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metals leaching from gla

updated fri 16 oct 98


Monona Rossol on thu 15 oct 98


---------- Forwarded message ----------
Date: Tue, 13 Oct 1998 09:26:54 EDT
From: John Hesselberth
Subject: Metals Leaching from Glazes--Status Report (Long)

Wow, John, how I wish more people would get as interested in this problem as
you have. I have a couple of comments.

=3E SNIP ... American Ceramic Society to do a literature search for me.
=3E Their capability to do a computer search goes back to 1976 and they gave=
=3E excellent service. Who should know the ceramic literature better than
=3E they? The output provided a good start for me to dig farther back. Part =
=3E the problem is that much of this work was done a long time ago.=3C

The history is important. But many of the theories developed back then are
flawed. They also do not consider the loose way potters weight out glaze
chemical, the inadequate mixing, the little toaster electric kilns, the fuel
fired kiln with no oxidation/reduction control, and many other conditions in
small potteries. Read all the historic stuff and the theories, but withhold
judgement for a while.

=3E How much metal, of the various metals we use, can leach from a glaze and
=3E still have it be 3food safe=B2? Answer: No one knows. Of course, in =
=3E countries lead and cadmium are regulated and we must follow the laws =
=3E respect to those. However, those are the only two metals addressed by
=3E regulation and there seems to be a complete void of information on other
=3E metals. =3C

Actually, a great deal of information is available on how much additional
metals we can tolerate in our diets from sources such as leaching from
ceramics. For this information, you need to switch to data bases in chronic
toxicology and environmental toxicology. In this field, they are really
narrowing down how much of each kind of metal people are getting in their
diets already from natural and pollution sources, and how much additional
amounts from other sources could cause health problems. This is the data =
is used by EPA, OSHA, and FDA to justify standards.

The best lists of references for this kind of information I find in the
proposed rule and prerule discussions in the Federal Register. This is =
industries comment on a proposed regulation and present their technical data
and where government argues their points with their data. Here in one place
you usually find the best resources for current thinking on both sides of =
=22how much is too much=22 question.

For individual metals, try the EPA prerule discussions for each MCL and HA
from the Federal Register. I have huge files of this stuff in tattered hard
copy since I get the Federal Register daily, but I understand its all
available on the net somewhere.

It is also crucial to remember that final regulations bare no relationship
whatever to what is safe=21 Regulations are always a compromise between
economic consideration, political forces, and human safety. Usually safety
loses. For example, OSHA knows their PELs (permissible exposure levels) are
inadequate and they have tried repeatedly to update them. Industry usually
takes them to court and gets the new standards thrown out over economic
factors. That's why most industrial hygienists in many countries follow the
ACGIH TLVs (threshold limit values) because they are more current and

=3E Monona Rossol challenges us to use allowable drinking water
=3E levels as a conservative, safe side level, =3C

As I point out, this is lousy toxicology, but makes some good sense. If we
don't add more metals to our customers food than they would get from a bad
municipal water supply, they shouldn't be too upset. Bottled water users =
think otherwise.

=3E Another bit of information: Copper concentrations of 10 ppm can give =
=3E juice an unpalatable, bitter taste. Levels of 2-20 ppm are regularly =
=3E from copper-colored lead-free green glazes. Do we want our pitchers to =
=3E orange juice a bitter taste? I think not. =3C

I also have a sister with Wilson's Disease who could be seriously harmed by
such concentrations.

=3E Each potter will have to determine for her/himself what levels they are
=3E willing to accept. =3C

No. Each potter will have to determine for her/himself what levels they
are willing to feed their uninformed customers. A very different thought.

The ethics of this decision should not even allow in the discussion the
fact that some customers don't care about =22burgers and fries,=22 but =
first consider pregnant and lactating mothers, children, sick, elderly, etc.

=3E I would like to begin accumulating a data base of glaze recipes where =
=3E standard acid leaching test has been done on the resulting glaze. Do =
=3E have any data? Are you willing to share it with me and help? I have had
=3E several glazes tested myself and more are on their way to the Alfred
=3E Analytical Laboratory. =3C

As you know, I have been in discussion with Alfred Analytical Laboratory for
some time. They also will be collecting this data and trying to make some
sense of it. You have hit on the crux of our problem. Only with a lot
of data can we draw any conclusions at all about what is a safe glaze

=3E If you have, and are willing to share, such data (I hope you will =
=3E me) please send it to me at: =3C

There only are bits and pieces of data out there at this point. No flood
will occur. And missing from the little data we have are precise firing
temperatures, ox/red levels, trace elements present (e.g. copper and
surely some other metals), effect of basic foods on glazes (some glazes may
leach more in alkaline solutions than acid), effects of use (surface
abrasion, time, detergents, etc.) and much, much more.

And as long as we are in the dark, the customer must come first--before the
=22art=22 or any financial consideration. We must err on the side of
caution--because the risk is not ours, its theirs.

Monona Rossol

181 Thompson St., =23 23
New York NY 10012-2586 212/777-0062