amy parker on thu 27 may 99
At 12:46 PM 5/26/99 EDT, you wrote:
>----------------------------Original message----------------------------
>A few days ago I sent in a transparent glaze recipe which contains Lead
>Bisilicate. Of cause one should not use lead in any form in functional
>pottery, because it is soluble in alkalies and also in some acids and
>lead is poisonous for the human body. One should only use it on
>non-functional ware like tiles. I guess that one can look at a fritt as
>a substitute, but this might be a too soft glaze to use for dinnerware.
>So ,sorry Amy Parker, I just learned from Tom Buck that you are a
>functional potter. This glaze will not work for you. I will look into
>THE POTTER`S BOOK OF GLAZE RECIPES of Emmanuel Cooper and see if I can
>find something for you to test. So you just let me know if you are
>interested.
>Take care.
>Antoinette.
Perhaps I was a bit harsh on Antoinette about the lead glaze...Sorry If I
hurt any feelings, but we should always include a special disclaimer about
lead or other known "bad" chemicals (like barium), since many on this list
do not know the hazards, and the above post obviously "assumes" that
"fritted lead is safe". NOT TRUE!!!
I would like to add an excerpt from the document that John Hesselberth was
kind enough to post on lead - please refer to the archives for the entire
document, dated 4/30/99.
------------John's post in part -----please read all following!------
Myth: Using lead in fritted form makes it safe
Fact: Lead-containing frits are indeed safer for the potter to handle
than the various forms of raw lead. The use of frits, however, has
ABSOLUTELY NOTHING to do with the safety of the final glaze. Once the
glaze is melted in the kiln, the lead no longer knows it was contained in
a frit. The only things of importance from this point on are the
composition of the glaze and the firing conditions.
Myth: If I use a commercial lead-containing glaze which indicates that
it is food safe, it is.
Fact: This is only true if your firing conditions and things like
underglazes are exactly the same as those used by the glaze manufacturer.
In fact, most kilns have significant variation in temperature and your
firing conditions, at least in part of your kiln, are probably
significantly different from those used by the glaze manufacturer. A
copper-containing underglaze may significantly affect lead release. The
only way to assure that a lead-containing glaze is safe for use with
foods is to have representative pots from every firing tested by a
qualified laboratory. In addition FDA regulation in the United States
requires that potters have a qualified laboratory periodically check
their ware. Ware that might be used for food, but does not pass the
leaching standards must have a hole drilled in the bottom or be
permanently labeled that it is not safe for food use. California
requirements are even more specific and stringent.
Myth: I only use lead-containing glazes on things like flower pots,
planters and bird baths so I don't need to worry.
Fact: Lead is toxic to animals just as it is to humans. Plants raised
in a lead glazed pot will absorb lead. If that pot is used for growing
something like herbs the lead has just worked its way into the food chain.
Myth: I only use lead-containing glazes on my sculptural work. I don't
need to worry about my functional work.
Fact: If your sculptural work and your functional work are fired in the
same kiln--even at different times--you need to worry. It is well known
that lead can migrate to the walls of the kiln and then contaminate
future firings. Of course, if functional and non-functional pottery is
fired at the same time, contamination is even more certain.
Myth: I use a lead-containing blue glaze which I have had tested and
know is safe. In fact it was way below the allowable limit. I wanted to
get a blue-green color so I added some copper. Because it was so far
below the allowable limit in my first test, I don't need to retest it.
Fact: Addition of copper to a lead-containing glaze is widely reported
in the literature to significantly increase the release of lead. COPPER
SHOULD NEVER BE USED IN A LEAD-CONTAINING GLAZE.
In Summary:
Although lead-containing glazes have been a favorite of potters for many
years and have many desirable properties, IT IS PAST TIME FOR POTTERS TO
STOP USING LEAD-CONTAINING GLAZES ON FUNCTIONAL WORK. Val Cushing, a
long time user of lead glazes, speaks for many potters when, after
reviewing all the data on lead glazes he wrote in Cushing's Handbook:
"It has been hard for me to accept the fact that, generally speaking, the
world is a safer place if we potters eliminate lead glazes from our
repertoire. Good-by lead glazes -- we shall miss you..."
-
amy parker Lithonia, GA
amyp@sd-software.com
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