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Bisque Fire Cracks


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#1 AmyJ

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Posted 12 August 2010 - 10:07 AM

I am new to the art of pottery. What are the major reasons for your pots to crack during a bisque firing? I have been working with brown clay. One pot cracked from the rim along the side in an L shape.




#2 Chris Campbell

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Posted 12 August 2010 - 10:14 AM

Could you post a picture and tell us a little bit about how you made it and dried it?

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#3 AmyJ

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Posted 13 August 2010 - 07:05 AM

Could you post a picture and tell us a little bit about how you made it and dried it?



#4 Oliva

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Posted 13 August 2010 - 09:46 AM

I am new to the art of pottery. What are the major reasons for your pots to crack during a bisque firing? I have been working with brown clay. One pot cracked from the rim along the side in an L shape.




Amy,
I am new to using this web forum so I hope this gets to you. My experience was with porcelain cracking. Mostly large platters. They started at the rim and cracked in various patterns in the biqsue. I know porcelain temperments so I had compressed rims and bottoms, dried exceedingly slow and out of drafts, tried elevating the pots off the shelf floor while firing, etc. What worked for me was so simple I felt foolish. I slowed down my bisque firing. It might work for you.

#5 Seasoned Warrior

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Posted 13 August 2010 - 05:00 PM

I am new to the art of pottery. What are the major reasons for your pots to crack during a bisque firing? I have been working with brown clay. One pot cracked from the rim along the side in an L shape.


Hi Amy:

Welcome aboard. When you say "brown clay" is this a commercial clay body you purchase from a pottery supplier or is it a clay that you process from nature. I dig my own clays and frequently I have to blend them with other materials to get a good workable pottery clay. One thing that works for me is to add a little fire-clay readily available from masonry suppliers. The other thing that works well for me is to add grog. I have heard of some people using sand but I usually just use broken pottery which I break up finely and then sieve to the gradation I want. If you make it too coarse it can affect your finish and you may not like the end result. Usually with an uinknown clay that I found I make up test forms or coupons and then test them for various parameters. After I'm through with my testing then I try additives and ornamentation. I try to lilmit the variables so that I can decide what the effect actually is. I find keeping a notebook of everything I do in the studio has helped me immesely. Good luck to you.

There are many here who can help you and many who have different areas of expertise as well as exceptional credentials. A picture sure would help as would the construction technique; was the pot thrown or built. Are the pots cracking in a specific pattern?

Best regards,
Charles

#6 Marcia Selsor

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Posted 16 August 2010 - 02:23 PM

Here are some possibilities:
rim crack..could be caused by too thin or weak lip when trimming the platter.
remedy: Keep the rim substantial, use a damp sponge to support the platter's center when it is upside down.

The "L" shaped crack could be from trimming unevenly, or stressing the lip when trimming.
Also dry slowly and bisque slowly.

Marcia




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