Monona Rossol on sat 22 mar 97
Jacquelyn Lumsden wrote:
> I have been following the thread about manganese poisoning. My question is:
> Will the manganese leach from a glaze? I have a Carleton Ball Blkack
> Metallic glaze that I will be trying to alter as there is 4% manganese
> along with copper and cobalt, but I need to know how dangerous it is to
> eat off plates glazed in this. <
Sometimes when we get into details about leaching we forget that some people
still aren't aware that glazes themselves--the whole glazes--are dissolving
or solubilizing slowly when in contact with liquids (even plain water). The
only difference between acceptable and unacceptable glazes is the speed at
which this is happening.
Since the whole glaze is dissolving, every chemical in the glaze will be
leach. In some cases, certain chemicals solubilize more easily and leave
some matrix behind, but eventually the matrix will follow.
The picture that must be removed from our heads is one of toxic metals
somehow migrating to the surface of the glaze and jumping into our food. It
doesn't happen.
> I will likely have my hammer ready!! <
Nah. If it is a beautiful plate, use it for things that don't put it in
contact with liquids that facilitate leaching. Chips, cookies, whole fruit,
lots of stuff can be in contact with leaching pottery safely. As long as
you know what the problem is, just make sure you or family members don't use
the plates the wrong way. The hammer is in order if testing shows that the
pottery leaches really *large* amounts of manganese or similarly toxic metals
or if it leaches lead or cadmium which are just too toxic to deal with in
this manner.
Monona Rossol, industrial hygienist
Arts, Crafts and Theater Safety
181 Thompson St., # 23
New York, NY 10012-2586 212/777-0062
http://www.caseweb.com/acts/
Evan Dresel on tue 25 mar 97
At 07:24 PM 3-22-97 EST, you wrote:
>----------------------------Original message----------------------------
>
>
>Sometimes when we get into details about leaching we forget that some people
>still aren't aware that glazes themselves--the whole glazes--are dissolving
>or solubilizing slowly when in contact with liquids (even plain water). The
>only difference between acceptable and unacceptable glazes is the speed at
>which this is happening.
>
>Since the whole glaze is dissolving, every chemical in the glaze will be
>leach. In some cases, certain chemicals solubilize more easily and leave
>some matrix behind, but eventually the matrix will follow.
>
>The picture that must be removed from our heads is one of toxic metals
>somehow migrating to the surface of the glaze and jumping into our food. It
>doesn't happen.
>
>Monona Rossol, industrial hygienist
Well if this is the scenario we are concerned with then I think that a leach
test of new ceramics is even more inappropriate for considering chronic
toxicity. Yes glazes dissolve slowly but I sincerely doubt that can explain
the amount of barium or lead or whatever release seen in some leach tests
that have been described. It would be easy enough to test if you measured
the silica or alumina leached. Bulk dissolving of the glaze would give you
the same ratio of barium to silicon as you see in the glaze analysis. And I
don't think looking at limit formulas would have nearly the value if bulk
dissolution were the dominant process.
Migration of certain elements to the surface of glass or ceramic materials
most certainly occurs. I agree the process slows down to a negligible rate
at low temperatures (and even oven temperatures are pretty low in my book)
once a thin (but measurable if you really are interested) rind forms on the
surface. But I guess that's my concern: If you want to look at long term
toxicity, then look at long term behavior.
I agree wholeheartedly that the scenario that someone presented of people
scrubbing dishes so hard that the surface was reactivated by scratching
should be taken very seriously. Then you get into considerations of the
hardness of the surface. I don't have any easy answers but that IMO is why
science is still relevant (and fun!).
-- Evan Dresel who's day seems to have evaporated. Goodnight.
Gavin Stairs on wed 26 mar 97
Monona:
>>The picture that must be removed from our heads is one of toxic metals
>>somehow migrating to the surface of the glaze and jumping into our food. It
>>doesn't happen.
Evan:
>Migration of certain elements to the surface of glass or ceramic materials
>most certainly occurs. I agree the process slows down to a negligible rate
>at low temperatures (and even oven temperatures are pretty low in my book)
>once a thin (but measurable if you really are interested) rind forms on the
>surface.
I'm not sure that we really have a handle on this. Fired bodies and glazes
are very complex systems. To the extent they are glasses, they are still
very thick, viscous liquids, even at room temperature. It would not
surprise me to find that some elements in the glass are more mobile than
others. Glass is recrystallizing continuously, even at room temperature.
That's why old glass is more fragile than new glass. This involves the
migration of atomic species in the glass. So transport phenomena are not
ruled out.
Also, the ability of water and certain ions to penetrate a glaze to some
depth is certainly a function of the glaze structure. A glaze which is
stressed by virtue of competing thermal contractions, for example, may well
have atomic sized channels by which molecular water may enter and dissolve
some fraction of the glaze. Perhaps helium studies could shed light on this.
Also, I suspect that more systematic studies are required to understand just
what influences solubility. Ron Roy has referred to a study by Janet De
Boos (the post is in today's set, so I won't reproduce it) that links Barium
availability to both glaze concentration and fit. This makes sense. There
was a prior post from Bill Walker telling of a glaze that changed color in a
short time of immersion in acid. I don't know how deep the color layer was,
but I suspect that it was somewhat deeper than a few atomic layers, so that
argues for some sort of transport phenomenon.
So that seems to say that we have more work to do to understand this
completely. But in the meantime, considering that most potters do not know
how well their glazes fit, and what influence that may have on the
solubility of glaze fractions, I would consider it prudent to at least have
a test done by someone competent, and better, to avoid questionable glaze
components whenever doubt exists. If you can't bear to give up your
favorite glaze, have it checked, and maybe use a low fire transparent
overcoat. But have that checked, too.
Gavin
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