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Pot Cracked And Became Soft In Water

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#1 PatJ



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Posted 29 May 2011 - 10:57 PM

Hi Folks,

I'm new to this forum and to pottery making so please forgive me for not knowing some of the basics or some of the terms.

I was testing the suitability of a new clay source I found out in the woods by making several small test pots. I dug up the clay, added varying amounts of Tuff (volcanic ash rock) temper and fired them in a pit (no kiln). They were not slipped or polished. Today I decided to test how waterproof the pots were so i filled one (this one was made with 40% temper) with water and left it sitting for about 10 minutes. Now I totally expected that the pot would be super porous and water would leak out big time, but what I didn't expect is that the pot cracked in multiple spots all starting at the rim and working their way down. Also when I tested the strength of the wall of the pot, a whole fragment/sherd broke off along a crack. The wall body had become soft, which I never would have expected since it was fired a couple of months ago.

Does anyone know why this pot cracked and the paste body had become soft? Was there too much or not enough temper? Is it the clay itself? Was it not fired hot enough?

Any suggestions would be appreciated.


#2 Marcia Selsor

Marcia Selsor

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Posted 30 May 2011 - 10:03 AM

I would guess not hot enough. Try some clay with less temper. Get a good bed of coals in the pit, place the pot upside down after preheating on the edge of the pit. When you cover with whatever you are using..i.e. sawdust, cow dung whatever, also seal the whole pile with sheets of steel or license plates or some type of metal. This is how Maria martinez did her pit firings. I do not think she bisques but would have to check.

Marcia Selsor, Professor Emerita,Montana State University-Billings

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