Bill Walker on fri 16 may 97
Richard Gralnik wrote:
Subject: Lead in commercial ceramic ware
>----------------------------Original message----------------------------
>I noticed a
>little sign on the counter with a yellow triangle. It read something
>like "In compliance with Prop. 65 (the one requiring disclosure of hazardous
>substances in things the public can buy), this symbol indicates dishes
>which use a glaze that contains lead, a substance known to cause cancer
>and birth defects."
>
Lead in glazes is an interesting topic. Glazes can be made
badly and leach lead into food, and they can be made very well and
leach such low amounts that standard testing cannot even detect any
lead. And the amount of lead that leaches doesn't necessarily have
anything to do with the amount of lead in the glaze, despite the
implications of California's labeling law.
When well made, lead glazes and glasses are amoung the most
chemically stable materials known. Some of the oldest artifacts made
by humans that have survived with the least deterioration are lead
glasses. The reason is that big, fat atoms (or ions) like lead fit
into glass structures in such a way to make them really stable -- if
you do it right. That is why they use glass to encapsulate high level
nuclear waste -- so those big fat radioactive isotopes are trapped and
cannot get away, into the ground water for example. Zillions of
dollars have been spent on this, because they _really_ want to be sure
that the radioactive stuff stays put.
When poorly made, lead from glazes can leach into your food and may
cause cancer and birth defects.
If you look through old issues of Ceramic Industry magazine, going
back about ten years, you can follow the whole story of California's
labeling requirement. I believe that it all started by someone
getting lead poisoning from dishes.
There may be a lesson here for users of other toxic materials such as
barium. It is much better to be careful and regulate yourself so no
one is ever poisoned by using handmade dinnerware, otherwise the State
of California may step in and set regulations that may very well put
you out of business, or really scare your customers.
By the way, at one point they talked about the little yellow triangle
being permanently affixed to the product, as a fired-on decal. Does
anyone know if this requirement made it into the final regulation?
Does anyone catch the irony here? -- Requiring the use of cadmium yellow
to warn about the dangers of lead.
And here we are back at the original question -- why do they use lead
in all the glazes. One reason is because it is the most reliable way
(sometimes the only way) to get the colors that people want and have
come to expect on their dinnerware. The companies are working on
developing new glazes and colors, but they still have a ways to go.
Bill Walker
Alfred NY
walkerw@bigvax.alfred.edu
Anita-Maria K. Parolla on sat 17 may 97
> Lead in glazes is an interesting topic. Glazes can be made
> badly and leach lead into food, and they can be made very well and
> leach such low amounts that standard testing cannot even detect any
> lead. And the amount of lead that leaches doesn't necessarily have
> anything to do with the amount of lead in the glaze, despite the
> implications of California's labeling law.
Can anyone explain what is "glazes made badly" and "glazes made very
well"
Anita
Bill Walker on sun 18 may 97
"Anita-Maria K. Parolla" wrote
----------------------------Original message----------------------------
>> Lead in glazes is an interesting topic. Glazes can be made
>> badly and leach lead into food, and they can be made very well and
>> leach such low amounts that standard testing cannot even detect any
>> lead. And the amount of lead that leaches doesn't necessarily have
>> anything to do with the amount of lead in the glaze, despite the
>> implications of California's labeling law.>
>
>Can anyone explain what is "glazes made badly" and "glazes made very
>well"
The reason I said "glazes can be made very well" and "glazes can be
made badly" is because there are a number of factors that can
influence the amount of lead that can be released. These include
formulation, firing conditions and even some batching, storage and
application practices which all go together to "make" the glaze.
Some examples:
A glaze may be formulated following the rules for minimizing lead
release, but still not be safe. Or the same glaze fired on the top
shelf might be safe, but on the bottom shelf it might not be. If the
person who batches the glaze is not too careful about accuracy, some
batches might not be safe. Glaze might be stored in a tank and not
mixed enough before applying, so what goes on the ware is not what
it is supposed to be, again the glaze might not be safe.
Bill Walker
Alfred NY
walkerw@bigvax.alfred.edu
John Baymore on sat 24 may 97
------------------
------------------------------------------------------------
=3Csnip=3E ....... Lawrence in his book shows a pretty dramatic difference =
in
lead
release depending on whether the atmosphere in the kiln is moving or not as
well. Moving =3D less, not moving =3D more ........ =3Ccut=3E
=3Cclip=3E...... Sounds reasonable given lead's tendency to volitize but =
I've
never
heard anyone else mention it (in the MANY discussions regarding lead on
this list) =3Czap=3E
---------------------------------------------------------------
Russel,
During firing, lead volitilizes out of the glaze into the kiln atmosphere
like many of the materials which are put into glazes (see Ron's post on
chrome fumes a few days ago). If the atmosphere is circulating, the
=22atmospheric=22 lead =3Cg=3E is pulled off with the exiting kiln gases. =
If the
atmosphere in the kiln is static, the =22humidity=22 (easy way to think of =
it)
of lead in the kiln increases quite a bit.
As the kiln is cooling, the lead in the kiln atmosphere, which has likely
been changed to the oxide form, percipitates out. (using the humidity
analogy....it =22rains=22 lead oxide as the cold front comes in =3Cg=3E.) =
Early in
the cooling cycle some of this lead goes back into the melt on the still
molten outer surface layer of the glaze, changing its chemical composition
slightly from that of the =22average=22 of the rest of the melt. As the =
kiln
cools further, some of the lead oxide sits as a =22dust=22 on top of the
surface of the already hardened glaze.
So this can cause a lead test to show higher leaching in wares fired in
static atmospheres as opposed to dynamic ones. The =22surface dust=22 can
cause high initial tests, but once it is washed away the results go down.
This is also true for the little =22extra=22 lead in the outer surface layer=
of
the glaze too.
This is also why non-lead bearing glazes fired in a kiln whose bricks and
furniture are impregnated with lead from previous firings can test positive
for lead, even though none was added into the batch.
This extremly fine =22dust=22 can get on your hands when you unload and =
handle
the kiln furniture too.
Best,
.....................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
Joseph Herbert on sun 25 may 97
There is a similar story of vapor deposition with arsenic in power plant
boilers. The arsenic present in the coal is vaporized when the coal is
burned but condenses on the parts of the furnace that are below 1800 degrees.
this coats the tubes in the boiler walls with aresnic, coats fly ash
deposits in the boiler with aresnic, causes problems for people who have to
work in there.
The arsenic content of coal is low but a significant portion of that which
comes in with the coal doesn't leave. Since thousands of tons of coal are
burned in a power plant boiler, the amount of aresnic that eventually ends up
there is significant.
The situation described is found in a suspended flame, water wall boiler
where coal dust is blown into a suspended fire ball surrounded by water
filled tubes that absorb the radiant heat.
To generalize from these situations, a vaporization - condensation cycle can
occurr when there is hot gaseous material that can condense on a relatively
cool surface. While one may or may not find things in the kiln chamber -
thought to be nearly all one temperature - the flue is much lower temperature
and could be a site for condensation of various vaporized oxides. Something
to think about when re-doing the chimmney.
Joseph Herbert
JJHerb@aol.com
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