The following safety information is used with kind permission from Frank E. Woolley.
The glass studio can be a dangerous place for the uninformed or careless artist. This chapter presents guidelines for minimizing the dangers caused by exposure to toxic chemicals and to intense radiation produced by high temperatures.
The information here is not intended as recommendations for your circumstances. You should consult a safety specialist, industrial hygienist, physician or eye care professional for specific recommendations.
Toxic Elements Used in Studio Glasses
The table on the next page lists some of the most commonly encountered elements for which OSHA has established permissible airborne concentrations for full-time worker exposure (8 hours a day, 5 days a week, 52 weeks a year). Although studio workers are not typically exposed for such long times, these values provide a rough guide to the relative toxicities of the elements. The most hazardous are listed first.
These elements usually are present in glass batches as compounds, not as the isolated elements which are listed in the table. Often the other elements that make up the compound, such as the nitrogen and oxygen in nitrate, are relatively harmless, so use this table as a rough guide to the toxicity of all the compounds of a given element.
Silica (the chemical name for sand) is an exception, since it is a compound (silicon dioxide) and since it is not chemically toxic, but rather is a particulate irritant. Long exposure to silica dust or fume can cause silicosis, a serious lung disease in which crytalline silica particles cause lung scarring and loss of elasticity. Overexposure to respirable silica also increases the risk of lung cancer.
Chemical Hazards in Studio Glassmaking
To create a danger to you, two events must occur:
- the toxic elements must be released from the glass or batch
- they must enter your body
The key to working safely with glasses is to prevent both the release and the entry of toxic elements.
Safe working conditions result from application of three principles: do everything practical to prevent release of toxic materials, contain whatever unavoidable escapes, and protect yourself from whatever is left.
Toxic elements are released by dusting, volatilization and dissolving. Toxic elements enter your body by inhalation and ingestion. Dusting and volatilization produce airborne particles. Dissolution produces liquids you may ingest.
Glasses that contain toxic elements usually are not hazardous in normal use at room temperature because the elements are tightly bound in the glass structure. However, they can be released by evaporation or by dissolution.
Dusting
Causes
Batches and cullet contain fine powders because the crushing processes which are used to break up oversize particles unavoidably also produce very small particles. These fine powders are released whenever batch or cullet is allowed to drop through the air, or whenever air is blown against it.
Glass grinding and polishing also produces fine powders, either in the air with dry grinding, or in a water slurry with wet grinding. The slurry from wet grinding becomes fine powder if it is allowed ti dry out.
Preventing Dusting
The first step is to avoid producing fine powders. Cullet should only be broken as small as needed for filling; overcrushing greatly increases the fine friction. Batch materials which have a minimum fines fraction should be chosen. Very fine batch materials are usually unnecessary for melting, and increase dusting, flow and agglomeration problems.
Wetting (adding a few percent water) is an effective way to prevent dusting of both batch and cullet, but they must be kept covered to prevent the surface from drying out.
Containing Dusts
Escape of dust can be minimized by enclosing and exhausting powder transfer points. The effectiveness of an exhaust depends on the air velocity at the capture point. A small volume of air moving through an exhaust located very close to the source is much more effective than a large air volume through an overhead hood. Dry grinding should be done only with a very good forced exhaust.
Spilled batch can be cleaned up by wet sweeping or by vacuuming with machine with a HEPA filter.
Others can be exposed to dust if you bring dusty clothes into your home; it is a good precaution to wash and change at the studio.
Volatilization
Causes
When glass batch is exposed to the high temperature of the melter, some of the compounds evaporate. Dropping batch into the melter causes dusting, and the airborne particles of volatile salts evaporate immediately. In addition, some of the glass components volatize from the melt surface. Volatilization also occurs when a piece is being reheated in the gloryhole.
The vapors produced by evaporation from both the batch and the glass surface either react with the hot refractory, shortening the life of the furnace, or they condense on cooling in the stack or in the exhaust gas to form extremely fine particles called fumes. Fumes also form when the surface of glass is strongly heated in a flame. Although the amount of material volatilized in flameworking is generally low, the exposure times can be long, so good exhaust is important in flameworking.
Preventing Volatilization
Gentle loading of wetted batch prevents mot dusting in the furnace.
Since volatilization rates increase very strongly with increasing temperature, the most effective way to prevent volatilization and fumes is to keep the batch and glass surface from being overheated. Good practices include lowering temperature and firing rate during charging, and in larger furnaces charging well away from the burners. Burners should never be aimed toward the melt surface. Reducing flames (low air/gas ratio) can dramatically increase the volatilization rate of heavy materials.
Containing vapors and fumes
The stack of melters should be large enough and dampers or stack covers adjusted to minimize stingout; static pressure inside the furnace should be just very slightly positive (0.05 inches water) at glass level; this will make it negative near the crown. If you are melting lead, cadmium or other particularly toxic glasses, forced exhaust systems may be needed with close-fitting enclosures to exhaust stingouts and combustion gasses.
The station where powdered colored glasses are coated on gathers, and where tin chloride or other sprays are applied to hot glass, can be a major source of toxic fumes. It should be well exhausted with shields designed to achieve high air flow toward the exhaust.
Good exhaust is also needed over flameworking stations. The draft should be away from yourself, and cooling air should not disrupt the air flow of the exhaust. A smoke test is a simple way to verify that air flows are correct.
Dissolved Glass
Most glasses dissolve in strong acids or bases. The rate at which glass dissolves depends on its durability, on the acidity or basicity (pH) of the solution, and on its surface area. Some glasses, like hard borosilicates and some high temperature lamp glasses, have excellent durability (resistance to strong acids and bases). Common glasses and especially soft colored glasses often have very poor durability, dissolving even in weak acids and bases. The toxic elements in the glass are released when the glass surface dissolves. The amount of toxic material released depends as much on the rate at which the glass is dissolved as on the concentration of toxic elements in the glass.
If glass is powdered, the surface area and the dissolution rate are greatly increased. Grinding and polishing produces a slurry of fine glass particles; the high surface area results in rapid dissolution, releasing any toxic elements in the glass. Grinding sludge may be a hazardous material; you should consult environmental control agencies to determine how to dispose of waste from your processes.
Acid etching also produces acid fumes. Good exhaust over etching stations is important, and waste acid should be treated as a hazardous material.
Preventing Inhalation
There are two types of particles to be concerned about:
- those that dissolve in your nose, throat or lungs, allowing toxic materials to enter your bloodstream
- those that enter your lungs and do not dissolve, even after long times
The first type are dangerous if they contain toxic elements, and if they dissolve in body fluids a reasonable time. All sizes of particles are dangerous. Three common sources are:
- toxic powders in the batch
- fumes produced by condensation of materials which have volatilized at high temperature
- fine powders of glasses which contain toxic components and which have poor durability
Fumes from condensation of vapors are extremely fine and easily inhaled, and are also often quite toxic. These fumes are present in the melter exhaust gases. They also form a layer of fine dust all over the superstructure and stack of a melter or gloryhole, and are released when repairs are being made.
The second type, insoluble fine particles, come from the batch (examples are sand, alumina, and zirconia), and from grinding compounds. The smallest particles are the most dangerous for inhalation. Particles larger than about 5 micrometers diameter are captures in mucus in the nasal and bronchial passages are expelled; smaller particles can enter the lungs, where they remain and irritate the tissue. The most dangerous particles are the ones that hang in the air (a 5 micrometer sand particle will settle about 20 feet in an hour).
Crystalline forms of silica (quartz, cristobalite and tridymite) are the most common respirable (inhalable) particles in glassmaking. The greatest silica hazards are dusts from batch and from old refractories removed when furnaces are repaired.
Protection against dusts
For dust that cannot be prevented or exhausted at the source, personal respirators with fine-particle filters are available. These should be used to supplement, not replace, the other measures to reduce exposure to dusts.
Personal respirators are of three types: disposable masks that cover your nose and mouth (dust masks), air-purifying cartridge respirators, and powered air-purifying respirators into which filtered air is pumped (air helmets). The latter type are much more expensive ($150-500, compared with $5-40 for cartridge respirators and $0.50-3 for disposable masks), but work with beards and are more comfortable for long use.
Masks and filter cartridges are rated by the National Institute for Occupational Safety and Health (NIOSH). New standards were issued in 1995 and will be fully implemented by mid-1998 that require capture of at least 95% of particles 0.3 micrometers diameter (larger and smaller particles are easier to capture). For dusts without oil mists present, the designations N95, N99, and N100 mean that 95%, 99% or 99.7% of 0.3 micrometer particles will be removed.
Preventing Ingestion
Good personal hygiene is the key to avoiding ingesting toxic materials. Keep food, cigarettes, and anything else you plan to put in your mouth away from glass batches, glass powder, and grinding dust and slurry. Water in which glass powder has been soaked may have a dangerous level of dissolved toxic elements. Grinding slurry sticks tenaciously to your hands, and may become an unwanted part of your lunch.
More Information
A good source of detailed information in relatively easy-to-use form is the OSHA homepage: www.osha.gov
A source of safety equipment and training videos is the Julius Kraft Co., 7 Pulaski Street, P.O. Box 918, Auburn, NY 13021-0918, phone (315) 252-7252 or (800) 447-6775, fax (315) 252-6386.
If you are working with lead-containing glasses for food or beverage applications, the International Crystal Federation (phone 202-342-8400) can be a valuable source of information and guidelines.
Eye Protection
Flying Glass
The obvious hazard in glass making is from flying glass particles. The value of your eyes is so vastly greater than the inconvenience of wearing glasses that it is utter foolishness to not have on safety glasses with side shields whenever you are in the studio.
Lenses labeled with "ANSI Z87.1 1989" meet the strength requirements for occupational eye protection.
Radiation
Glass workers are also subjected to a less obvious hazard due to the intense visible, ultraviolet (UV) and infrared (IR) radiation that emanates from melting furnaces, gloryholes and flameworking burners. UV radiation has a shorter wavelength than visible light, and can cause sunburn-like damage to your eyes (as well as your skin); this type of burn is signaled by pain like a headache behind the eyes. IR radiation has a longer wavelength than visible light, and can cause deep general burns. "Glass makers' cataracts" are the well-known result of radiation damage to the lens of the eye, caused primarily by the IR radiation.
As the temperature of a hot source increases, the damage caused by radiation increases much more rapidly; even a small increase in temperature can make an important difference. The intensity if radiation increases with the fourth power of the temperature. The wavelength at the maximum intensity is inversely proportional to the absolute temperature, so higher temperatures mean a larger proportion of the total energy is UV intensity: oxygen/gas flames may be nearly invisible, but are strong sources of IR radiation. The yellow "flare" seen both in the flameworker's burner and in melting furnaces is the result of a strong emission in the visible by sodium in the flame; it may obscure your view, but it is not necessarily accompanied by IR or UV radiation.
Protective Eyewear
Protective lenses are available which will absorb most of the UV and IR radiation. A dark green lens with strong IR absorption, like those used for gas welding, gives the best protection, but it makes it difficult to judge color temperature during working - these are most useful for gathering, charging, or doing other prolonged work in the melter or gloryhole. This type of lens is widely available at automotive and welding supply stores; they are relatively inexpensive. The absorption power of these green lenses is designated by "shade" numbers which increase with darker glasses: Shade 4 or 5 is dark enough for melters with soda-lime glasses (up to Shade 15 might be used for electric welding).
Good quality green sunglasses are similar to these welding glasses. Cheap sunglasses may be worse than no protection, since they may absorb in the visible, causing your pupils to open, but may not absorb the UV or IR.
Lenses designed specifically for glass makers also absorb the sodium flare - these are the "didymium" glasses, in which the rare earths neodymium and praseodymium absorb most of the sodium yellow radiation, but only a part of the near UV and very little of the IR. For flameworkers, lenses are available that absorb well in the IR and UV, but still have good transmission in the visible to see small details. The combination of didymium with the UV and IR absorbing welding glasses gives the best protection for melter and gloryhole use.
To obtain protection from flying particles, the lenses need to be strong as well as absorbent, so they should meet the strength standards of ANSI Z87.1 1989.
One source of protective eyewear tailored to the needs of glass workers is:
- Aura Lens Products, Inc.
- 51 Eighth St. N, Sauk Rapids, MN 56379
- (320) 393-3182
- www.auralens.com
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