Maougna Posted February 20, 2021 Report Share Posted February 20, 2021 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 ? Link to comment Share on other sites More sharing options...
Sorcery Posted February 20, 2021 Report Share Posted February 20, 2021 What's your firing schedule like? I use some irony and dirty clays that only slump if fired too fast through burnout. I'd start there then maybe add higher fire clay in small amounts until it's cured. Sorce Link to comment Share on other sites More sharing options...
Maougna Posted February 20, 2021 Author Report Share Posted February 20, 2021 Thanks for the quick reply . I fire at about 150 ° / hr but slow down from 900 up . I don't want to add commercial clay unless really necessary . What would happen if I add silica ? Link to comment Share on other sites More sharing options...
Sorcery Posted February 20, 2021 Report Share Posted February 20, 2021 Test it! You'll probably end up with too much silica, but then you're also just some alumina away from adding clay. I'd try silica sand as a grog maybe. I had a backyard clay hold up at cone 6 until I removed the sand. Sorce Link to comment Share on other sites More sharing options...
Maougna Posted February 20, 2021 Author Report Share Posted February 20, 2021 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 Link to comment Share on other sites More sharing options...
Min Posted February 20, 2021 Report Share Posted February 20, 2021 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. Link to comment Share on other sites More sharing options...
Maougna Posted February 20, 2021 Author Report Share Posted February 20, 2021 Thanks for your reply . Yes , I would like to continue firing at 1250 if possible as I like the glazes at this temperature . Not all of my pots deform , only a few and especially on the top shelf . Link to comment Share on other sites More sharing options...
oldlady Posted February 20, 2021 Report Share Posted February 20, 2021 do you fire with cones? many of our european members only mention a temperature. are you familiar with using cones to tell how the length of time affects the clay, called heatwork? Link to comment Share on other sites More sharing options...
glazenerd Posted February 21, 2021 Report Share Posted February 21, 2021 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 Link to comment Share on other sites More sharing options...
Mark C. Posted February 21, 2021 Report Share Posted February 21, 2021 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 Link to comment Share on other sites More sharing options...
Maougna Posted April 3, 2021 Author Report Share Posted April 3, 2021 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 ? Link to comment Share on other sites More sharing options...
Mark C. Posted April 3, 2021 Report Share Posted April 3, 2021 Yes as Nerd said 5% -10% can help. Are you stacking plates in bisque fire? Link to comment Share on other sites More sharing options...
Maougna Posted April 3, 2021 Author Report Share Posted April 3, 2021 Thank you . Yes I'm stacking the plates. Would the addition of 5% kaolin effect the firing temperature ? Link to comment Share on other sites More sharing options...
Maougna Posted April 3, 2021 Author Report Share Posted April 3, 2021 I'm undecided about how much kaolin to add . If I add too much what happens ? Link to comment Share on other sites More sharing options...
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