Jump to content


  • Content count

  • Joined

  • Last visited

  1. Pyro-plasticity of clay

    I was looking at a forum about bonsai and there was a thread about pots and pottery. I noticed a post about slumping where the author noted a solution which was to support the pot using a bed of granular sand. This would make it pretty easy to just position the bisque ware on the shelf, apply sufficient pressure just to compress the sand 'enough' and bob's your uncle. It sounds like a good idea to me. The only problem would be the differential in movement at the feet vs the base supported by the sand, but for a small - 10mm - foot this would be unlikely to be a problem. Has anyone had a parallel experience with using grit or sand to provide temporary support to overcome slumping? Cheers, Happy Potter
  2. Pyro-plasticity of clay

    Thanks again for some useful ideas. Mark, the EPK addition should be good and I will test it along with firing temperature and porosity as the guide. The pots are press molded and I'm keen on the stoneware to get low porosity for durable behavior in all conditions outside.. It's not the glaze that softens the clay but I think just the temperature for the glaze firing. Interesting test on Talc, Matt and good to see the picture. Yes I enjoy Digitalfire very much, a most helpful site. John the pots you haven't seen in the gallery show visible horisontal droop of the pot floor between the feet, only perhaps 2mm over a span of 20mm but too much to accept and very noticeable. I have been making additional feet each time I make a pot and take them through both bisque and glaze firing, using them as props under the pot base. In the glaze firing the props mature and shrink at the same rate as the pot so everything comes down and stays in place and the props do not bond to the pot itself. So this fix does the job but I'd rather not have to do it. The CTS#30 does look interesting as does the YG and I can see a time when I get some to test. Happy Potter
  3. Pyro-plasticity of clay

    John, One other thing. I am measuring shrinkage and with the firing cycle I'm using I'm getting pretty much the full extent of shrinkage the supplier is suggesting for the clay. Cheers, Happy Potter
  4. Pyro-plasticity of clay

    Thanks John, Lots of good questions. Yes I'm taking the maturity temperature from the supplier data which I've just checked again and they say the range is 1260 to 1320C.. I've asked them for test data but I don't think they have much to offer. I haven't actually tested for porisity but perhaps that might be a good place to start. The supplier info actually says the range is 1260 to 1320C. I'm firing in oxidation with a firing cycle of 50C/hr to 100C, 200C/hr to 1080C, 85C/hr to 1230C, Hold 15 Min, -275C/hr to 1000C, Hold 5 min, -70C/hr to 760C and then off. I'm aiming for cone 6. The tinting is done by the supplier and I do not know exactly what the iron agent is. The decision on the additives came from a variety of sources - this one in particular: http://thaiceramicsociety.com/download/Pyroplasticity_in_porcelain_tile.pdf I appreciate that some of the additives will increase the problem but I was interested in that too, to gauge responses. Many thanks, Happy Ppotter
  5. Pyro-plasticity of clay

    Hello, I'm looking for some advice on Stoneware clay slumping during firing. I'm making bonsai pots which by design have a large flat horisontal base suspended in the air by 4 feet. During vitrification the stoneware glaze I use softens as it vitrifies and the floor of the pots can sag if they it is not supported. I have a blog and posted a story on a test I did: http://bonsaipotterycoy.blogspot.com.au/2012/08/post-28-pyro-plasticity-of-clay.html As you can see from the picture the clay has matured but in doing so gravity has done its job. The clay is a local stoneware clay with some RIO included to give it a buff colouring. I'm only firing it to 1240C which is well below its nominated maturity temperature of 1280C. It was suggested that the RIO additive may be fluxing the clay and so I repeated the test with both the untinted base clay and the tinted. As this picture shows there is no perceptable difference. I am tempted to start a whole series of tests on all sorts of other clays that may be available but I only need 200gms of each to start with and that's not really practical. So the question is do all high and intermediate firing bodies slump during vitrification? The other question I have is 'is there anything I can add to the clay to limit the impact of this characterisic?'. I have done some research into body formulation and the work done in the commercial ceramics industry to overcome this problem and it seems the answer may be yes, but does any additive actually solve the problem by simpoly elevating the vitrification temperature? I still want the body to reach maturity. And so I have prepared some samples for a further test. I've taken 200 gms of my stoneware clay for 8 samples and then added 10gms of either Silica, Aluminium Hydrate, Kaolin, Magnesium Carbonate, Woolastonite, Rutile flour, Talc, Dolomite. I'm most hopeful of the talc test. These are drying now and so will be a little while before I get to fire them to maturity. Can we predict the results? What will be the unintended consequences? Looking forward to hearing from you. Happy Potter