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Square Cracks On Outside Of Thrown Bowl


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#1 kecpotter

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Posted 16 September 2010 - 11:15 PM

Hi, we recently fired my friend's gas kiln. Pieces (B-mix, cone 10) came through the bisque firing well. During the glaze firing, we had two pieces with very unusual cracks: one is a bowl (appx 10" across and 8" high) which developed square shaped cracks on the outside of it. I have never seen anything like it. Another is a 12" plate, the rim of which actually developed a long crack which bent up -- like an airplane wing. Again, never seen this one before. Does anyone have ideas about why this happened? It doesn't seem like a throwing problem -- I've made many, many of these in the past and never had this issue. Ideas appreciated!

#2 APHenix artist

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Posted 17 September 2010 - 08:33 AM

Hi, we recently fired my friend's gas kiln. Pieces (B-mix, cone 10) came through the bisque firing well. During the glaze firing, we had two pieces with very unusual cracks: one is a bowl (appx 10" across and 8" high) which developed square shaped cracks on the outside of it. I have never seen anything like it. Another is a 12" plate, the rim of which actually developed a long crack which bent up -- like an airplane wing. Again, never seen this one before. Does anyone have ideas about why this happened? It doesn't seem like a throwing problem -- I've made many, many of these in the past and never had this issue. Ideas appreciated!

Sometimes if you work near plaster it can get into the clay and cause cracking if wedged in right (powder not clumps). Otherwise if that is not the case then it sounds like stress fractures that can come out of either throwing too quickly or stacking incorrectly. B-mix is a porceliana which has the properties of porcelain, without the white color and translucency when thrown thin enough. The grog is what affects it so. When throwing porcelain, different techniques are used in regard to the touch a potter must have. Throwing dry is a great technique, but very difficult to adapt. So, on that note the square fractures sound like stress from throwing. Too much going in and out with the form or perhaps not taking the flange and reinforcing the clay into the form. I have had sinuous cracks form on the base of my plates from not reinforcing and sometimes from trimming too fast and perhaps not reinforcing after. I do not know why it happens to some pieces and not others, but i do know it happens when i do a lot of work and quickly. As you can see there a number of factors and without seeing you throw and trimm i cannot say with definity.

#3 Deb Evans

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Posted 18 September 2010 - 12:01 AM

Sounds like you're a wet thrower, I've seen those cracks even w/ raku clsy but mostly w/ fine stoneware or wheiteware. You need to be consistent in getting slime off of both inside and outside the pot which = even water consistency, slow dry and that should solve your problem.

#4 Pres

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Posted 20 September 2010 - 02:38 PM

Hi, we recently fired my friend's gas kiln. Pieces (B-mix, cone 10) came through the bisque firing well. During the glaze firing, we had two pieces with very unusual cracks: one is a bowl (appx 10" across and 8" high) which developed square shaped cracks on the outside of it. I have never seen anything like it. Another is a 12" plate, the rim of which actually developed a long crack which bent up -- like an airplane wing. Again, never seen this one before. Does anyone have ideas about why this happened? It doesn't seem like a throwing problem -- I've made many, many of these in the past and never had this issue. Ideas appreciated!


Interestingly enough the type of crack you describe (spare shape cracks on the outside of the bowl) is drawn up in Frank Hamers The Potter's Dictionary of Materials and Techniques. Page 82 shows squarish brick shaped cracks on a bowl. He says "The crack is probably more than a haircrack in a few places and results from a biscuit dunt that was unnoticed at the discuit stage. At biscuit stage some dunts do not penetrate through the pot. A slip coating could dunt seperately from the pot body and vice versa." It seems like to me your bisque schedule may have to be changed to deal with the type of cracking you are having, and maybe you didn't notice the cracks when the bisque was done. Years ago I had a series of really strange cracks in pieces like spirals all the way around the pots-looked like springs instead of pots. I also had a series of pots with brick shaped cracks as you describe, in the bisque, and they were extreme. Made a change in the firing schedule, tweeked it, and things worked out well. You may be cooling too fast.

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#5 kecpotter

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Posted 20 September 2010 - 11:53 PM

Thank you all very much for taking the time to coach me through this!



Hi, we recently fired my friend's gas kiln. Pieces (B-mix, cone 10) came through the bisque firing well. During the glaze firing, we had two pieces with very unusual cracks: one is a bowl (appx 10" across and 8" high) which developed square shaped cracks on the outside of it. I have never seen anything like it. Another is a 12" plate, the rim of which actually developed a long crack which bent up -- like an airplane wing. Again, never seen this one before. Does anyone have ideas about why this happened? It doesn't seem like a throwing problem -- I've made many, many of these in the past and never had this issue. Ideas appreciated!


Interestingly enough the type of crack you describe (spare shape cracks on the outside of the bowl) is drawn up in Frank Hamers The Potter's Dictionary of Materials and Techniques. Page 82 shows squarish brick shaped cracks on a bowl. He says "The crack is probably more than a haircrack in a few places and results from a biscuit dunt that was unnoticed at the discuit stage. At biscuit stage some dunts do not penetrate through the pot. A slip coating could dunt seperately from the pot body and vice versa." It seems like to me your bisque schedule may have to be changed to deal with the type of cracking you are having, and maybe you didn't notice the cracks when the bisque was done. Years ago I had a series of really strange cracks in pieces like spirals all the way around the pots-looked like springs instead of pots. I also had a series of pots with brick shaped cracks as you describe, in the bisque, and they were extreme. Made a change in the firing schedule, tweeked it, and things worked out well. You may be cooling too fast.





#6 PotterPaul

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Posted 21 September 2010 - 12:30 PM

I have recently had a similar problem with B-mix ^5. A local friend gave me a physics lesson on silica going through some kind of conversion at a specific temperature range causing it to shrink rapidly. In short slow the cooling cycle at around 1150 deg. I relight the pilot light at about 1500 and then shut it off at about 500.

It worked. Good luck

#7 kecpotter

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Posted 22 September 2010 - 12:30 AM

Thank you!!







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