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Strengthening home made clay


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Hi everyone , I have had the good fortune  of discovering some very nice clay . I have managed to fire it up to 1250°C ; it's a beautiful colour and the glaze holds well . It is really the limit though as some pots deform slightly so I'd like some advice as to how I can strengthen it . I make it by throwing it in the cement mixer with lots of water , then I filter it several times so I'm left with a very smooth slurry . I put the slurry in large plaster containers which absorb the moisture so I'm finally left with a usable clay . I' wondering if I can mix something like silica in the slurry  but have no idea about the chemistry , quantity , etc. Can anybody help me please ?

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I like to say that my pots are made which local clay . If I add another clay I might as well just buy the clay in the first place. I finish off aclot of my pots with terra sigalatta so grogged clay would be no good . Yes , I'll have to test it but I'm not sure how so I thought maybe somebody could give me a clue . Thanks anyway

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Just to clarify, you want to keep firing to 1250C but the clay is slumping at this temperature? To raise the firing temperature of a clay body you obviously can't remove some of the existing flux in a commercial or wild clay so you will need to raise the clay content (probably kaolin) and the silica content which will in effect be diluting the flux content. Is firing to a lower temperature an option?

@glazenerd has helped a few people process wild clay, he might chime in here with his thoughts.

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Clay that slumps indicates the lack of alumina. Wild unprocessed clays usually do not have any appreciable amounts of natural fluxes: some rare exceptions like clays harvested along a coast line (salt migration). Slumping can also be an indication of ultra fine sub micron particle size. If you only have a mild issue: 5-10% kaolin (any type will work) will fix the issue. Kaolin has 37% alumina content and will spike cone firing values quickly. 

Tom

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EPK is the cheaper way to get alumina for sure as Tom says.

We did that with a cone 11 porcealain slip body that slumped slightly when it had a large hiole cut into the side-really worked great 

Wow that was 26 years ago-I'm getting old

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  • 1 month later...
Posted (edited)

Thanks all for your help . I haven't tried the advice yet because I've had further problems . Pieces , particularly plates have been cracking in the biscuit  firing . I have checked the thermocouple and the thermometer but it's still happening . The only conclusion is that it's the clay . Cups and small bowls come through ok but plates and larger pieces suffer . I have had no problems with my clay up to now and can only guess that I have hit a different vein . I'm wondering about the advice to add kaolin . Would that help the situation ?

Edited by Maougna
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