John Baymore on wed 3 jul 96
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From: Don Jung
Subject: Re: The Barium hazard
To all the glaze gurus out in clayart, I was wondering if there are any
other ingredients in a glaze that could/should be tested for?
........................... ....Should there be any concern
regarding other
ingredients in a similarily 'weak' glaze leaching out? This is especially a
concern when we make a 'mystery' glaze out of the scrap and leftover glazes.
thanks Don Jung ***Happy Canada Day***
.................
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Don,
Pretty much everything that you put into a glaze COULD leach out given the right
circumstances. Many are of no concern due to lack of significant toxicity.
Many are. The only ones that are LEGALLY controlled right now are lead and
cadmium........ by the FDA. (Uranium was considered for regulation, but the FDA
concluded that noone was still using uranium )
However other oxides could be a problem too, and have just not been studied
effectively. A.C.T.S. in NYC (Monona Russo) feels that the figures for
contaminent levels in drinkling water are a good place to start with this
subject. She feels that if the release in the test exceeds the levels permitted
in drinking water, it's not safe. Labs can test for most any and all ceramic
material release. The tests are expensive.
In a legal sense, even if there are no regulations concerning leaching of a
specific substance, if a medical problem is caused to someone by your pot, you
are pretty much "responsible". You would have to demonstrate that there was no
reason for you to have been concerned about the hazard, and that you exercised
"due diligence" in researching the subject. And in light of the volume of
information "around" about leaching problems of all sorts (particularly
barium!), you would have a hard time defending such a case, IMHO.
Morally, you just don't want to cause anyone harm.
Any of the metals used in ceramics should be of possible concern........ even
iron. I have heard (from Monona Russo) of a case of a high iron glaze which was
oversprayed with iron wash causing a medically significant poisoning from a
coffee mug which was in daily use. (Once the archives are up, this was
discussed here, I think.)
There seems to be this misconception amongst ceramists that if it looks like a
glass, it's insoluble. Not true! Even if it is a nice gloss (lead glazes are a
easy example of that ). Ron Roy is "dead on" on that post about barium
(perfect bullseye Roy! ), and hopefully a lot of people will have noticed
that point! Barium matts are not the only barium glazes of concern.
People doing "experimentation" in glaze labs all over the world are doing tests,
and then evaluating them for "sucess" totally by eye. If it looks good, it's a
glaze. Now, this approach is FINE as long as you are making totally
non-functional wares. Or maybe when you are using wood ash, local clay, and
local granite. (However....... do you really know what is in that ground
"rock"?) Pretty non-toxic sources, in general. These sources generally produce
sodium, calcium, magnesium fluxed alumino-silicate glass, with trace levels of
other stuff.
(It is important to note here that I have an ash glaze formulated by good ole
"traditional" methods that shows significant "etching" by food acids over time!
I don't use it on food contact surfaces for that reason, even though it doesn't
release any significant "toxics".)
Now if you are doing experiments doing triaxials off of refined "chemically
pure" things like lithium carbonate, barium carbonate and a frit, that is
another story all together! Even if you get something that looks good, that
doesn't make it a "good glaze". It may be a nice visual finish for a piece, but
it may not stand up even to weathering (for sculpture) or certainly to the
action of food acids. And the concentrations of the oxides are certainly high;
high enough to be of concern.
All these "new" glaze formulas get circulated around and quickly lose their
context. Pretty soon, a less experienced potter gets one out of a book,
magazine, or off someplace like CLAYART, and assumes that it is "OK", since it
is a published formula that "has been around".
Another factor here is when the glaze is attributed to "A NAME". A well known
ceramic artist develops a "glaze" for his/her use in a specific context. So you
invariable have a glaze called "Famousperson'sname Blue Matt" for cone 9
reduction that was shared at a workshop. That formula gets around. It was
intended for use in a sculptural environment (which was the theme of the
workshop). It then eventually gets used by some potter for the inside of mugs!
[OK....... got my flameproof suit all ready here ]
Just because "the name" is well known, came from the best school, and has
exhibition and workshop credentials up the wazoo, that doesn't make him/her a
glaze chemist! The glaze you got from them might be a stable one........ but
there is NO GUARANTEE that the "famous" person knows any more glaze chemistry
than a Ceramics 101 student. (A sad comment on our education system.) In fact,
THEY may have gotten it from someone else who doesn't know glaze chemistry
either.
[Now ...........apologies and due respects to those many who DO have a solid
technical background to complement and support the more aesthetic side.]
"Mystery Glaze" has a long historical tradition in the ceramics field, and is a
nice way of dealing with the accumulation of glaze scrap. Unfortunatly, the
tradition usually calls for the (usually dark brown or black) glaze to be used
on the inside of pots! Bad practice. This stuff is very suspect for oxide
release. Unless you have it tested, you don't have a clue as to the
composition! The black ones particularly so. The black comes from a saturation
of coloring oxides. A likely prospect for release of metals, I would think.
Use mystery glaze on sculpture.
In a school situation, "Mystery Glaze" is even more suspect. God knows what has
been washed out in the "glaze washing only" bucket. Maybe that old package of
blood red lead glaze from home that has been kicking around for years!
There is no way around it......... when you get into the glaze lab, they don't
call it "lab" for nothing. To be a "potter" in the modern world is more than
just dealing with form and surface. You gotta have a little of the science end,
at least. And there certainly is a significant "science" component involved
here. More than most of us are willing to devote to it. That's why god
created ceramic engineers and industrial health specialists .......... they
delve into this stuff.
Probably the definitive source on this subject at the moment available here on
the "Net" is Monona Russo. She can be reached by e-mail at
75054.2542@Compuserve.com. She has tons of information on this subject (and
many other health related things) available. She is glad to help anyone.
........................................john
John Baymore
River Bend Pottery
Wilton, NH
76506.3102@Compuserve.com
http://www.CraftWEB.com/org/jbaymore/rivrbend.shtml
PS: It is interesting to note that the renowned "folk potter" Hamada Shoji
studied ceramic science and was employed as a glaze chemist BEFORE he ventured
off to learn how to make pots by hand. Leach brought him to England with him
because he realized that after seven years doing clay in Japan he didn't really
have the technical education to set up the pottery at St. Ives.
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