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Pottery/Functional Ware and Clay

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Question
Is midfire clay ok to use for cups, bowls and plates? I heard it is better to use a highfire clay, but I only have an electric kiln. I just want to make sure that when someone buys a piece from me they will be happy and safe. Also, how do I know if what I make will be ok for a microwave or dishwasher?

Answer
Dear Marko,

    Thank you for your recent question. Yes, you can use mid fire clay for functional ware. However, you must make sure the clay is fired to it's maturing temperature at glaze. Bisque fire the clay to cone 05 and then glaze the ware with a mid fire glaze and fire to the maturing temperature of the glaze.

    There are some rules of thumb when choosing a glaze for functional ware. Always choose a glaze that is food safe. It must not contain any barium, lead or other toxic ingredients that may leach from the glaze when used with food. All commercial glazes will have a notice on the glaze bottle that lets you know if it is food safe. Studio glazes are sometimes difficult to determine if they are food safe. But they can be tested by creating a piece and the performing a leach test on the ware using a leach test kit. You can also send a copy of the glaze recipe to me and I will let you know if the glaze is safe to use on functional ware.

    One of the most important facts about functional ware when using glazes is to make sure the glaze is not a crackle glaze or a glaze that creates a unique texture, such as foaming, crazing and pitting. These types of glazes allow bacteria to enter the cracks of a glaze of this type. You should also make sure the kiln cools very slowly to prevent glaze crazing or cracking. You will know if your pieces are cracking if you raise the lid too soon and the ware begins to make a cracking or pinging noise.

    Finally, your glaze should fit the clay body. The glaze should have the same thermal expansion rate as the clay body. If a glaze begins to craze (crazing means the glaze will separate from the surface of the clay and pool, allowing the clay to been seen through the glaze), then you must use a different glaze. A glaze for functional ware should be smooth throughout the surface of the clay body, without any cracking, pooling or crazing.

    If a glaze and clay has been properly matched in thermal expansion, does not contain any toxic ingredients and does not crack, craze, pit or foam, and is fired to the proper maturing temperature, the ware should be fine for functional ware and will be microwave and dishwasher safe.

    I believe this information will assist you in your discovery of functional ware clay and glazes. However, if you have further questions or would like to send me examples of what you are going to use, I will be happy to assist you further. If you choose to send samples of the clay and glazes you will be using, please be clear on the name of the clay and glaze, maturing temperature or cone, ingredients if they are studio made or the complete name of any manufactured clay or glaze, to assure I give you the correct response as to the use of your materials. I hope to hear from you soon and much luck to your in your new adventure. Always at your service.

Sincerely,
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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