Saturday, August 13, 2011

Are your glazes safe

Between garden, kitchen and studio there hasn't been much time leftover to do a daily blog.

This morning I've been doing some glaze chemistry, looking at some old recipes from the the past 40 years of more. It's amazing how many glazes used then and now are either not durable or not safe or both.

Whenever I design a glaze or check out one that look promising enough to test, I run it through my Insight Glaze program to check the limits for it's cone range, expansion, etc. I find, in many instance, glazes listed as gloss, are actually mattes under supplied with alumina and silica and over fired for their cone; or the reverse, glazes over supplied with some materials and then under fired for their cone giving a false matte. In either case, they will probably not be safe and/or durable over time.

Running a glaze through a glaze calculation program, or just doing the calculations by hand which I did for years, can give a good indication of whether a glaze will craze, be a matte or a gloss or something in between, as well as other important information, like the mole limits of that oxide for the cone you're firing.

If you're not sure of the safety of your glazes, Alfred University has a service and for about $15 per oxide will test your glazes for leaching and let you know if the leaching is within accepted safety limits. High copper glazes are notorious for leaching, mainly because there isn't enough silica even at cone 10 to keep that material in suspension. So, over time the glaze will bleach out, releasing that copper into any food stored in that pot. Better to get copper naturally through eating peas than getting over supplied through toxic copper carbonate.

Well, time to think about breakfast and get into the studio. Yesterday pots need attending to and more pots need to be made. Things are going slowly because I pugged this batch of clay way too soft.

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