Disintigrating Clay

[Nollart]

I'm totally stumped on this one.

I live in Ajijic Mexico and am teaching a pottery class to a mostly adult class of expats. Finding a suitable clay has been a real challenge. Clay in this area is low fire earthenware used for making garden pots, bricks and tile.

I did find a local supplier who manufactures kitchenware. He suggested I use a the same product they use for slip casting, I fire it with gas in a fabic lined brick kiln only to about cone 010 as we burnish a lot of the work and don't want to fire away the shine. We saggar or pit fire the work later. It's not a great solution but it gets us by - most of the time.

 

The Strange Case of Disintigrating Clay

On several occassion after firing, two or three of the pieces will begin to disintegrate. The disintiigration takes place gradually, usually starting a day or two after firing, some entirely falling apart into small crumbly pieces and dust. Coming out of the kiln they look just fine.

I can not find anything that the crumbled peices have in common or think of what could have caused it. One piece that disintigrates maybe sitting on the kiln shelf next to piece that comes out just fine.

They are work from different students, all done with the same clay, some are thin, some are thick.

I thought I may have been firing too fast (crystabolite conversion, trapping carbon?) and so I slowed the firing down to about 200 F per hour, raised the end temperature to about 1850 and let it soak for about an hour. The work has a nice ring to it when done although at that temperature the burnished look is mostly gone. Still, several of the works are crumbling.

Have you ever heard of clay reacting this way?

Any ideas on this one?

Daniel Noll

www.danielnollstudio.com

[faber]

your pots could be getting wet or absorbing moisture. Have you tried sealing them when they are finished? Just a guess.

[Nollart]

your pots could be getting wet or absorbing moisture. Have you tried sealing them when they are finished? Just a guess.

 

The pieces are kept dry and the climate here is very dry so I don't think that's it. What puzzles me is the randomness of the occurance, same clay same artist, same kiln and same firing yet on occassion some just disintigrate, fortunately very few but enough to create some stress.

Thanks for the suggestion.

Dan

[hansen]

Daniel: It sound to me like maybe there is something he (Mr. Kitchenware) is not telling you. Try mixing the low fire red and the "mystery clay" to see if it raises or lowers the maturing point. It could be that you need the alkaline fluxes from the low fire clay to make the other clay work

h a n s e n

 

I'm totally stumped on this one.

I live in Ajijic Mexico and am teaching a pottery class to a mostly adult class of expats. Finding a suitable clay has been a real challenge. Clay in this area is low fire earthenware used for making garden pots, bricks and tile.

I did find a local supplier who manufactures kitchenware. He suggested I use a the same product they use for slip casting, I fire it with gas in a fabic lined brick kiln only to about cone 010 as we burnish a lot of the work and don't want to fire away the shine. We saggar or pit fire the work later. It's not a great solution but it gets us by - most of the time.

 

The Strange Case of Disintigrating Clay

On several occassion after firing, two or three of the pieces will begin to disintegrate. The disintiigration takes place gradually, usually starting a day or two after firing, some entirely falling apart into small crumbly pieces and dust. Coming out of the kiln they look just fine.

I can not find anything that the crumbled peices have in common or think of what could have caused it. One piece that disintigrates maybe sitting on the kiln shelf next to piece that comes out just fine.

They are work from different students, all done with the same clay, some are thin, some are thick.

I thought I may have been firing too fast (crystabolite conversion, trapping carbon?) and so I slowed the firing down to about 200 F per hour, raised the end temperature to about 1850 and let it soak for about an hour. The work has a nice ring to it when done although at that temperature the burnished look is mostly gone. Still, several of the works are crumbling.

Have you ever heard of clay reacting this way?

Any ideas on this one?

Daniel Noll

www.danielnollstudio.com

 

 

 

 

[Nollart]

I'll give that a try.

I just had another student tell me that her piece, which she had taken home and put on the shelf about three weeks ago disintigrated overnight and was nothing but particle and dust in the morning. Strange!

On a good note I found another source of clay from a large ceramic factory that some artists in Bucerias have been using successfully for a long time. I bought some today.

 

Thanks for your comments.

Daniel

[Marcia Selsor]

I have seen this in Spain and in Italy. In Montelupo, Italy they use this type of clay. We use this clay in my architectural ceramics classes.

To prevent disintegration, you must SOAK the clay in water within 24 hours of firing. It seems to be some kind of reaction to calcinating

some of the ingredients. This happened in Spain when I was teaching a workshop. I didn't know about soaking it at the time.

Soaking it does prevent the disintegration.

[Peter Horsburgh]

Sorry, double post of the one below.

[Peter Horsburgh]

Daniel,

 

And here was me thinking that I was the only one who had the problem.... (not that there is anything new in the world). I have had exactly the same problem. However, in this case, the clay is body that I have dug and processed myself from the hill on which I live. This hill has a history of potters running back to 700 BC, so the clay must have been good at some stage. Thinking of the possible problems, when I processed the clay, there were small inclusions in the body. I decided to treat these as a random form of grog as they gave an interesting finish to the final pieces. I fired them (with other pieces made from purchased body) to cone 05. Exactly the same happened as it did to you. Day 1 - Perfect, lovely, interesting pieces. Day 2 - Broken pieces Day 3 - Almost dust. In my case, it appears that the break-down of the pots is centred around these 'interesting inclusions', which are very small white stones that had disintegrated during the firing and formed pockets of white powder. Where these stones are close to the surface, there is spalling, where it is further into the body, the pots disintegrate. As the rocks on the hill are partially limestone, it is possible that these inclusions are small pieces of limestone, which is a form of plaster of paris. Remembering the warnings from the mould making gurus about not getting plaster of paris into your clay body, it is possible that these (I think) limestone pieces are forming plaster of paris when heated to cone 05 and thus causing the piece to fall apart.

 

Marcia's earlier response that soaking the pots immediately after firing may then be curing / hardening the plaster powder thus preventing the disintegration. I shall try as soon as I get home - I am away at the moment for a week. Marcia, many thanks for your input. The thought of using the very same clay body as potters on this same hill 2700 years ago is both humbling and exciting. I would hate to miss the opportunity.

 

Returning to your clay Daniel, is it possible that your supplier is getting limestone / plaster into his body?

 

On a slightly different note, when the thicker pots cracked, I was able to see that the center of the clay mass was a blue-black colour whilst the outer 5 mm was red/brown earthernware colour. The thinner protions were also red/brown. Is this possibly due to the fact that the thicker portions did not reach the full cone 05 temperature? The raw colour of the clay is standard earthernware orange/red.

 

Regards and thanks - Peter

 

Bredon Hill, England

[Marcia Selsor]

Dear Peter and Daniel,

Let me know if this works for you.

Marcia

[Nollart]

Boy, It' s good to know that this is not a unique occurance. I was beginning to think it was some kind of Karma thing going on.

The clay I was using is a commericially prepared body that is used widely for slip-casting. I used a thicker version for hand-building. It's used commerically all around Mexico so I doubt that the clay itself is the source of the problem.

 

 

I will try soaking the clay in water for twenty four hours after firing and see if that works in this case - Thanks Marcia.

 

Peter I noticd the same coloration in the broken pieces - a dark blue black core.

 

Here's what odd about it all - it's a comlpletely random occurance. One of my students made a figurative piece and the head started to disintegrated and the body was just fine. It was all made at the same time with the same clay.

 

Thanks to all for your comments, I've since found another clay body that I am going to start using.

 

Daniel

[hansen]

or it could be "marl" with lots of calcium carbonate in it. If you fire seashells into a pot they look fine until they turn to dust.

h a n s e n

 

Boy, It' s good to know that this is not a unique occurance. I was beginning to think it was some kind of Karma thing going on.

The clay I was using is a commericially prepared body that is used widely for slip-casting. I used a thicker version for hand-building. It's used commerically all around Mexico so I doubt that the clay itself is the source of the problem.

 

 

I will try soaking the clay in water for twenty four hours after firing and see if that works in this case - Thanks Marcia.

 

Peter I noticd the same coloration in the broken pieces - a dark blue black core.

 

Here's what odd about it all - it's a comlpletely random occurance. One of my students made a figurative piece and the head started to disintegrated and the body was just fine. It was all made at the same time with the same clay.

 

Thanks to all for your comments, I've since found another clay body that I am going to start using.

 

Daniel

 

 

 

[Seasoned Warrior]

I use local clays which I dig from various sources around me. For more difficult clay bodies I have found that adding fireclay to the mix improves the properties of the clay. Sometimes you have to experiment with the amounts to get the best properties. Fireclay should be available in most masonry supply places and I am sure that since you are close to Guadalajara you should have no problems finding a source of fireclay. I hope that helps.

[Nollart]

I use local clays which I dig from various sources around me. For more difficult clay bodies I have found that adding fireclay to the mix improves the properties of the clay. Sometimes you have to experiment with the amounts to get the best properties. Fireclay should be available in most masonry supply places and I am sure that since you are close to Guadalajara you should have no problems finding a source of fireclay. I hope that helps.

 

 

 

I will go to the local supplier and see what' savailable in fireclay and give your suggestion a try.

Thanks for the tip

[Peter Horsburgh]

To give some feedback on the disintegrating pots, there seems to be some resolution. Having fired some more experimental pots this week to about cone 08, I soaked them for varying times ranging from overnight to several individual 5 minute sessions. The latter seems to have the best effect. After 3 days, all are still in one piece (!) If the inclusions are greater than about 4mm, they are too big and cause spalling. However, the rest are fine.

So, Bredonware, based on the work of the potters of the Saxon Dobunni tribe, is a possibility.

 

Thanks to Nollart for bring the subject up and to Marcia for suggesting the fix. There is no substitute for experience.

 

Peter

 

Bredon Hill

England

 

[Marcia Selsor]

YAY! Glad that was a fix!. Thanks for letting us know it worked for you.

Marcia

[Nollart]

To give some feedback on the disintegrating pots, there seems to be some resolution. Having fired some more experimental pots this week to about cone 08, I soaked them for varying times ranging from overnight to several individual 5 minute sessions. The latter seems to have the best effect. After 3 days, all are still in one piece (!) If the inclusions are greater than about 4mm, they are too big and cause spalling. However, the rest are fine.

So, Bredonware, based on the work of the potters of the Saxon Dobunni tribe, is a possibility.

 

Thanks to Nollart for bring the subject up and to Marcia for suggesting the fix. There is no substitute for experience.

 

Peter

 

Bredon Hill

England

 

 

[Nollart]

THANKS FOR ALL THE RESPONSES TO THIS SUBJECT.

I'm taking a little break to get a new studio set up but will be anxious to try the suggestions above.

Daniel Noll

www.danielnollstudio.com