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Green Ware Cracks


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#1 PianoSal

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Posted 01 November 2013 - 10:59 AM

A thrown cylinder shaped piece developed a vertical crack at the rim AFTER it dried to the green ware stage. Another thrown small bowl developed a vertical crack at the rim in glaze firing both pieces are 1/8" thick or less at the rims I'm using Alberta plainsman M370 clay. Any ideas why?

#2 bciskepottery

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Posted 01 November 2013 - 11:08 AM

Is the wall thickness pretty uniform for the whole piece? The top might be drying/shrinking faster than the bottom, resulting is stress on the top when the bottom finally dries and shrinks. As for the bowl, the crack was probably there after bisque firing, just not visible to the eye. Contraction/expansion during glaze firing and/or uneven cooling (a thinner top would cool faster than a thicker bottom) brought it to the surface. Excessive working of the rims while finishing throwing could have weakened the rim and made it more susceptible to eventual cracking.

Every potter should have a copy of either Hamer and Hamer's The Potter's Dictionary of Materials and Techniques, or Harry Fraser's Ceramic Faults and Their Remedies, or a similar reference book. These have great pictures of most faults you will come across, as well as good explanations and suggestions for preventing their recurrence.

#3 PianoSal

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Posted 01 November 2013 - 01:13 PM

Thank you - you have confirmed another potter's thoughts on this.  And thank you for the suggestions for reference material.






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