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Brittany

Clay Thickness Before It Explodes?

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I made a face jug and I'm worried that there may be parts of it that are too thick and may explode in the kiln. It's a very nice piece so I really want to avoid that. It's almost 1 1/2 inch and my teacher has always told me to never fire a piece past 1 inch, but she's been wrong before. Is this a safe thickness to fire?

P.S. It's lowfire clay if that helps any. :) ~Thanks

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Its not the thickness but the time allowed to preheat and fire...thicker ware needs a longer preheat and low cycle. The key is to get all the moisture out of the clay before the molecules start firm up in the kiln. The moisture needs to escape before the passages start to close. An over night preheat and a long low cycle could insure the survival.

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If the clay has sandy particles in it, called "grog", then it has a chance, e.g. "rough" Raku clay which can withstand thermal shock.

Otherwise it is not a good idea, unless you very, very slowly ramp up the temperature.

If steam builds up in the middle of such thick clay and it cannot escape then it will explode.

If you are making sculptures it helps if you poke thick parts full of holes with a skewer, and then cover the holes on the outer side, but you can only do that when the clay is wet anyway.

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Brittany ... If your teacher was right no children's work would ever get fired! I think what she really means is she won't be responsible for doing it in her kiln.

If work is thick then as mentioned here you have to be careful. As an adult you can learn techniques for hollowing it out and re-assembling ... or poking those breathing holes in it ... or just being extremely careful that it is dry and then fire it slowly.

But no, as far as I know there is no 1 1/2 inch rule on the books.

What might make your piece break is that it has different thicknesses which is very tricky to fire successfully.

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1.5" in really thick for something made with clay intended for pots. It sounds like it's probably not uniformly that thick, either. I'm more worried about it cracking rather than exploding. Give it a good long preheat, and fire as slow as you can, like a good 24 hour firing if you can.

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 I used heavily grogged clay that has survived Raku firing, so from what you're telling me, I should be fine. Anyways the only place with the 1 1/2 " thickness is where the eyes were added. So hopefully it goes well, because if it does I'm going to add it to my art portfolio for competition.Thanks for your help! :)

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technically speaking clay can be VERY thick, you just have to fire it very slowly - think of a brick, those are 2.5" thick and they don't explode in the kiln.  also, it's not just the preheat that needs attention to drive off physical water, you also need to fire very slow since the chemical water in your clay needs to escape the core of the work.  lastly, there is down-firing involved with thick ceramics or your pieces will crack when cooling.  i've got a decent amount of experience firing thick work - usually firings take 3-5days when you get into the 2" thick range for life-size sculpture.

 

for this particular piece since you added the features to another form, my guess is that there is less chance of "explosion" (unless you trapped air when applying the face) than there is for the added features to heavily crack (or crack and fall off).  you'll be fine, just fire slow.

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Norm Stuart,

The inherent problem with firing clay is that as it converts from kaolin to meta-kaolin between 986 F and 1.166 F, clay loses 12% of its weight as steam in the process.

 

Norm, Fascinating posting on big pieces. Are there applications for meta-kaolin in pottery? I ask as it's becoming more readily available (because of its use as a pozzolan in cements).

Regards, Peter

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Ugh ... Now that sounds worse ...

Those eyeballs are gonna pop! ... Especially since you might want to add it to your portfolio.

Is there no way of hollowing out those eyes?

I'll hollow them out during class tomorrow then, and how do face jugs normally not crack with the added features? Because I've seen face jugs that were made exactly like mine was, but they don't crack or break. Do all of them hollow out the eyes?

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The original face jugs were tiny - about 3"-4" -- and were fired in the little spaces between larger pieces that would normally go to waste. The one you're copying may have been much smaller than the one you've created!

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hey there @Brittany, I am interested to know, how much grog was in your clay? everything was smooth or the piece was damaged during the firing?

@Frederik-W, @Mart 40% grog (0-0.5mm size), 5% firing shrinkage like we have here could be a good solution for building solid pieces? 

Thanks,

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How THIN can you make the clay when you are planning to fire it?  I have a pretty nice mask that I am planning to give a  relative for his birthday but am fearful that I may have made some of hte clay too thin.  Can anyone give me an idea?  Thank you!

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Thin can cause slumping issues during the firing. Post firing - clay products much less than 1/8” thickness tend  to be fragile to handle especially as their size increases. Open shapes, (Masks etc...) are likely more fragile than closed shapes (Mugs, Bowls.......)

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Slumping is definitely a possibility, but it will depend a lot on the shape of the piece and the clay body you're using. Slab platters, for instance are prone to slumping when too thin. A mask, which has the opposite curve of a platter, may not. Glazing can be an issue when a piece is too thin. The clay has to be thick enough to absorb the water in the glaze. If it's too thin, it can't take in enough water, and you can't get a thick enough glaze application without doing several coats. And each coat will take a long time to dry.

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