CLAY
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 as a clay slurry into molds. Clay slip is made by adding talcs which themselves can be contaminated with fibrous materials. Ensure you read the SDS for talcs and whether or not it contains any fibrous materials. Pfizer has some fiber-free talcs.
Potential Clay Hazards
- Silica dust exposure is not hazardous by skin contact or ingestion; however, it is a potential inhalation hazard. Repeated long-term exposure may result in silicosis.
- Kaolin is an inhalation hazard and may result in kaolinosis, a disease in which the lungs become mechanically clogged.
- Sand, perlite, grog, and vermiculite contain free silica and are a potential inhalation hazard.
- Adding clay or water while the mixer is in operation is dangerous and may result in serious injury.
- Bags of clay and glaze materials can be very heavy, and lifting can cause back strain.
- Be aware if mold grows in wet clay that is being soured or aged in a damp place, or in slips that stand for months. There are artists that are sensitive to skin and/or inhalation exposures to mold. If mold growth is present consider disposing of the material.
- Throwing on a potter’s wheel for long periods of time can result in hand and wrist fatigue or injury. Back problems can occur from bending over the potter’s wheel for long periods.
- 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, stools 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 greenware can create silica dust.
Precautions
- Use premixed clay to avoid exposure to large quantities of clay dust. Good handling practices will also reduce the production of clay dust during mixing. Utilize local exhaust ventilation whenever possible.
- Clay storage and mixing should take place in a separate room. Bags of clay (and other pottery materials) should be stacked on pallets 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 do not 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.
- Keep wrists and hands in a straight position as much as possible to avoid strain and fatigue. 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 greenware 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
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. An assortment of metal oxides or other metal compounds produce particular colors when fired. These are added in such small amounts to the glaze, and do not pose an increased hazard. Luster or metallic glazes are fired in a reduction atmosphere. These glazes can contain mercury, and arsenic, solvents such as aromatic and chlorinated hydrocarbons, and oils such as lavender oil. The common metals are often resins of gold, platinum, silver, and copper. Some underglazes and overglazes use mineral spirits as the vehicle instead o 3858653540 f water. Glaze components are weighed, sorted and mixed with water. These materials are often in fine powdered form, and result in 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 hazardous due to 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 or probable human carcinogens.
- Use a ventilation fume hood 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 must 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, 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 combined, tested, and used as a glaze.
KILNS
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 like a chimney to the outside Firing temperatures can vary from as low as 1382°F for raku and bisque wares, to as high as 2372 °F for stoneware, and 2642 °F for certain porcelains. Galena, cornish stone, crude feldspars, low-grade fire clays, fluorspar, gypsum, lepidolite and cryolite can release gases and fumes during glaze firings.
Hazards
- Chlorine, fluorine, sulfur dioxide, nitrogen dioxide, and ozone are a potential inhalation hazard. Bisque firings of high-sulfur clay result in the production of sulfur dioxide.
- Many metal fumes generated at high temperatures are a potential inhalation hazard. 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 can cause oxygen deficiency.
- Hot kilns produce infrared radiation, which is hazardous to the eyes, and may cause cataracts.
- 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 temperature, 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 Facility Operations Dispatch at 801-581-7221. Test for leaks with soapy water or use approved leak-detection solutions.
SPECIAL PROCESSES
While most glaze firings refer to firing a glaze-coated pot in the kiln, special processes are sometimes used. Salt glazing and raku firing are two examples. Raku Firing Raku involves first firing bisque ware at a low temperature in a regular gas kiln, and then removing the still-hot pieces and placing them in sawdust, leaves or other organic materials for a reduction phase.
Hazards
In addition to the hazards associated with whatever kiln is used the following hazards may also be present:
- The reduction step produces large amounts of smoke and carbon monoxide.
- Treated wood or other materials can yield exposure to 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
Lead Leaching Lead leaching into food and drink from pottery fired with lead glazes is a real concern. The U.S. Food and Drug Administration (FDA) has 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 the ceramics industry, and lead leaching is more of a problem.
According to United States regulation, ceramic ware that does not pass the lead leaching tests must have a permanently fired decal stating: “NOT FOR FOOD USE – MAY POISON FOOD. FOR DECORATIVE PURPOSES ONLY.” A hole can also be drilled in the pottery so it cannot be used for liquids or food. The best way to control this hazard is to not use lead glazes, especially for food and drink vessels. Any foodware finished with lead glazes should be tested by a certified lab.
Other Leachable Metals
Other metals such as cadmium and barium can also leach into food and drink. Use of glazes that contain these materials should only be used for decorative ware not used for food.
It is best for food and drinkware to use only food-safe products and colorants.
