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monona on food safety (fwd)

updated sun 16 apr 00 on wed 12 apr 00

> --------- Forwarded message ----------
> Date: Tue, 11 Apr 2000 14:46:14 EDT
> From: "Earl C. Brunner"
> Reply-To: Ceramic Arts Discussion List
> Subject: Re: Monona on food safety
> Resent-Subject: Re: Monona on food safety
> ----------------------------Original message----------------------------
> Fine, shut down the kilns, pack up your equipment and send it to the dump.
> Clay: Al2O3-2SiO2-2H2O
> pay particular attention to the AL2O3 part = ALUMINA
> that's the only aluminum that I know of that is going to get out of my
> pots and
> into anything. Tell me where the Aluminum is going to come from?????
> Because if we are talking about Alumina then we can give it up
That's what we are talking about. The kaolins, bentonites, and other clays,
the aluminum-containing spars, and other sources of aluminum in the glaze
fuse with the other glaze ingredients during firing to become glass-like.
Then all the substances in the glaze can leach out with time and exposure to
liquid. The good glazes are those that leach very slowly so that very little
gets into food.

I don't think aluminum leaching is a big problem from a toxicological
standpoint as long as exposure is low, because aluminum is clearly far less
toxic than lead, cobalt, chrome, etc., etc. But aluminum's significance is
that no glaze can be made completely "nontoxic." Even when all the highly
toxic metals are taken out of glazes, there still is aluminum to consider.
And we need to respect those who don't want any sources of aluminum in their
diets by letting them know that it is there.

I don't understand what all the anger I'm seeing in some of these responses
is coming from. This is just another thing to consider and deal with.

Monona Rossol
181 Thompson St., # 23
NYC NY 10012-2586 212/777-0062 on sat 15 apr 00

> ---------- Forwarded message ----------
> Date: Fri, 14 Apr 2000 13:42:54 EDT
> From: Michael
> Reply-To: Ceramic Arts Discussion List
> Subject: Re: Monona on Food safety
> Resent-Subject: Re: Monona on Food safety
> ----------------------------Original message----------------------------
> Monona,
> All this about food safe is very interesting to me. I am a potter and I
> develop my own glazes. I am very interested in safety and leaching. I have
> an engineering background, and am currently an organic farmer... I better
> stop there lest you think I have a bit of a multiple personality disorder!

Join the club and meet the president.
> Have your tests shown trends of safe vs high leaching from various glaze
> recipes? Is there any difference between oxidation and reduction firings of
> the same glaze? Is high silica content a way to trap and prevent leaching?

People have been trying for some time to draw conclusions. There is nothing
really clear yet at all. And I have seen one glaze that was all over the map
depending on very slight differences in temperature of the endpoint.

> I have been adding extra silica and zirconium and firing at higher ox
> temperatures trying to make harder and more food safe glazes. <

Higher is not necessarily better. Just look for good fusion. In usually (not
always) shiny leaches less than matt.

> I do not yet
> have a lab to do the metals testing but I do so rudimentary brute force
> testing just for my own info - like boiling water immersion of pieces
> frozen to 0F for glaze fit, microwave oven testing at high for 10 minutes
> (will blow gold rims off if there is even a tiny crack) and my acid test
> using full strength muriatic acid (HCl) and some lye tests (using NaOH). I
> am looking at these extreme tests just for any visible change - so they are
> pretty crude. <

Sure are. You are looking for physical durability more than chemical
stability. And some lead glazes are very good in terms of withstanding high
heating/cooling. They may also still have a nice looking shiny surface after
acid leaching takes a lot of the lead from the glaze. You could be fooled by
these tests.

> The prospect of doing sub ppm tests of leaching is intriguing and I can
> already see several hundred tests I would run on multiple samples and this
> seems expensive - maybe I need to build a lab next to the pottery. There
> are so many variations I can see now that I will have a new test to run
> with every line blend and firing experiment.
> I ask all this because I have some intuition that higher fire higher silica
> glazes are going to leach less, and this is in conflict with my
> environmental economies of wanting to fire lower. I am at a cross roads of
> development: to pursue moving glazes to lower temperatures or to pursue
> making harder and more insoluble high temp glazes.
> Thank you for your contributions!
> Michael
There are some real glaze gurus here. Maybe one of them will jump in.

Good luck.

Monona Rossol
181 Thompson St., # 23
NYC NY 10012-2586 212/777-0062