Bill Walker on fri 4 apr 97
"Karl P. Platt" wrote:
----------------------------Original message----------------------------
>I think you will have trouble finding a case of Barium poisoning in the USA<
>They simply don't exist outside of really whacky extremes.....snip
I'll give you the benefit of the doubt, even though I am sure barium
poisoning in not something doctors look for unless they know that the
person has been exposed. We would like there to never be a case of
barium poisening attributed to ceramic glazes, and the way to be sure
is to exercise caution.
>I'm a lousy potter and I will speak on behalf of the presence of Ba in
>ceramic glazes.
>
>Leaching of Ba from a ceramic glaze has a couple of aspects worth
>considering. Ba can exist in a glaze either as a component in a
>crystalline material, BaSiO3, for example or it can be involved in the
>glassy material present as an ion. Let's be clear that Ba isn't sitting
>about in the glaze freely waiting to leap off and corrode someone's
>health.
I suspect that BaSiO3 is not the only crystalline barium compound
found in glazes, considering the wide range of formulations that
people might try.
>The Ba ion is very large. it's the largest ion you'll meet in a glaze. As
>such it isn't the most likely candidate to be extracted by any corroding
>solution. It's fat, er, gravitationally challenged, and has a hard time
(how about inertially challenged)
>being squeezed out of a glaze. Among the our monovalent friends (the
>"RO" materials) Mg, because it's tiny, is the most likely to go into
>solution. Solubility is also a very strong function of temperature. If
>the "standard"leaching test referred to above was of the "accellerated"
>variety which uses hot acetic acid (vinegar), then the results were
>seriously skewed -- indeed, Ba tends to be released more readily than
>the other Ro components. Of course no-one drinks hot vinegar -- or cold
>vinegar for that matter.
It is the acidity that counts. pH of 7 is neutral. The lower the pH
the more acidic. The Handbook of Chemistry and Physics lists the
following pH values:
vinegar 2.4 - 3.4
tomatoes 4.0 - 4.4
oranges 3.0 - 4.0
lemons 2.2 - 2.4
cider 2.9 - 3.3
SOFT DRINKS 2.0 - 4.0
sauerkraut 3.4 - 3.6
wines 2.8 - 3.8
So we see that leaching by vinegar is a reasonable approximation of
what might happen with some of the foods that a person might put in
contact with a ceramic glaze.
>Noting the above, one should be clear that BaO, CaO, MgO, etc, are the
>least likely characters to come out of a glaze and go into leaching
>solution. Actually, the first to go are the alkalies.
>
>Leaching, apart from the chemical constitution of the glaze, depends on
>a variety of factors such as:
>
>The ratio of leaching solution to the surface of the container
>
>The "specific surface" presented by the surface -- Shiny glaze surfaces
>actually present a vastly smaller surface area to reaction than matte
>glazes.
>
>Whether or not the solution is agitated has huge effects.
agitating would increase the release rate
>The time of contact is a critical factor, too. Here remember that the
>residence time of any sort of foodstuff in contact with the glaze is
>usually limited to the course of a dinner --- not hours. However, one
>wants to remember that ovenware is subject to the higher reaction rates
>one sees at elevated temperatures -- ie, leaching, if it can occur, will
>do so at an accellerated rate.
Food is frequently stored or cooked in ceramic containers.
>I hate being redundant, but I want to mention again my experiences
>making chemical vat lining tiles. The glaze contained about 4% BaO and
>the tiles never failed, showed color changes or spoiled product owing to
>leaching. These were used in a variety of applications which included
>heated inorganic acids, etc., including applications in the foor
>processing industry.
Potters often use barium because of its influence on color and texture --
when used in large amounts it produces beautiful effects that would
not be considered appropriate for chemical vats or the food processing
industry where a smooth, usually white surface is desirable.
>The point is that condemning this or that glaze component as a matter of
>ideology is, well, dumb. i. "Barium is BAD" Remember that the US
>tableware industry used fritted lead glazes for decades and decades
>without any recorded incident.
Because they test regularly!
>Out of the Millions of items of trouble
>free tableware made by US producers, the only documented cases of
>leaching problems (poisoning) have been with foreign products.
Because the importers did not have the stuff tested!
And probably did not know that they should!
>Especially rustic stuff from Latin America where smearing litharge onto
>a pot passes for glazing in a lot of places. Untold thousands of
>man-hours have gone into studying making stable glazes and this wealth
>of information is free to anyone who wants to go look it up.
But you still have to test!
>>>the EPA set a barium maximum contaminant level for your
>drinking water at 2 ppm.<<
>
>This is practically 2 orders of magnitude higher than PB or Cd leaching
>standards in the TCLP test -- which is another useless exersize.
Are you suggesting 0.02 ppm as the safe level?
>If so desired, I'd be happy to take this out to more detail -- anyone
>interested in studying the thermodynamics of these corrosion reactions?
>It's very worthwhile from the perspective that they show quite plainly
>that under all but real extreme cases there's more danger walking across
>NeW York's West Side Highway at 4:30 PM or in breathing in LA on a hot
>August afternoon.
The extreme cases being those glazes that release 3000ppm that no one
ever tested because someone doubted, was ignorant of or chose to ingnore
the toxicity barium.
Or are we only considering glazes that are in thermodynamic
equilibrium?
>OK, all that said, I'm off to eat.
>Karl P. Platt
>27 degrees south studying silver in glassy things
Bill Walker
Alfred NY USA
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