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toxic loose ends

updated sun 30 jun 96


Monona Rossol on fri 14 jun 96

I'd like to tie together three threads that I saw this month:

1. Raku Sickness - effects of raku smoke

2. Smoke and Mirrors - white "smoke" emitted during bisquing

3. Residual Lead in Electric Kilns


Smoke from raku is hazardous simply because ALL smoke is hazardous. For
example, "The Health Hazards of Leaf Burning" were discussed in The American
Journal of Public Health (October 1994). The author referenced three studies
all showing that traditional open burning of leaves in the fall generates
dangerously large quantities of carbon monoxide, particulates, and at least
seven proven carcinogens. Several medical journal papers also showed that
during fall leaf-burning, the air pollution severely increases breathing
problems in a majority of asthmatics in the area.

Toxic smoke is emitted by all natural organic substances. It matters little if
you burn leaves, coal, oil, wood, incense, hamburger, or tobacco. Just because
we like the smell of incense or burning autumn leaves does not make their smoke
healthier. And Raku involves heavy exposures to smoke.

The incomplete combustion, noted in the comments about the "yellow" smoke,
produces the most hazardous substances. Just as incomplete combustion produces
coal tar, wood tar, or cigarette tar--raku can create airborne "tars" from
leaves, straw, paper, sawdust, etc.

One criticism: Posts suggesting wearing a repirator with "smoke blocking
filters." There ain't no such thing. Firefighters wear tanks of compressed air
because no filter is useful in smoke.

One reason is that thousands of chemicals are created when things burn. Over
4000 chemicals have been isolated from cigarette smoke--and that's just smoke
from a leaf and a piece of paper. As soon as anything is burning, your only
methods of protection are air-supply or ventilation. The same is true for kiln
emissions which are the result of a very special kind of "burning."

One additional hazard in raku is incurred by standing near the kiln and removing
hot pots with tongs. Here you can be exposed to toxic metal fumes. While these
fumes are usually so dispersed that they are invisible, Pat Wehrman was
privileged to see fume created in visible quantites during a bisque firing. She
wrote about it in her post: smoke and mirrors.


The fine white powder Pat observed was a fume. Fume results when an element
first forms a vapor, reacts--usually with oxygen in the air--and condenses into
a particle. The individual particles are so small that they can be inhaled deep
into the alveoli (air sacs) of your lungs.

Pat called the clay company and she found out that it was something in the clay
that was fuming. It is most likely to be a metal oxide fume or a
sulfur-containing compound. But DAMN--I wish Pat had DEMANDED TO BE TOLD WHAT IT

You can be sure the company that mines the clay knows what it is. They have to.
By law, they must prepare a material safety data sheet (MSDS) on the clay for
their miners. The MSDS must identify all the mineral and chemical components to
which the miners are exposed. It is easy from this data to figure out what is
capable of fuming at bisque temperatures.

This MSDS information must, by law, be passed on to the people who mix and sell
your clay. And they, darn their hides, SHOULD PASS IT ON TO YOU.

You need the information not only for health information but for quality. You
have a right to know what is now contaminating your kiln. This stuff may be
refuming onto the surface of bisque or glaze ware for some time to come. This
is the same process by which lead fume contaminates a kiln after firing lead


The fume from lead behaves similarly to Pat's fine particles. When you fire lead
glazes, lead fume settles into every little crevice and cranny in your fire
brick. Even if you have a negative pressure downdraft ventilation system, there
will be some fume adhering to the brick. And forget trying to purge the kiln of
lead if it is lined with ceramic fiber bat. You'll never get it out.

Every time you fire a kiln that has been used to fire lead glazes, some of the
deposited fume can recondense on your wear. Methods of reducing the lead
contamination include repeated firing with non-lead glazes, using downdraft
ventilation, and by firing very high. How long it takes before there is no
significant contamination of non-lead glazed surfaces can only be determined by
repeated firing and testing.

And a word to the brave souls who whipped out their copies of the Handbook of
Chemistry and Physics (one used the 43rd edition, mine is the 71st--brag, brag).
I used to do that--and still do on occasion. But I have learned that there are
so many wild and wonderful variables in that "soup" inside the kiln, that you
can never be sure about predicting results. Testing the only way to really know
what's happening.


Thank you, ClayArt people, for providing me with so many graphic stories to
explain the problems of clay and glazes to my students and OSHA Right-to-Know
trainees. All the theory in the world is not as helpful as telling them about
Pat's white mirror or about Julie Elver's high schools students getting coughs
and rashes after raku firing.

Monona Rossol, industrial hygienist with Arts, Crafts and Theater Safety