Jacquelyn Lumsden on thu 20 mar 97
Hi folks--
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. I will likely have my hammer ready!!
Thanks.
Jackie Lumsden
Waterloo, Ont.
CDANIELLE on sat 22 mar 97
Jacquelyn Lumsden wrote:
>
> ----------------------------Original message----------------------------
> Hi folks--
>
> I have been following the thread about manganese poisoning. My question is:
> Will the manganese leach from a glaze?
What about leaching from a claybody fired at ^6? I haven't heard any
talk of this and am wondering if it is posible. I have twenty mugs I
threw with a claybody containing manganese. I would like to know if
there is the slightest possiblity that they could be a health hazard
before I do anything with them. Expert advice would be much appreciated.
Gabe Thomas
Springfield OR
Cdanielle@prodigy.net
Monona Rossol on mon 24 mar 97
Gabe Thomas wrote:
> What about leaching from a claybody fired at ^6? I haven't heard any > talk of
this and am wondering if it is posible. I have twenty mugs I > threw with a
claybody containing manganese. I would like to know if > there is the slightest
possiblity that they could be a health hazard > before I do anything with them.
Expert advice would be much appreciated.<
What an interesting question! I have never seen any data at all on clay body
leaching. It seems possible, but only testing would tell.
Monona Rossol
Monona Rossol on thu 27 mar 97
Monona wrote:
>>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. <<
Evan wrote:
> 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.
Initial leach tests, indeed, usually will look different than tests over
time. There are several reasons.
First, surface metals fumed from other glazes appear in early tests. This is
why, for example, lead-free glazes fired in kilns with lead glazes will often
test positive for lead initially.
Second, some glazes--not all--are not uniform in composition after firing and
more colorant is found at the surface.
Third, glazes leach just like geological specimens--because they actually
are similar to materials like obsidian. The more soluble materials near
the surface will dissolve out of the matrix. Microscopically, this often
leaves the matrix a bit more porous and less resistant to abrasion.
Fourth, continued exposure will continue to work on the glaze slowly
solubilizing the matrix and further exposing the more soluble elements. Use
and abrasion will speed this process.
I have often thought that leach tests for either silica or aluminum
should be run on pots that have been used for a while. It would be
fascinating to see if the dissolving of the matrix can be quantified this
way. And soluble aluminum is also a substance that some people what to avoid
in their food.
It also might be fun because I don't think a leach test for soluble aluminum
has ever been done.
> 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.<
My guess is migration is insignificant. But you are right about the "rind."
Exposure to gases in the kiln to form a layer of slightly different
composition at the surface.
But no where is it written that this surface "rind" is less soluble in acid.
It may be more soluble depending on the glaze AND whether the gases which
were reacting with the surface were from oxidation or reduction
atmospheres. And whatever the rind is, it will not survive surface abrasion.
These are some more reasons why we are going to need many tests before we
know which glazes are safe and which are not. Look at all the variables we
have covered so far in these discussion:
*Amount of a toxic metal in the glaze
*surface area--matt, shiny, etc.
*Stability of the glaze as a whole to acid/water
*Matrix structure which allows selective dissolving soluble elements
*Migration of colorants to the surface during firing
*Maturity of glaze (both temperature and time may be factors here)
*Oxidation/reduction
*Surface metals deposited by fumes from other ware in the kiln
*Changes in leaching over time
*Changes in leaching with use--and type of use (hot food, microwave, etc)
*Changes in leaching with cleaning processes--abrasives, hot dish washers and
strong alkaline detergents, etc.
The ceramic industry found out that glaze leaching problems are not solved
simply by finding the right glaze formula. Their kilns have vented
atmospheres, their firing cycles are highly controlled, the exact
compositions of their ingredients are constantly monitored, etc. AND STILL
THEY TEST.
> 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 think there are some C-1 ASTM standards for surface hardness and
resistance to abrasion used by the ceramics industry. I will take a look
when I get time. If the tests use affordable equipment, they also might make
a good research project for someone.
> I don't have any easy answers but that IMO is why science is still relevant
> (and fun!). <
Couldn't agree more!
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/
Monona Rossol on thu 27 mar 97
> I'm not sure that we really have a handle on this. Fired bodies and glazes
> are very complex systems. <
Dead on.
> 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. <
The people I work with who research and restore historic stained glass say:
" 'tain't so." They said there are a lot of myths about changes even in the
shape of glass with time (e.g. bowing out or becoming thicker at the bottom
of a pane), but they say there is no actual evidence of this at all.
They also say that old glass is more brittle because it was made differently
and/or the glass is actually thinner since the surface has erroded over time
and acid rain exposure. Notice: this is glass leaching!
If you know of some solid research on glass elements migrating or pieces
flowing or changing in any way, I'd love to post it to the International
Guild of Glass Artists forum that I'm on.
> 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. <
This is very true. The glass or glaze may even appear "etched" because the
errosion has made the surface irregular.
> 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. <
Its going to take a lot of tests checking for many different variables before
we will know for sure.
> 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. <
I know that blue barium glaze--or at least one very much like it. The
surface is matt and the solvent penetrates into the surface. Also, the glaze
that leached over 3000 ppm barium also changed color slightly from greenish
brown to a slight pinkish cast. Hmmmmm. Both obviously had some copper
there also. And there was, I think, lithium in the blue glaze also. Any
ideas?
> 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. <
From your lips to God's ear.
Monona Rososl, industrial hygienist
Arts, Crafts and Theater Safety
181 Thompson St., # 23
New York, NY 10012-2586 212/777-0062
http://www.caseweb.com/acts/
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