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SECTION 12: Ceramics
Ceramic art and pottery has a wide variety of hazards. The specific hazards and precautions can be
divided into four area:
- working with clay
- glazing and coloring
- firing in a kiln
- potential leaching
of finished ware
Clays are minerals composed of hydrated aluminum silicates,
often containing large amounts of crystalline silica. Other impurities
may include organic matter or sulfur compounds.
Sometimes, grog (ground firebrick), sand, talc, vermiculite, perlite,
and small amounts of minerals such as barium carbonate and metal oxides,
are added to modify clay properties.
Clays can be worked by hand or on the potter's wheel, or cast in
a clay slurry into molds.
Clay is made by mixing dry clay with water in clay mixer.
Clay slip is made by adding talcs which themselves can be contaminated
with fibrous asbestos or asbestos-like materials.
Geographical sources of talcs are relevant, for example, New York
State talcs are notoriously asbestos-contaminated, while Vermont talcs
are not. Pfizer has some fiber-free talcs.
Hazards
- There have been known cases of silicosis, or "potter's
rot, from chronic inhalation of large amounts of free silica during
clay
mixing. Symptoms of silicosis
include: shortness of breath, dry cough, emphysema, and high susceptibility
to lung infections such as tuberculosis.
The disease may take years to develop.
Silica dust exposure is not hazardous by skin contact or ingestion.
- Chronic inhalation of kaolin is moderately hazardous, and can result
in kaolinosis, a disease in which the lungs become mechanically clogged.
- Asbestos is extremely toxic by inhalation and possibly by ingestion.
Asbestos inhalation may cause asbestosis, lung cancer,
mesothelioma, stomach cancer, and intestinal cancer.
- Sand, perlite, grog, and vermiculite contain free silica and
are, therefore, highly toxic by inhalation.
Vermiculite is also frequently contaminated with asbestos.
- There is a danger of accidents if clay or water can be added
while the mixer is in operation.
- Bags of clay and glaze materials can be very heavy, and lifting
can cause back problems.
- Hypersensitivity pneumonia, asthma, or other respiratory problems
may occur with exposure to molds growing in wet clay that is
being soured
or aged in a damp place, in slips that stand for months,
or with inhalation of dry aged clay. Molds
can cause or exacerbate skin problems and change the workability
of clay.
- Throwing on a potter's wheel for long periods of time can result
in carpel tunnel syndrome because of the awkward position of
the wrists.
Pain, numbness and/or pins and needles in the thumb
and first three fingers, are common symptoms. Back problems can occur from bending over
the potters wheel for long periods of time.
- Hand contact with wet clay can result in abrasion and dryness of
fingertips and hands. Moving
parts of kickwheels can cause cuts and abrasions.
- Clay scraps on the floor, bench and other surfaces can dry and pulverize, producing an inhalation
hazard due to the presence of free silica.
Similarly, reconditioning clay by pulverization and
sanding finished green ware, can create very high concentrations
of hazardous
silica
dust.
Precautions
- Use premixed clay to avoid exposure to large quantities of clay
dust.
- Clay storage and mixing should take place in a separate room.
Bags of clay (and other pottery materials) should be
stacked on palettes or grids off the floor for easier clean-up.
- All clay mixers should be equipped with local exhaust ventilation
to remove fine silica dust particles from the air.
- Clay mixers should be equipped with proper machine guards so
that they cannot be opened to add clay or water while the mixer
blades
are
turning.
- Wear separate work clothes while in the studio. Choose clothes of material and design that
don't trap dust. Wash
these clothes weekly, and separately from other laundry.
- Avoid contact of clay with broken skin. Use a skin moisturizer.
- To prevent back problems, always lift with knees bent. Also, use
a standup wheel (Cranbrook style treadle wheel), or elevate electric
wheels to a height that doesn't require bending over.
Exercise and massage may relieve minor muscular pain.
- Keep wrists in unflexed position as much as possible to prevent
carpel tunnel syndrome. Take
frequent work breaks.
- Be careful of the moving parts on kickwheels.
- Recondition clay by cutting still-wet clay into small pieces,
letting them air-dry, and soak in water.
- Finish green ware while still wet or damp with a fine sponge instead
of sanding when dry. Do
not sand greenware containing fibrous talc.
- Wet mop floors and work surfaces daily to minimize dust levels
and prevent dry scraps from becoming pulverized.
Glazes used to color or finish clay pieces are a mixture
of silica, fluxes and colorants. Common
fluxes include lead, barium, lithium, calcium and sodium, and are used
to lower the melting point of silica.
The actual colorants, which are an assortment of metal oxides usually
account for less than 5% of the glaze by weight.
Originally, soluble raw lead compounds including red
lead, white lead, galena, and litharge were used as fluxes in low-fire
glazes. In fact, over 400 cases of lead poisoning were
reported in British potters in 1897.
Lead frits and good housekeeping greatly lowered the number of
potters that had been poisoned by these highly toxic lead compounds. Frits are made of melted minerals and metal
compounds that are sintered and ground into powder form. While lead frits are sometimes assumed to be
insoluble and nontoxic, leaching tests with acids have shown that many
frits are as soluble as raw lead compounds and, in fact, there have been
cases of lead poisoning from both inhalation or ingestion of these.
High fire porcelain and stoneware techniques eliminate
the need for lead as a flux. Also,
alkali earth or alkaline earth fluxes can be used for low-fire conditions
instead of lead. Silica may also be removed from leadless type glazes. The substitution can be based on boric oxide
as the glass-former, instead of silica.
Alkali earth fluxes include sodium, potassium, and lithium oxides;
alkaline earth fluxes include calcium, magnesium, barium, and strontium
oxides. Minerals containing these fluxes include certain
feldspars, nepheline syenite, petalite, bone and plant ashes, whiting,
and dolomite.
An assortment of metal oxides or other metal compounds
produce particular colors when fired.
These are added in such small amounts to the glaze, that they aren't
usually a great hazard. Luster
or metallic glazes are fired in a reduction atmosphere.
These glazes can contain mercury, arsenic, highly toxic solvents
such as aromatic and chlorinated hydrocarbons, and oils such as lavender
oil. The common metals are often resinates of gold,
platinum, silver, and copper. Some
underglazes and overglazes use mineral spirits as the vehicle instead
of water.
Glaze components are weighed, sorted and mixed with water.
These materials are often in fine powdered form, and result in
high dust exposures. Glazes can be dipped, brushed, poured, or sprayed
on the ceramic piece.
Hazards
- Lead compounds are highly toxic by inhalation or ingestion. Symptoms of lead poisoning include: damage
to the peripheral nervous system, brain, kidney, or gastrointestinal
system, as well as anemia, chromosomal damage, birth defects and miscarriages.
- Lead-glazed foodware can leach lead if not fired properly, or if
the glaze composition is not correctly adjusted.
For example, the addition of copper to lead frits renders a higher
solubility of lead in the final fired ware.
Acidic drinks and foods such as tomato juice, citric juices,
sodas, tea, or coffee, can increase this hazard.
- A glaze label marked "lead-safe" means that the finished
ware, if fired properly, will not release lead into food or drink. The actual glaze is still hazardous to handle and fire and may contain
lead. Adequate control
over firing conditions is very difficult in the craft studio.
- Other fluxes such as barium and lithium are also highly toxic
by inhalation, but less so than lead.
- Certain colorant compounds of particular metals are known or
probable human carcinogens, including: arsenic, beryllium, cadmium,
chromium
(VI), nickel, and uranium.
- Antimony, barium, cobalt, lead, lithium, manganese, and vanadium
colorant compounds are highly toxic by inhalation.
- Antimony, arsenic, chromium, vanadium, and nickel compounds
are moderately toxic by skin contact.
- Free silica occur in many of the clays, plant ash, flint, quartz
feldspars, talcs, etc. used in glazes.
See the discussion above for the hazards of silica and
the disease silicosis. Weighing and mixing glazes can result in the
inhalation of these toxic materials.
- Soda ash, potassium carbonate, alkaline feldspars, and fluorspar
used in glazes are skin irritants.
- Spray application of glazes is very hazardous because of the
potential inhalation of glaze mists.
- Dipping, pouring, and brushing certain glazes may cause skin
irritation and accidental ingestion due to careless personal hygiene
habits.
- Glazes containing solvents are both flammable and hazardous.
Precautions
- Use lead-free glazes. If
the glaze does not state "lead-free" or "leadless" on
the label, assume it contains lead until proven otherwise.
- Lead glazes should only be used on non-foodware items. Design lead-glazed pieces so that they won't be used for food or
drink. Lead-glazed
pottery should be labeled as lead-containing.
- If possible, don't use colorants that are known human carcinogens
and avoid probable human carcinogens.
There is no known safe level of exposure to carcinogens.
- Consider wearing a respiratory when weighing and mixing powdered. Wet glazes are not an inhalation hazard.
Good housekeeping procedures and cleanup of spills reduce
the risk of inhalation or ingestion of toxic dusts.
Wet mop spilled powders.
- Gloves should be worn while handling wet or dry glazes.
- Good dilution ventilation or local exhaust ventilation should
be available when applying solvent-containing glazes.
- Basic personal hygiene rules should be followed including restricting
eating, drinking, or smoking in the studio, and wearing
personal protective equipment such as gloves, and separate work
clothes or
coveralls. Wash hands after work. Leftover glazes and
glaze scrapings can be homogenized, combined, tested, and used
as a glaze.
Electric kilns and fuel-fired kilns are used to heat
the pottery to the desired firing temperature.
The most common type are the electric kilns. Heating elements heat the kiln as electric current passes through
the coils. The temperature rises
until the kiln is shut off.
Fuel-fired kilns are heated by burning gas (natural or
propane), oil, wood, coke, charcoal or other materials. Propane gas or natural gas is used most often.
These kilns can be either located indoors or outdoors.
The fuels produce carbon monoxide and other combustion gases. Fuel-fired kilns are usually vented from the top through a chimney.
Firing temperatures can vary from as low as 1,382°F for
raku and bisque wares, to as high as 2,372 °F for stoneware, and 2,642
°F
for certain porcelains.
The early stages of bisque firing involves the oxidization
of organic clay matter to carbon monoxide and other combustion gases. Sulfur breaks down later producing highly irritating
sulfur oxides. Also, nitrates
and nitrogen-containing organic matter break down to nitrogen oxides.
Galena, cornish stone, crude feldspars, low grade fire
clays, fluorspar, gypsum, lepidolite and cryolite can release toxic gases
and fumes during glaze firings. Carbonates,
chlorides, and fluorides are broken down to releasing carbon dioxide,
chlorine, and fluorine gases.
At or above stoneware firing temperature, lead, antimony,
cadmium, selenium and precious metals vaporize and the metal fumes can
either escape from the kiln, or settle inside the kiln or on ceramic ware
in the kiln. Nitrogen oxides and
ozone can be generated from oxygen and nitrogen in air.
Hazards
- Chlorine, fluorine, sulfur dioxide, nitrogen dioxide, and ozone are
highly toxic by inhalation. Bisque
firings of high-sulfur clay have caused the production of great amounts
of choking sulfur dioxide. Other
large acute exposures to gases are not common. Inhalation of large amounts of these gases
can result in severe acute or chronic lung problems. Long-term inhalation of low levels of these gases can cause chronic
bronchitis and emphysema. Fluorine
gas can also cause bone and teeth problems.
- Many metal fumes generated at high temperatures are highly toxic
by inhalation. Since
lead vaporizes at a relatively low temperature, it is especially
hazardous.
- Carbon monoxide from fuel-fired kilns or the combustion of organic
matter in clays is highly toxic by inhalation and can cause
oxygen starvation.
One symptom of carbon monoxide poisoning is an intense
frontal headache, unrelievable by analgesics.
- Hot kilns produce infrared radiation, which is hazardous to
the eyes.
There have been reports of cataracts, from years of
looking inside the hot kilns.
- Heat generated by the kiln can cause thermal burns. The Edward Orton Jr. Ceramic Foundation reported
that when a kiln was operated at 2370 °F, the surface temperature,
was at and above 595 °F, and the temperature one foot away from the peephole
was 156 °F.
- Heat produced by even small electric kilns can cause fires in
the presence of combustible materials or flammable liquids.
- If an electric kiln fails to shut off, the heating elements melt
which can cause fires. Gas kilns
also generate a lot of heat, and room temperatures often exceed 100
°F.
Precautions
- Infrared goggles approved by the American National Standards Institute
(ANSI) or hand-held welding shields should be worn when looking into
the operating kiln. Shade
number from 1.7 to 3.0 is recommended, but a darker shade may
be required if
spots appear in front of one's eyes after looking away from
the kiln.
- Do not use lead compounds at stoneware temperatures since the
lead will vaporize.
- Lumber, paper, solvents, or other combustible and flammable
materials should not be stored in kiln areas.
- Always check that the kiln has shut off.
- If gas leaks are suspected (e.g. gas odor): shut off gas at
the source; shut off power to the kiln room at the circuit breaker;
and call the
gas company. Test for leaks with nonfat, soapy water or
use approved leak-detection solutions.
SPECIAL PROCESSES (top)
While most glaze firings refer to firing a glaze-coated
pot in the kiln, special processes sometimes are used. Salt glazing and raku firing are two examples.
Salt Glazing
This process involves throwing wet salt (sodium chloride)
into the heated kiln while the bisque ware is being fired. Wet salt at high temperatures decomposed to
sodium and chlorine. The sodium
reacts with the bisque ware to form a glaze.
Large amounts of hydrogen chloride gas and possibly chlorine are
also formed.
Sodium carbonate (washing soda) can also be used. Carbon dioxide is generated instead of hydrogen
chloride.
Hazards
- Hydrogen chloride gas is highly toxic by inhalation. Health effects are both similar and more irritating
compared with most other kiln gases. Often, local environmental protection laws ban salt kilns.
- Hydrogen chloride and water vapor form hydrochloric acid, which
can corrode metal fittings in the area.
Precautions
- Substitute safer sodium carbonate for sodium chloride.
- Sodium chloride salt glazing should only be done outdoors. Kilns should be equipped with canopy hoods and chimney stacks that
are tall enough to disperse the hydrogen chloride safely.
- All gas piping, and metal fixtures should be routinely checked
for corrosion.
Raku Firing
Raku involves first firing ware at a low temperature
in a regular gas kiln, and then removing
the still hot pieces and placing in them in sawdust, leaves or
other organic materials for a reduction phase.
Hazards
- See above for the hazards and safety precautions used with gas
kilns.
- The reduction step produces large amounts of smoke and carbon
monoxide.
- Treated wood or other materials can yield an exposure to highly
toxic preservatives or pesticides, such as arsenic and chromium
compounds.
Precautions
- Raku should only be done outdoors because of smoke. Be careful to not locate raku near air intakes or open windows
of buildings.
- Do not use materials that have been treated with preservatives
or pesticides for the reduction phase.
LEACHING OF FINISHED CERAMIC WARE (top)
Lead Leeching
There is a real concern about lead leaching into food
and drink from pottery fired with lead glazes.
Both the U.S. Food and Drug Administration (FDA) and the Canadian
Consumer and Corporate Affairs have regulated how much lead can leach
from foodware into food and drink. Acidic
liquids are of particular concern. Similarly,
continual microwave reheating, (e.g. a coffee mug at work) can
yield greater leaching of lead glazes.
Many cases of lead poisoning, and even some fatalities, have occurred
from the leaching of lead from lead-glazed pottery.
While commercial ceramics companies routinely test their
ware for lead leaching, craft potters do not have the same quality control
as does the ceramics industry, and lead leaching is more of a problem.
According to United States regulation, ceramicware that
does not pass the lead leaching tests must have a permanent fired decal
stating:
"NOT
FOR FOOD USE - MAY POISON FOOD. FOR
DECORATIVE
PURPOSES ONLY." |
As mentioned earlier, you can also drill a hole in the
pottery so it cannot be used for liquids or food.
Preferably, do not use lead
glazes, especially for food and drink vessels.
Any foodware finished with lead glazes should be tested regularly
by certified laboratories.
Other Leachable Metals
Other metals can leach into food and drink. Cadmium is the single metal besides lead presently
regulated in the United States and Canada. However, other possible toxic metals in glazes can leach. Barium has been seen in some tests to leach
in hazardous amounts from certain glaze formulations. If a barium glaze, or other glaze, changes color from contact with
food, do not use the vessel for food.
Try and use only glazes with calcium, magnesium, potassium, and
sodium fluxes and minimize the amounts of toxic metal colorants. Routine testing for other metal leaching should
be done. More research needs to
be done in this area.
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