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Slip Glaze


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#1 Burtis Bills

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Posted 04 April 2010 - 08:33 AM

I found some Utah deep red native clay that fires to a cone 5. Straight off the hill, screened once it throws wonderfully up to 18 inches without grog. Is there a quick and easy test for lead?

I would also like to explore using the clay as a slip glaze. What would I add if the glaze test appears under fired or over fired at cone 10?


BURTIS BILLS

#2 Sherman

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Posted 06 April 2010 - 10:27 AM

I found some Utah deep red native clay that fires to a cone 5. Straight off the hill, screened once it throws wonderfully up to 18 inches without grog. Is there a quick and easy test for lead?

I would also like to explore using the clay as a slip glaze. What would I add if the glaze test appears under fired or over fired at cone 10?



Burtis,

You don't need to worry about lead in your local clay. Lead in ceramics is a concern when it is intentionally used as a glaze ingredient, but is not fully contained within a stable glass.

If it's over fired, add some higher-firing clay, like a stoneware or even kaolin, for the refractory properties of alumina and silica. If it's under fired, add either frit or feldspar. These will contain some mixture of silica, alumina, and flux, and this flux will likely lower the melting point. It may not take much.
Sherman Hall
Editor, Ceramics Monthly
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http://www.ceramicsmonthly.org

#3 Marcia Selsor

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Posted 18 July 2010 - 06:41 PM

I used a 50 cherry wood ash and 50 Albany slip as a ^10 glaze back in 1971 in Upstate NY. It was a sparkling golden glaze.
I had an infinite supply of cherry wood ash. Try mixing your local clay with ash. If too runny add a little stoneware clay or kaolin.
Try some line blends with ash, local clay and maybe koalin if you need to keep the glaze from running.

#4 bciskepottery

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Posted 18 July 2010 - 11:10 PM

I believe lead can, in some instances, be absorbed through the skin. So, it might be safe to have the clay tested. When I sell pottery at fairs and other events, customers often ask if the clay and glaze are non-toxic and lead-free. It would be a plus to be able to provide them that assurance.

#5 Sherman

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Posted 19 July 2010 - 08:45 AM

I can't imagine a situation where clay would naturally contain lead. Testing your clay alone for lead content is really not a wise use of your time or energy.

In terms of using as a slip glaze, follow Marcia's suggestions of adding ash. You could also add a frit or feldspar. Her suggestions of 50/50 is a good place to start.

All the best,

Sherman
Sherman Hall
Editor, Ceramics Monthly
Co-host, Ceramic Arts Daily
http://www.ceramicsmonthly.org




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