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Pottery/Refiring cone 6 Amaco glaze


I had a bad outcome in my pottery class with several plates. The glaze was half dull and half shiny. The plates were otherwise smooth and level. We refired them at home in our electric kiln, cone six with a glaze firing. We sprayed the plates with clear to make sure the dull areas would shine just in case the glaze didn't improve. It was a medium ramp hold ten minutes. When it cooled to 200 we peeked and thought everything was fine. Well all plates cracked , pimpled, warped and bubbled. We have refired pieces before and never had things appear that weren't there before. The pimples were underneath as well, where there is no glaze. Any ideas? Don't want to repeat this lesson. Also, the non refired pieces were fine.
Thank you,
Frances Fowler

Dear Fran,

Thank you for your recent question. There can be many reasons for faults in a glaze firing, everything from a badly mixed glaze recipe to incompatible clay and glazes. Although I don't really like to speculate when I don't have exact information, it almost sounds as though the glaze was bad, causing stress breaks on the plates.

A suggestion when re-firing is to fire at a much slower rate. This is especially true when firing plates which always have stress problems anyway. I would do some test firings on the glaze you used on tiles made of the same clay body you used. Also, try a different cone 6 glaze on another tile to test the tile itself. Glaze defects are usually caused by under or over firing or incompatible clay and glazes. Therefore, testing is the only sure way to figure out what went wrong.

I'm sorry I can't be of further help, but without the exact information about the clay body used, the exact glaze used and the firing technique used in the school studio, it is unlikely anyone could give you a satisfying answer.

If you have further questions, please feel free to contact me again any time. I am always at your service.

Ms. Ti Phillips
Earth Stoke 'N Fire Pottery Studio and Artist Retreat


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Ti Phillips


Will answer any questions on hand building, wheel, glaze, firing. Speciality questions to include those in glaze calculation and development, firing techniques. Please do not send questions on identifying pottery. Although I would love to add this to my question topics, I have a retreat to run as well as the studio and volunteering on AllExperts, and therefore do not have the available time to research indentification and marks. Thank you for understanding.


Experience includes 30 years in pottery design and education. Have taught online and studio classes worldwide for the last 20 years. Own a pottery retreat specializing in firing techniques. Have 12 years solid experience in glaze calculation and formulation as well as problem solving in glaze chemistry. I am the first potter in the United States to have developed a complete package of pottery equipment blueprints for a studio. The blueprints include wheels, kilns, studio furniture, wedgeboards, raku kilns, slab rollers, ball mills and studio tools.

Alliance of Pottery Artists Worldwide Association

Ceramic Industry - PPP Wyndstryder Press - Pottery Journal

University of Sciences and Art's of Oklahoma, studied under Professor Jaymes Dudding.

Awards and Honors
Potter of the year with APAWA, various awards for showmanship and design.

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Available upon request.

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