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Pyro-plasticity of clay


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#1 bonsaipotter

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Posted 16 September 2012 - 05:47 PM

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://bonsaipottery...ty-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.

Attached File  P1130402a small.jpg   83.95KB   25 downloads

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

#2 JBaymore

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Posted 16 September 2012 - 06:16 PM

Welcome to the forums Happy Potter.

No, not all bodies slump.

The first question I have in tackling this problem is, "How do you know that the maturing temperature of that clay body is 1280 C?" Did you do apparent porostity testing to help assess that fact, or are you believing the supplier's stated range? What IS the apparent poriosity figure for the fired clays that you are using that is slumping in your firing cycle?

Did you add the red iron oxide to the body after the general formulation (1280C "rated") was made? Was it added AS red iron oxide... or some other materials supplying iron? Are you fing in reduction?

How did you decide on the "additives" there for your tests? I'm guessing that some of them will have the opposite result from what you desire.

Temperatures of 1240 C and 1280 C do NOT tell the story on firing at all. If I go to 1280 C in 2 hours I get a very differnt maturity (heat work result .... or "cone") than if I go to 1280 C in 60 hours. Ditto for 1240 C. What is your firing schedule?

best,

................john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#3 bonsaipotter

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Posted 16 September 2012 - 06:38 PM

Welcome to the forums Happy Potter.

No, not all bodies slump.

The first question I have in tackling this problem is, "How do you know that the maturing temperature of that clay body is 1280 C?" Did you do apparent porostity testing to help assess that fact, or are you believing the supplier's stated range? What IS the apparent poriosity figure for the fired clays that you are using that is slumping in your firing cycle?

Did you add the red iron oxide to the body after the general formulation (1280C "rated") was made? Was it added AS red iron oxide... or some other materials supplying iron? Are you fing in reduction?

How did you decide on the "additives" there for your tests? I'm guessing that some of them will have the opposite result from what you desire.

Temperatures of 1240 C and 1280 C do NOT tell the story on firing at all. If I go to 1280 C in 2 hours I get a very differnt maturity (heat work result .... or "cone") than if I go to 1280 C in 60 hours. Ditto for 1240 C. What is your firing schedule?

best,

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



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://thaiceramicso...celain_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

#4 bonsaipotter

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Posted 16 September 2012 - 06:43 PM

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

#5 Mark C.

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Posted 16 September 2012 - 10:49 PM

We had slumping issues in a porcelain custom slip body at cone 10. Our cure was 15 % addition of EPK which has alumina in it and was cheaper than adding straight alumina . I would add some EPK in smaller amounts as tests to see if this helps. Also many formulated cone 5 bodies are fired to cone 6 which can lead to the top end of their stability.
A few other easy things are lower your firing temp. You really do not need the high end for planters. Why do you feel the glaze softens them?? I am very familiar with bonsai pots as I had a friend who slipped cast them for awhile (red stoneware cone 10)
How are these made? slip ?pressed? slab built?.
I'm a little rummy after 12 hours on the street at art show this weekend.
Mark
Mark Cortright
www.liscomhillpottery.com

#6 Matt Oz

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Posted 17 September 2012 - 09:58 AM

One material I’ve read about that helps with deformation is Pyrophyllite

more info here on Digtalfire...http://digitalfire.c...llite_1167.html If you don't already know about Digitalfire check it out, very useful.

I have done tests with talc, here's one where I increased the talc and reduced the feldspar in the recipe, starting with 5% talc on the left, up to 20%, all fired to cone 6 with no apparent porosity. I haven't noticed talc having much slump resistance, but it will be interesting to see what your results are.

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#7 JLowes

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Posted 17 September 2012 - 10:57 AM

Greetings Happy Potter,

I took a look at your blog and gallery. Are the pots in the gallery representative of what you are experienceing problems with? The sides didn't seem to have the departure of line from horizontal, so is it the flat interior that is slumping? Have you tried adding extra feet to provide support, or vertical structural elements to resist the bending? Have you explained the problem to your supplier/manufacturer and asked for corrective suggestions?

Out of curiosity, have you made any planters or bar tests from the CTS #30 mesh?

John

#8 bonsaipotter

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Posted 17 September 2012 - 06:38 PM

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

#9 bonsaipotter

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Posted 23 September 2012 - 05:51 AM

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




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