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Base Of Pots 'chipped Edge' After Glaze Firing - Why?

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The attached picture shows the chipping that has occurred to the bottom edge of pots as they have come out of glaze firing (c1240 degrees centigrade – cone 6-7). The edge was perfectly smooth when it went in the kiln.

 

Larger pots affected worse than small ones.

 

Clay has some grog in it.

 

There is a layer of bat wash on the kiln shelves.

 

Any help gratefully received!

post-61816-0-11481800-1408353899_thumb.jpg

post-61816-0-11481800-1408353899_thumb.jpg

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Could be pluckng?

What kind of clay?

That's not a problem, a few passes with diamond pad and you're back to new.

Try putting some silica on shelf before next firing. See if it does it again?

Did all pots have this!

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The term is "plucking". It happens with porcelain quite frequently. When waxing the feet of my porcelain bowls, I put a bit of alumina hydrate in the wax. Problem solved?

Tom.

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I had this happen recently.  My chips were much larger and could not be sanded out too look decent enough for selling.  I have never had it happen before.  I lost about 4 mugs to this.  The studio manager thought it might have been due to the kiln wash being too thin on the shelf, or at least needing to be redone.

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I do not use Kiln wash I simply brush a light dry coat of Alumina Oxide on my shelves and call it good.  No more chips on my large porcelain bowls.  I believe this happens during cooling where the pot cools at a different rate than the shelves while the pot has stuck slightly to the shelf. 

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