Leaching of Finished Ceramic Ware
Ceramic art has a wide variety of specific hazards and precautions that can be divided into five areas:
Clay minerals contain crystalline silica, long-term exposure can lead to silicosis. Other impurities in clay include bacteria, molds, sulfur, small amounts of barium, dioxins and metal oxides.
- Inhalation of pulverized clay material is hazardous to the lungs. Silica and kaolin can clog the lungs and cause chronic health problems similar to smoking.
- Asbestos dust is contained in talc and vermiculite and can cause cancer.
- The diseases related to inhaling silica, asbestos and metals take years to develop. Symptoms: short of breath, dry cough, emphysema, and susceptibility to lung infections.
- Allergic hypersensitivity and asthma may occur from exposure to molds in clay.
- Bacteria in clay can cause or exacerbate skin problems. Course grains can also cut skin.
- To avoid dust buildup, clay mixing should be done in a separate room from studio spaces.
- NEVER PULVERIZE DRY CLAY INDOORS. Recondition clay by cutting into small pieces or crushing dry clay outdoors. Soak pieces and wedge to desired consistency.
- AVOID SWEEPING AND STIRRING UP DUST. Wet mop floors and work surfaces daily to minimize dust levels and prevent dry clay dust from becoming airborne.
- Refine greenware surfaces with a damp sponge instead of dry sanding. NEVER sand greenware that contains talc, it may contain asbestos.
- Wear rubber gloves to protect any cuts or damaged skin. Put on a skin moisturizer before working in clay to protect and seal hands.
- Wear separate work clothes while in the studio to avoid taking home dust.
- Wear an N95 dust mask when mixing clay or creating clay dust in closed spaces.
Ceramic glazes are a mixture of silica, fluxes and colorants. Common fluxes and colorants include many hazardous metals and minerals. Precautions are very similar to clay.
- Lead compounds are highly toxic by inhalation or ingestion. We do not use lead at FSU.
- A glaze labeled “food-safe” means the finished ware will not release toxins into food or drink. The actual glaze is still hazardous to handle before and during the firing process.
- Fluxes and colorants to AVOID ALL respiratory contact with are: barium, lithium, arsenic, beryllium, cadmium, chromium (VI), nickel, uranium, antimony, barium, cobalt, lead, lithium, manganese and vanadium.
- Fluxes and colorants to AVOID ALL skin contact with are: antimony, arsenic, chromium, vanadium and nickel.
- Skin irritants include: soda ash, potassium carbonate, alkaline feldspars and fluorspar.
- Powdered silica occurs in many glazes. See clay hazards.
- Spraying glazes is very hazardous because of the potential inhalation of glaze mists.
- Dipping, pouring and brushing glazes may irritate skin and accidental ingestion is possible with careless personal hygiene habits.
- Some luster overglazes can contain mercury, arsenic, highly toxic solvents such as aromatic and chlorinated hydrocarbons and oils. These materials should be used in accordance with manufacturer specifications; they are both flammable and hazardous.
- Assume any commercial glaze that does not state “LEAD-FREE” has lead.
- We do not use lead in glazes at FSU.
- If possible, don’t use colorants that are known human carcinogens.
- Consider wearing a respirator or N95 dust mask when weighing and mixing powders. Wet glazes are not an inhalation hazard and prompt cleanup of spills reduces the risks.
- Always wet mop spilled powders, don’t sweep!
- Gloves should be worn while handling wet or dry glazes.
- Good ventilation or outside spaces should be used to apply solvent-containing glazes.
- Basic personal hygiene rules should be followed including restricting eating, drinking or smoking in the studio. Wash your hands after using the glaze area!
Electric and fuel-fired kilns are used to heat clay and glaze. The most commonly used kilns are electric; these use heating elements to radiate heat. Fuel-fired kilns are heated by burning gas, oil, wood or other materials. Fuel kilns produce carbon monoxide and combustion gases and require frequent monitoring and adjustment.
Firing temps. can vary from as low as 1,382°F for raku and bisque firings to 2,642 °F for porcelains and stoneware.
- During bisque firing carbon monoxide and sulfur dioxide are released.
- At higher temperatures ozone, sulfur, chlorine and fluorine gases are produced; lead, antimony, cadmium, selenium and metal fumes escape from the kiln. Many metal fumes generated at high temperatures are highly toxic.
- Galena, cornwall stone, crude feldspars, low grade fire clays, fluorspar, gypsum, lepidolite and cryolite can release toxic fumes during glaze firings.
- Chlorine, fluorine, sulfur dioxide, nitrogen dioxide, and ozone are highly toxic by inhalation. Inhalation of large amounts of these gases can result in immediate respiratory distress. Fluorine gas can also cause bone and teeth problems.
- Carbon monoxide can cause oxygen starvation. One symptom is an intense headache.
- Permanent eye damage and cataracts are caused from looking into hot kilns.
- A kiln at 2370 °F has a surface temperature above 595 °F.
- If an electric kiln fails to shut off, the heating elements melt and may cause fires.
- Always turn on overhead ventilation or Envirovents when kilns are in use.
- If spots appear in front of one’s eyes after looking into a hot kiln, goggles are needed. Use welding-type green glass goggles.
- Do not store combustibles near kilns.
- Always return at the end of a firing, even if a controller is set to shut it off.
- If gas leaks are suspected: shut off gas and call the FSU Service Center at 644-2424 and FSU Police at 644-1234.
Leaching of Finished Ceramic Ware
If a glaze changes color from contact with food do not use the vessel for food! A good test for this is a placing a lemon wedge on a suspect glaze overnight, any change means that it is not food safe. Acidic liquids and continual microwave reheating (e.g. a coffee mug at work) can yield greater leaching metals from glazes.
- The U.S. Food and Drug Administration (FDA) regulates how much lead can leach from dinnerware into food and drink. Commercial companies test their ware for lead leaching, but potters do not have the same resources to test if lead leaching is a problem.
- It is best to never use lead on dinnerware.
- Lead and cadmium are the two metals in dinnerware presently regulated by the FDA.
Other Leachable Metals
Barium has been seen in some tests to leach in hazardous amounts from certain glaze formulations. 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.
Soda & Salt Firing
This process involves throwing salt (sodium chloride) or soda ash (sodium carbonate) into the kiln during a fuel firing. Sodium and chlorine fumes are produced.
- Hydrogen chloride gas is highly toxic. Health effects are similar but more irritating than most other kiln gases. Some local environmental laws ban salt kilns.
- Hydrogen chloride and water forms hydrochloric acid corroding metal fittings in the area.
- Salt glazing should only be done outdoors. Kilns should be equipped with chimneys, tall enough to disperse the hydrogen chloride above roof levels.
- When injecting soda or salt, one should avoid standing in or creating unnecessary fumes.
- All gas piping and metal fixtures should be routinely checked for corrosion.
Raku involves firing bisqueware in a fast-fire low temperature kiln. When the glaze has melted, the kiln is shut off and the red-hot pieces are placed in a metal can with sawdust, leaves or other combustible materials for a reduction phase.
- See above for the hazards and safety precautions for gas kilns.
- The reduction step produces large amounts of smoke and carbon monoxide.
- Because red-hot ceramic and open fire is used, an experienced team of people with raku gloves, tongs, fire-resistant clothing and face protection should conduct the firing.
- Burning treated wood or other non-plant materials can yield exposure to highly toxic preservatives or pesticides, such as arsenic and chromium compounds.
- Raku must be conducted in an approved outdoor location, be careful not to setup near air intakes or open windows.
- Do not use plastics or materials that have been treated with preservatives or pesticides.