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Clay Thickness Before It Explodes?

Face Jug

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#1 Brittany

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Posted 10 November 2013 - 11:28 PM

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





#2 Mart

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Posted 11 November 2013 - 02:45 AM

How much and what size grog you got there? No grog and >1.5 cm, is asking for trouble.

#3 ayjay

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Posted 11 November 2013 - 06:36 AM

 No grog and >1.5 cm, is asking for trouble.

 

 

........and 1.5" is more than double 1.5 cm.



#4 PSC

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Posted 11 November 2013 - 06:59 AM

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.

#5 Frederik-W

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Posted 11 November 2013 - 07:44 AM

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.



#6 Marcia Selsor

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Posted 11 November 2013 - 07:52 AM

Robert Arneson, CA funk ceramicist, fired his sculptures for several days. Thick requires a slow firing.
Marcia

#7 Chris Campbell

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Posted 11 November 2013 - 11:25 AM

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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#8 neilestrick

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Posted 11 November 2013 - 05:48 PM

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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#9 Diane Puckett

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Posted 11 November 2013 - 06:25 PM

If by chance the thickness is in the bottom, put three or more very short kiln posts under it in the kiln.
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#10 Brittany

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Posted 11 November 2013 - 06:35 PM

 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! :)





#11 Chris Campbell

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Posted 11 November 2013 - 06:48 PM

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?

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#12 perkolator

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Posted 12 November 2013 - 01:41 PM

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.



#13 kathleenMK

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Posted 12 November 2013 - 05:41 PM

Agree with Chris, get a sharp loop tool or greenware cleaning and hollow them out from the inside. Go slow.



#14 PeterH

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Posted 12 November 2013 - 07:58 PM

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



#15 Brittany

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Posted 12 November 2013 - 08:47 PM

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?





#16 Chris Campbell

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Posted 12 November 2013 - 10:36 PM

I would suspect that the eyes are hollow ... but I don't know much about making face jugs. When I got the pottery assignment to make a face jug I put the dial face of a watch on it.

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#17 Nancy S.

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Posted 14 November 2013 - 09:02 PM

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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