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Pottery/Bubbles in Thick Glazed Areas

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Question
I single-fire in electric kiln at cone 6 and hold at highest heat for 1-3 hours.  For bowls I put glaze in bowl and pour out, brushing some of the glaze out.  My favorite glaze tends to bubble in bottom of pots where glaze is thickest.  I have been bursting the bubbles and re-firing after some grinding and swipe of new glaze.  Sometimes on re-firing it smoothes out and sometimes not.  The re-firing produces some additional flowing and a brighter gloss and bright red coloring (12% iron oxide) and additional striping.  On one bowl a small amount of pitting remains but the rest of the bowl is so appealing that I am wondering if there is some way to independently smooth out the pitted part, e.g.,say by blow torch) so the rest of the pot stays the same.  Or should I fill the pits with glaze and refire again?  Holding for 2 hours rather than 1 hour accentuates  a hare-fur effect where glaze is thickest, which I just love. This hare's fur effect was noticed by my Chinese friend who had a recovered 1000 yr old Jian ware teacup from his hometown in China and the the similarity in the hare's fur or partridge feather effect is obvious.  And I don't understand why the bubbling only happens sometime when glaze thickness seems similar.

Your thoughts will be appreciated.  This glaze and these effects have made potting even more fun and increased the surprise and unpredictability factor associated with this great craft.

Answer
Dear John,

Thank you for your recent questions. It is impossible to answer your questions without knowing the formulation of your glaze. Many factors can cause bubbling of a glaze body, especially in the thicker areas, but can be corrected by altering or adding ingredients or adjusting the firing technique, only if the glaze formula is known.

If you are willing to share the glaze recipe, I will gladly calculate the glaze formulation and work the firing schedule to assure you are not over firing or the glaze does not have too much or too little of an ingredient or lacks an ingredient.

If you wish to share the glaze with me but not publicly, you may email it to me at earthstokenfire@earthstokenfire.com I will not publish the recipe nor use it in any way other than to work your results for an answer for you.

It would also be nice to see a photo of the bowl in particular that is giving you the problem. There is a difference between bubbling and pin holes and other undesired effects. I would be interested in seeing the effects you are receiving from the firing.

I look forward to your reply.

Sincerely,
Ms. Ti Phillips
Earth Stoke 'N Fire Pottery Studio and Artist Retreat
www.earthstokenfire.com

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Ti Phillips

Expertise

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

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.

Organizations
Alliance of Pottery Artists Worldwide Association

Publications
Ceramic Industry - PPP Wyndstryder Press - Pottery Journal

Education/Credentials
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.

Past/Present Clients
Available upon request.

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