone 9-10 porcelain clay body is something about here:

 

Kaolin 25%

 

Low Iron Ball Clay 25%

 

Potash Feldspar 25%

 

Flint 25%

 

 

That puts the clay content at about 50%. For cone 6 you might want to substitute Nepheline Syenite for some of the potash spar to give you a bit more fluxing power and allow you to get the clay content up. HOWEVER, Neph Sy is slightly soluble in water.... so the clay could become thixotropic over time as it is stored. The bentonite suggestion is good, There is also the possiblity of adding some Macaloid instead. Select your kaolin for maximum plasticity... and ditto on the ball clay.

 

Cone 6 and "porcelain" is a a bit difficult combination to formulate. The trade-offs are legion to get the cone range down. You fix one aspect... and you trade off another.

 

 

best,

 

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

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The usual "starting point" for a cone 9-10 porcelain clay body is something about here:

 

Kaolin 25%

 

Low Iron Ball Clay 25%

 

Potash Feldspar 25%

 

Flint 25%

 

 

That puts the clay content at about 50%. For cone 6 you might want to substitute Nepheline Syenite for some of the potash spar to give you a bit more fluxing power and allow you to get the clay content up. HOWEVER, Neph Sy is slightly soluble in water.... so the clay could become thixotropic over time as it is stored. The bentonite suggestion is good, There is also the possiblity of adding some Macaloid instead. Select your kaolin for maximum plasticity... and ditto on the ball clay.

 

Cone 6 and "porcelain" is a a bit difficult combination to formulate. The trade-offs are legion to get the cone range down. You fix one aspect... and you trade off another.

 

 

best,

 

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

 

 

 

Greetings John:

 

I'm familiar with thixotropic clays in other industries, in paticular attapulgous clays. I've always understood the defintion of thixotropic as a mechanical property: changes in shear. Would the clay turning thixotropic affect the workability since the property is mainly shear related? I have been told that clays, porcelains in particular increase in workability when they are aged and in many cases start to host mold. I have had this discussion with a credible source but the source was not able to support the claims with actual science.

 

 

regards,

Charles

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Guest JBaymore

The thixitropy occurs mainly because of the sodium ions leaching /disolving out of the slightly soluble Neph Sy affecting the water chemistry over time. The clay with neph Sy actually can go thru all sorts of slow changes to the rheology/shear as the water chemsitry slowly changes over time. The higher the concenttration of the Neph Sy in the body... and the more the likelihood that this stuff will happen.

 

best,

 

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

 

PS: And oh yeah... makes the clay bonkers to try to work with.

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The thixitropy occurs mainly because of the sodium ions leaching /disolving out of the slightly soluble Neph Sy affecting the water chemistry over time. The clay with neph Sy actually can go thru all sorts of slow changes to the rheology/shear as the water chemsitry slowly changes over time. The higher the concenttration of the Neph Sy in the body... and the more the likelihood that this stuff will happen.

 

best,

 

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

 

PS: And oh yeah... makes the clay bonkers to try to work with.

 

 

 

Thanks John:

 

I really apreciate the explanation, I never thought if it in those terms.

 

Best regards,

Charles

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Thanks,John

I really don't know how to find all those products! I live in Brazil, and here the places where they sell those materials don't know much about its chemistry, and I'm a begginer also... So I'll try my best to find all these 'low iron ball clay"... I do have the Potash feldspar, which I've used...

What means thixotropic?

Thanks for answering!

Pat

 

 

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