Gavin Stairs on sat 29 mar 97
At 07:04 AM 27/03/97 EST, Robbie Hunsinger wrote:
>Dear Monona, Gavin, Ron, Evan, Tom, Tony, Ric, Linda, Richard, Vince et al-
>There seems to be a gap between current thought on safe clayworking practices,
>and the methods I've seen used at various group studios and classrooms. Some
>Clayart contributions could help me create change in this area. I know it's a
>vast topic, but anything in writing might help. ...
Well, this is my third attempt to draft a useful reply to your questions.
Generally, when this happens it means that I don't know what I'm talking
about. In this case the reason I don't know is that toxicology is a moving
target, and a sensitive one at that. So there is no once for all answer to
what's bad and what's good.
At the same time, I detect a note of hysteria in the general discussions we
are having on this list. This list is an appropriate forum for discussion,
and there are numerous issues of concern. It is well to remember that
ceramics, pottery and glazes are old, very old, in the experience of
humankind, and we haven't been killed off by it yet. When we discuss risks
and toxins, we tend to do so with emphasis on the dire consequences, without
regard for the fact that most of what we are discussing has been in common
use for a long time. There is an experiment which has been proceeding all
the while, called life, and the general outcome is not wholly bad. Life
expectancy is up, on the whole. We should not get mesmerized by the
problems, but try to incorporate them into the whole picture.
Toxicology as it is now practiced is complex. Most of the easy problems
were solved back in the 19th century and before. We know that strychnine
and cyanide are generally bad for you, although the former is used as a
helpful drug. The purest water, completely deionized, is not good to drink
in large quantities, because it upsets the salt balance and tends to do bad
things to the gut. Oxygen, a necessity for life, like water, is best taken
with nitrogen, because pure it can lead to excessive tissue oxidation, even
combustion. So toxicology is generally relative. Most of the metals we are
concerned about are required by the body in trace amounts. Barium, for
example, seems to have a place in nerve chemistry. Sodium and potassium are
ubiquitous and highly tolerated, although I tend to rip the roof off my
mouth with excess salt when I eat corn on the cob. Potassium is a key
synapse carrier. Iron, of course, is used as an oxygen carrier in the blood
haemoglobin. It seems that we grew up in this soup of elements, and our
bodies have evolved the means to keep most of them in proper balance,
according to the concentrations normally encountered in our ecosystem. It
is when the balance gets disturbed that things happen, and we get problems.
Our glazes and clay bodies are concentrated substances. This is the primary
source of danger. They are out of balance. So, as Monona says, even iron
can be an acute danger to a child. This is by no means to say that iron
glazes are dangerous. What it does say is that nothing is completely
without risk, contrary to what lawyers and litigants may believe. Almost
everything we handle in real life is similarly out of balance. The iron,
nickle and chromium in our knives and forks is highly out of balance, and
will eventually disintigrate into oxides and end up contaminating some
future potter's porcelain. The unbalance is what powers change and action.
When it flows through us, it can change us. This is all part of the natural
flow of matter and energy.
Those at first and greatest risk of poisoning from glaze substances are
those who process them at extraction and manufacture of our raw materials.
Next are those who handle them and use them in readily absorbed forms, like
potters. Last and least are those who use the ware. In order to absorb the
toxin from the ware you must first extract it from its largely inert state
in the glaze. This tells you that the insides of liquid storage and cooking
vessels are the highest risks to users. It is highly difficult and
improbable to get poisoned from the outside, or bottom of any ceramic vessel.
We aren't discussing the manufacturers at the moment, so we'll ignore them,
although historically this is where most of the damage has been done.
The potter is at higher risk than the user, in general. Preadolescents are
somewhat more sensitive to poisons in general than late teens and adults,
because whatever they are exposed to becomes a part of them very rapidly and
directly. They become what they eat, breathe, etc. Generally, concentrated
substances of all but the most inert kind should be kept away from young
Clay is a concentrated substance, but it is largely inert. Its dust is a
respiratory irritant and risk for future silicosis and respiratory disease.
Clay is a common contaminant in our everyday environments. It is just a
form of soil. However, it is a particularly dusty soil, so our concern is
to keep the dust levels tolerable. This means mainly good cleanliness, both
of the room and the students. Most schools are built to accommodate this.
Ordinary school clay of the ball clay/talc variety is normally chemically
non-toxic. Check to make sure: don't buy clay without an analysis that
shows the heavy metals, in particular.
Young children may try to eat some of whatever they handle. Probably they
should be given only certified non-toxic, oil based clay. Not that you
really want them to eat large quantities of this guck, but... at least there
are no dust problems, and what sticks to them is mainly oil.
The adolescent crowd is somewhat more able to avoid ingestion and breathing
of dusts. However, they are generally inattentive to matters of safety, the
risks of which are not readily apparent to them. They may not take
seriously warnings about the relative toxicity of glazes. Most clays,
including school clay, are safe for them, provided dust is controlled. Of
glazes, commercial or not, only the most non-toxic ones are truly safe (see
Ron Roy's post), and only after they understand that glazes are not to be
ingested or soaked in. Safety equipment must be provided when appropriate.
Personal hygiene is important, as is good supervision. This may be the time
to instill good work habits that will remain with them when they begin to
handle more toxic materials.
Post-adolescents are more able to control their own behaviors, to comprehend
rational and graded discussions of relative risk, and to apply the knowledge
to their own benefit. To the extent that they can absorb the information,
more materials can be entrusted to their handling. If they are taking
chemistry (which, God grant) then they may have been taught the basics of
lab safety. They should be taught that glazes, and glaze materials in
particular, are concentrated chemicals of the sort that they use in the chem
lab, and that they must understand the appropriate precautions in order to
handle any given material with safety. The specific precautions for each
material available to them must be provided positively and overtly, not just
after waiting for it to come up. This is part of what the teacher is there
This makes it incumbent on the teacher to study and comprehend the risks and
benefits of every material in use in the studio classroom. I would
recommend as an exercise to teachers that they (mentally) remove from the
scene everything, all materials. Then, one by one, consider each one to see
if it is worthy to be admitted. As a preliminary guide, something like Ron
Roy's recently posted list might be useful. Here it is again, pace Ron:
All clays except Barnard (Black Bird) which has a fair bit of MnO2.
All feldspars including Cornwall Stone and Neph Sy.
Frits 3110, 3124, 3134, 3195, 3278, 3269. (there are many more but these
are the ones I use - all have some boron.)(sic)
Iron oxide, Rutile and Titanium Dioxide
Strontium Carb (has a small amount of Barium 1to2%)
Soda Ash (soluble)
After that, there will likely remain a collection of stains and colors, and
a few mixed pots of gunk. This is where it gets more tricky. If the
contents are unknown, it should be removed from the classroom. If you can't
justify letting students use it, use it yourself or safely dispose of it.
If it's still undecided, put it safely away until you have learned enough to
judge. For each unknown, do a research project: You are dependent on other
peoples' work, but don't depend on only one voice. Learn all you can, then
decide. Remember, the first criterion is safe handling in raw form by
students or in the presence of students, not the end user safety: that comes
Finally, keep up to date. That means staying informed, and carefully and
rationally evaluating all new information. If you have done what I
suggested above, you will already have done this once. You need only decide
if the new information warrants reevaluation.
Now we get to the end user. The first thing is to recognize what I said
above, namely that poisoning risk comes almost exclusively from prolonged
food contact during cooking or storage. This means that making safe,
non-toxic ware is easy: do whatever you like (see caveat about transport of
toxins in kiln vapors) on the outside, and apply a safe, smooth, well
fitting glaze to the food contact area. If you are unsure about how to
formulate such a glaze for your body, see the many posts on this subject by
Ron Roy, Tony Hansen, Tom Buck and many others on this list. Or study some
of the many good texts. And do your own trials using the methods you will
find there. If you need to use a colorant glaze whose susceptibility to
toxic leaching you don't know, have a test done. See Monona's many posts to
find out what and how to do it. All of the experts have said over and over
that good fit and safety do not come from trusting others and received
recipes. They come from checking and rechecking, and matching glaze to
body. Commercial glazes are expensive, and they are matched to an ideal
body. Who has one of those? For that reason, if for no other, take Tony
Hansen's advice, and stick to a basic glaze for each body, and keep up with
changes in each batch. Then you will know.
Caveat: Some glaze toxins can be transported in the kiln by diffusion and by
fumes. Lead and other low temperature melting point materials are the
highest risks, but there are other rare and fascinating circumstances, no
doubt. They can also be spread by dusts, errors and pot contamination while
mixing. These are good reasons to totally exclude high risk materials like
The issue of crackles, mattes and crazed glazes is often broached in respect
of insanitary surface flaws. I do not know if this is sound or not. I have
certainly eaten out of and off of many cracked plates, bowls and mugs, and
have not suffered from it, to my knowledge. Even so, I incline to the use
of smooth, glassy, durable, well fitting glazes on the insides of pots. It
gives me a better feeling, if nothing else.
The final issue is regarding mechanical failure risks. In student ware, it
is almost inevitable that they will initially experience a fairly high
failure rate. The main thing is to give students to understand what to
expect of their ware. If they have made untrustworthy ware, they should be
told. Better, teach them to test their pieces by one of the methods like
hot/cold to stress unsound pieces to fracture, if the students intend to use
the ware for food, or to give it away to unsuspecting friends and relatives.
Best, learn how to make and teach the making of sound, safe ware.
I hope this is helpful. More, I hope that others on the list will add their
ret on sun 30 mar 97
Gavin, thank you for your excellent and comprehensive, well-thought out
and written summation. It should be required reading for all potters, esp,
teachers of all levels.
I can't think of anything to add.
On a related topic: the use of functional items that have served our
parents or grandparents without apparently doing them any harm. I have a
large ancient pewter collection, the old really soft kind. When my son was
small and rather rough on the dinner ware, he ate out of pewter
bowls and mugs. I am ashamed to admit that I never gave it a second
thought. He drank his juice out of a pewter mug and came down with serious
Nowaways, some 35 years later, I hope such ignorance has become extinct.
Although in Germany they still sell pewter wine goblets, the new harder
ELKE BLODGETT email: firstname.lastname@example.org
12 Grantham Place
St. Albert, AB T8N 0W8
403 (458-3445); 403 (727-2395)
Richard Burkett on sun 30 mar 97
In response to Gavin Stairs' well thought-out notes on safety, health, and
ceramics, I'd like to second his comments in general. I'd also like to
specifically urge a calm, rational approach over the mild hysteria on this
topic which surfaces from time to time.
We all have to pick our risks in life, and ideally pick them so that we
create the least likelihood of injury to others, too. I agree that it is
impossible to reduce risk to zero. Sadly, we're living in a very litigious
era here in the U.S. So now one does have to consider the very real risk
of a lawsuit, too. The resulting economic injury may be worse than the
It is very good to know what risks one takes. Even the list Ron Roy posted
is certainly not without its hazards for the potter or the end user of
pottery, and he posts his caveat "in a relatively balanced glaze." I would
argue that most common glaze materials are "safe" for the end user IF
properly incorporated in a well-designed and fired glaze. This may involve
testing. And "safe" to the potter IF propery handled. But safety is indeed
a relative thing as Gavin mentions - don't empty out your glaze room yet -
still those big 'IFs' remain. Making a well-designed "safe" glaze takes
The human race has survived a long time, and made and used an unbelievable
amount of pottery. That's not to say that some people haven't gotten
injured by some of it, but in general (lead glazes aside here), pottery
has been a pretty low risk factor in life.
Working with little (or flawed) knowledge of the hazards involved creates
the highest risk of injury.
Over the last few hundred years, those who study science have worked out
some very good (if not always perfect) methodologies to assess cause and
effect, risk and benefit. Hysteria has no part in this. This is a
rational, analytical approach which comes from careful examination of
empirical and theoretical data, not one based on hearsay or myth.
Richard Burkett - School of Art, SDSU, San Diego, CA 92182-4805
E-mail: email@example.com <-> Voice mail: (619) 594-6201
Home Page: http://rohan.sdsu.edu/dept/rburkett/www/burkett.html
Gavin Stairs on mon 31 mar 97
Elke thought that this reply to her ought to be seen by the whole list, so
here it is:
>At 07:52 AM 29/03/97 -0700, ELKE BLODGETT wrote:
>>On a related topic: the use of functional items that have served our
>>parents or grandparents without apparently doing them any harm. I have a
>>large ancient pewter collection, the old really soft kind. When my son was
>>small and rather rough on the dinner ware, he ate out of pewter
>>bowls and mugs. I am ashamed to admit that I never gave it a second
>>thought. He drank his juice out of a pewter mug and came down with serious
>>Nowaways, some 35 years later, I hope such ignorance has become extinct.
>>Although in Germany they still sell pewter wine goblets, the new harder
>My parents kept spirits (alcohol) and sherry in lead crystal decanters. Of
course you don't drink a lot of that stuff. We also had a fair amount of
pewter about, and a whole set of beer mugs in pewter. The Roman empire, and
most of Europe up until quite recently used lead pipe and high lead solders
in drinking water distribution systems. There is actually a measurable
increase in the atmospheric lead record (Greenland and Antarctic ice cores)
during the Roman era. Some have speculated that lead poisoning may have
contributed to the collapse of the Roman Empire, but I expect that is
stretching a point. We still use lead solder in copper water pipe systems.
Some people suggest that you not drink water that has been standing for some
time in such pipes. Lead oxides have a long history of use as antiseptics
and for other medicinal purposes. People have actually eaten it on purpose.
So-called sugar of lead is actually sweet, and good tasting, I'm told.
Arsenic too. And we all know about the majolica glazes.
>It is clear that lead can be a problem, especially when exposed to
chelating acids, such as those in your child's orange juice. Not only does
this dissolve the lead, but it also presents it to the body in a readily
absorbed form. Same thing with lead from leaded gasolines, and to a certain
extent with lead paints. The record shows us that young children are
especially at risk.
>Here we have two sets of somewhat contradictory evidence. A long history
of use and association without apparent damage, and episodes of relatively
acute damage in particular circumstances. That is why lead was not
discovered as a poisonous agent until relatively recently. So we can expect
more instances of the same sort to occur with other materials as time goes
by. Better to avoid anticipatable problems before the fact.
>Simple, safe liner glazes are easily formulated and used, with due skill.
Best to avoid unknown hazards. Go wild where there is no danger of
ingestion, but beware of raw material hazards, and
fume/diffusion/contamination hazards with materials like lead. Best to
avoid the lead group altogether (lead, arsenic, antimony, bismuth...).