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Post by shoshana on Jun 2, 2017 14:32:03 GMT -6
I have been working with crystalline glazes for about 4 years. I have done a lot of reading about crystalline glaze on surfaces used for food. I haven't found a conclusive answer, as some information says surface is not food safe and other articles are vague. In addition, have seen work with crystalline surface in form of dinnerware. I made the decision to note on my work that it is not food safe.
Another part of my question which I have not tried yet, but wonder if I glaze the interior crystalline surface with cLear glaze if that would make it food safe. Additionally, would it alter the results of the crystalline process?
Thanks in advance for your feedback.
Shoshana
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Post by Arnie Benton on Jun 4, 2017 7:31:26 GMT -6
Problems with crystalline glazes come from
1. crazing is believed to allow bacteria to grow in the cracks.
2. Firing a cone 6 to 10 clay to cooler than cone 10 or maybe 11 produces a piece that absorbs water - also related to bacteria growth.
3. Acids and alkalis easily change the color of crystalline glazes, showing that they are leaching chemicals. Just put a slice of lemon on a piece - you'll see the change. Dishwasher detergent works too.
4. Cu Co Fe Zn Ba and others will leach out. At least we don't use lead.
5. Legally saying they are safe opens you up to blame for whatever symptoms a user has. In a civil suit I would hate to have to prove that crystalline glazes are safe.
I don't know how you could put a clear glaze on a crystalline piece - if applied in one firing it will markedly change the composition of the crystalline glaze and its appearance. If applied in a second firing you'd have to heat it to a temp that will change the crystalline glaze dramatically - see my postings on striking.
Arnie
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Post by shoshana on Jun 5, 2017 21:49:52 GMT -6
Thanks Arnie for your detailed response. I really appreciate it. I have always stated that my pieces are on food safe and will continue to do so!
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gczop
Full Member
Posts: 202
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Post by gczop on Jun 8, 2017 8:23:25 GMT -6
One other addition to Arnie's comprehensive reply, the results that are obtained with etching, such as hot soaking a piece in vinegar and salt solution clearly demonstrate that at least some combinations of coloring oxides create leachable ( not food safe) glazes. Gordon
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