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Additives For Thermal Shock Resistance

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A friend makes coil pots from local, hand dug clay.  Lately the clay he's been digging from the same area isn't withstanding the trauma of pit firing as well as it used to.  He was advised to add volcanic ash, but it hasn't really helped much, if any.  (He wasn't told what percentage would be adequate, so he's been playing things on the safe side.)   He burnishes his pots with a rubbing rock, and adding grog would probably make the clay too rough. Is there anything that he can add to the clay other than grog to strengthen it?

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I'm newish so ymmv.

 

But I have been doing a lot of research in clay bodies.

 

Try a fine grit/grog. Extra fine.

 

Then I'd think about beefing up clay body.

Adding kyanite... ( thinking raku body )

Maybe epk.....

Or wedge in some raku capable clay.

What is the nature of dug clay?

Are you sure it's thermal shock?... Pit firing has other physical stressors.

 

Wait..... A guru will answer shortly

( clay and glaze section may be more appropriate for this question )

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Try kyanite-it comes in a few mesh sizes so fine would still be smooth-it has the one of the best thermal shock resistance's going.

I use it it in spray kiln coatings. It is in kiln shelves-Laguna clay and others sell it. From 200 mesh to 30 mesh.

I'm a big fan of this stuff.

Mark

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The pieces are heated to 600-800 degrees, then moved while still warm to a prepared bed of coals, his tribe's manner of firing pots.   This system has worked quite well until recently, so we're wondering if perhaps the clay vein (in use for at least 150 years) has some variations that are affecting the outcome.  Additives were never necessary before, but perhaps the makeup of the clay has changed? 

 

What percentage kyanite (or mullite or mica or super fine grog) would be appropriate?  I know that testing different additive-to-clay proportions is the standard recommendation, but there are so many variables (dryness of wood, outside temperature, humidity, wind, etc) that even a firing of a bunch of test tiles wouldn't necessarily yield reliable answers.)  So, since we have to start somewhere....any suggestions?

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My guess would be to start up to 10% additive (calculated on the basis of estimated dry weight of the clay body). I believe that kyanite/mullite would just be acting as a filler at the temps you would reach in pit firing.  I would probably try 100 mesh if using kyanite / mullite and if that doesn't work then move to the coarser mesh. 

 

You mention 600-800 degrees, I'm guessing you mean Farenheit? when the pots are moved from one area to the coals. That would be in the cristobalite range so maybe the cracks are occuring then and not during the pit firing part of the process? Moot point though since the clay body still has to withstand the heating/cooling.

 

Other people probably have better guesses than I do as I'm just going by logic and haven't ever adjusted a pit firing claybody.

 

Min

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Thanks, Min.  I don't think the cracks are happening during the transporting from kiln to coals, since we can hear the "ping" of the clay when it cracks in the fire!  It's certainly possible, though.....  Maybe I should do as Biglou13 suggested, and put this question to the clay/glaze forum for specifics on percentages. 

Jayne

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Without knowing about dug clay no one will be able to help you. There is a some science involved. And giving you percentages/ formula would be conjecture. You could send clay body out ti a ab for testing.

 

Other wise ...test

 

5 grams per 250 gm clay

 

10 gms per 250 gm of clay

 

Etc............. And see if it works

 

I'm interested in what tribe and what part of country, mostly for education and interest sake, but I'm also wondering if there would be local/ indigenous material to fix yor clay, or explanation why clay is failing. Any chance you have any pictures?

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Mollicite, is a good additive for porcelain type clays. Depending on what your local clay will determine what you need, as Biglou says.

If you are digging creek bed clay, it is most likely a low fire sedimentary clay. It may already contain said, for example.

 

Marcia

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mullite, kyanite, petalite, spodumene, etc - all the good stuff that effects thermal expansion in clays and glazes would be the first things i'd experiment with

 

even though your friend is pulling clay from the same location, there are too many variables that can effect the chemistry of the clay he's actually digging up.  happens all the time to big mining facilities - even though they're digging up the same "material" the actual batches may differ chemically and it can be enough to make your piece crack or not.

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A semi-related question ... has anyone tried adding kyanite when making cone 10 functional pots? Say for a wood-firing, where thermal shock resistance would be helpful too. I've always heard kyanite recommended in raku firings, is there some reason it is not suitable for functional ware?

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Grog, Take the broken pots and crush them. sift for desired particle size. Try 1:2, 1:1, 2:1 with clay and test fire.

Of course this will make the clay less smooth. The traditional way of overcoming this on burnished wares is to use the base clay as a slip (without the grog) on the leather hard pot and burnish the slip layer.

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