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A Problem With Cobalt?
Article by Jeff Zamek, author of Safety in the Ceramics Studio & What Every Potter Should Know.


Piet Stockmans, '100 Vases' exhibition Sotheby's (London, UK), 1998. Stockmans is well known for his exclusive and innovative use of cobalt blue. Website: http://www.pietstockmans.com
Cobalt oxide and cobalt carbonate are ceramic raw materials widely used by potters. They are frequently found in glaze and slip formulas contributing, under varied circumstances, a light to dark blue color depending on the amount employed. Cobalt has traditionally been used as a decorative blue colorant in everything from Chinese porcelain to American salt glazed stoneware. However, its use today reflects a wider problem potters face when employing any ceramic raw material. The dilemma is not with the specific raw material but with the lack of pertinent health statistics relating to the effects of ceramic raw materials on potters. The available statistics are based on industrial populations of workers whose exposure and duration rates are significantly different from potters working in craft centers, schools, and private studios. In the wider economic world large industries have the incentive and money to assemble and document health problems associated with the workplace. There have been numerous statistical records in commercial areas such as mining, paper production, and metal industries as to the effects of raw materials on workers. From the data obtained many procedures have been enacted for the safe handling of raw materials.

There is also an economic incentive for industry to protect its workers to prevent potential litigation. This is not the case with the population of individual potters who have no economic resources for such health related documentation. A central question that has not been addressed is: can an accurate extrapolation of industrial statistics be applied to potters who have lower exposure and duration rates?

Based on the lack of health and safety statistics as they relate to the small population of potters can it be possible that ceramic raw materials will become the next “asbestos type” material for potential litigation? Society in general has become more litigious, a description for taking legal action over a real or imagined injury to a person or damage to property. It is not surprising that this mania has inevitably come to the field of ceramics. It is present even in lawsuits generated from customers scalding themselves from a hot cup of coffee. The economic reactions to lawsuits, or even the possibility of lawsuits, can have a negative effect on ceramics suppliers and eventually on potters who purchase ceramics equipment, supplies, and raw materials. Ceramics is a small industry. In fact, the term industry really does not apply to ceramic supply companies, equipment manufacturers, glaze companies and clay producers who sell to potters involved in this creative activity. For the most part ceramics supply companies are staffed by few employees, and the potters to whom they sell to are individual craftsmen. With such a small market for goods and services, any negative influence on profit margins can have large penalties for producers and consumers of raw materials. Larger industry has the capital and personnel to bear the possibility of legal actions regarding their products or services. Ceramics supply companies do not have staffs of lawyers or high profits to pay for any type of legal action that a raw material complaint might produce.

Not only can an individual potter be sued but a ceramics supplier can conceivably be named as party to a law suit for selling items to the potter that caused the eventual user real or imagined harm. For example, if a potter sells a coffee cup with a glaze containing cobalt carbonate and the user claims the glaze transferred harmful amounts of cobalt into their body, a problem can exists on many levels. Conceivably the potter who made the pot and the ceramics supplier who sold the raw materials used in the pot could be named in the lawsuit. Such lawsuits are going after “deep pockets”, or defendants who can afford to pay damages. Depending on the circumstances of the case a party can be dropped from the suit, but the process does require a lawyer who expects a fee for services. Defending against a potential or actual lawsuit can be an expensive and time-consuming process for any individual or business. Aside from the costs involved in such actions, the tangible area of liability has to be evaluated as to the declared damage inflicted on persons or property. Most, if not all, ceramic supply companies and individual potters would not knowingly allow a situation to exist where a product could inflict damage or illness on their customers, but good intentions have little to do with the factors driving litigation. Even if negligence cannot be assigned, the time and cost to defend oneself is considerable. In many instances even if the case does not go to court legal fees are incurred.

Spode Willow Pattern in Cobalt Blue
During the past few years, there has been a steadily increasing amount of conjecture and embellished claims concerning the use of raw materials by potters. Many undesirable things can grow in a climate of ignorance and speculation. Ceramic raw material toxicology has become increasingly subject to exploitation and misunderstanding by the uninformed and misguided. Ignorance of ceramic raw materials by the legal profession and exploitation by individuals who make their income from distorting data has increased in the past few years. This disturbing trend is in part due to the revenue potential associated with any form of litigation. A steady flow of articles has been published warning of new ceramic “poisons of the month.” The topic of health factors relating to ceramic materials has attracted alarmist “experts” with access to undiscriminating print media. Rather than counter such biased reporting knowledgeable potters and centers of ceramic learning have consistently remained silent.

While it is understandable that individuals cannot access the resources to scientifically counter misconceptions regarding ceramic materials, it is inexplicable why places of “higher learning” have not undertaken ceramic related health studies to counter false claims and rumors. Research studies on the effects of ceramic raw materials as pertaining to potters are not to be found in colleges offering ceramics. They are frequently introspective institutions looking only toward their own activities and not to the wider population of potters.


Reliable deposits of cobalt are currently mined in Canada and Southern Africa. It is extracted as a by-product of nickel ore mining. Cobalt carbonate (CoC03) and the stronger cobalt oxide (Co02) are metallic coloring agents that have been employed by potters throughout the history of pottery making. It can be found as a decorative element in the earliest 12th century salt glazed ware produced in Germany. Individual artist potters in the United States have been using cobalt in both forms for over fifty years. Cobalt will produce stable blues in oxidation and reduction kiln atmospheres at various temperature ranges. It is one of the most potent colorants and produces a strong blue when added to slips, glazes, and overglaze/underglaze washes. One part of cobalt can tint 100,000 parts of a glaze or slip. This widely used metallic coloring oxide can also generate a range of blues and black in clay bodies. Cobalt can also be found in many commercial glazes, stains, and underglaze colors; large-scale industrial uses of cobalt occur in the formulation of steel and chromium alloys.1

A recent court case concerning cobalt carbonate has come about by a confluence of specific events driven by greed, ignorance and an atmosphere of misguided consumer protection. As in most accidents, a series of individual actions aligned themselves in such a way as to produce a flawed outcome for everyone involved. Eventually, ignorance of ceramic raw materials by laymen and “experts,” along with the legitimate health concerns of a potter, were brought together with lawyers who did not do their homework.

The Potter

A semi professional potter with over twenty years experience producing high temperature functional pottery in a garage studio claimed to have contracted cobalt poisoning. The potter used approximately 5 lbs. of cobalt carbonate every four years in various overglaze washes, glazes, and a blue slip. The slip formula contained two tablespoons of cobalt carbonate to one cup of slip. It was applied by a brush “spattering” method to green ware. Cobalt slip was loaded onto the brush and flicked onto the pots. The floor of the pottery studio was exposed to the excess cobalt slip from mixing and application operations. A glaze containing 3% cobalt carbonate was mixed in thirty-gallon containers, which were refilled every three years. The glaze was applied by dipping or pouring it on the bisque pottery. The potter glazed 6-8 hours a day 5 days a week for a 5-month period every year. An overglaze wash containing 60% cobalt carbonate was also brushed over some glazes. The slips, glazes, and overglazes were applied to a clay body which was mixed in the studio. All the pots were fired in a 30 cu./ft., sprung arch downdraft kiln to cone 9 (2300 F.). The potter has been a pipe smoker for thirty-three years using approximately 30 pipe fills of tobacco a day. He also consumed approximately four litters of diet soda a day. He ate, drank, and smoked in the ceramics studio, which was cleaned periodically with a garden hose.

Large Three-legged Nabeshima Platter depicting a Snowball Fight (in Underglaze Cobalt Blue). Japan, Genroku Period, (1688-1703)

The Symptoms

Applying underglaze blue pigment to a large bowl in Jingdezhen, China.
After noticing symptoms of fatigue, gastrointestinal problems (acid reflux), muscular pain in all major joints, shaking when fatigued, depression, anxiety attacks, headaches, sore mouth, scalp irritation, poor sleep, and prolonged infections, he visited his local doctor. After examinations by several physicians, he was placed on a number of medications to relieve the symptoms that had been developing over the last fifteen years of working in his pottery studio. On one of his visits to another doctor who happened to have taken a pottery course, it was suggested he might have developed heavy metal poisoning from the cobalt used in his studio. A hair sample was taken and it was found to contain cobalt. He was informed it was also possible that the range of his medical problems fit many of the symptoms associated with cobalt poisoning. At this time, the potter was given over ninety-five chelation treatments, which are used to draw out heavy metals from the system. The potter also did a significant amount of research on the Internet believing his symptoms fit cobalt poisoning.

He then went to the Mayo Clinic in Rochester, Minnesota and was evaluated by a number of medical specialists including internists, neurologists, gastroenterologists, rheumatologists, psychiatrists and a physician specializing in occupational medicine. Their findings did not indicate heavy metal poisoning. Significantly, blood and urine tests were negative for heavy metal poisoning, but a test did indicate an abnormally high level of arsenic in his blood. However, one doctor did tell him “symptomatically” his aliments fit cobalt poisoning, but this opinion was unsupported by the medical facts of the case. In fact, the symptoms for cobalt poisoning are characterized by functional effects on the ventricles and enlargement of the heart (cardiomegaly). There are no demonstrated musculoskeletal or neurologic effects as a result of exposure to cobalt. Gastrointestinal effects primarily from ingestion include, nausea, vomiting, and diarrhea.2

The Trial

The potter claimed he contracted cobalt poisoning because the ceramic supply company, which sold the cobalt carbonate, did not list safety instructions and precautions on the packages he purchased before 1998. His purchases of cobalt carbonate after that year did have safety information but he claimed the labeling was not adequate to inform him of the potential hazards when using the material. In the petition for damages filed by the potter’s lawyer’s they claimed the cobalt product sold to the plaintiff as designed, constructed, assembled, manufactured and distributed was defective and unreasonably dangerous when put to a reasonably anticipated use. At some point in the presentation his claim was extended to include manganese and arsenic poisoning because the initial test of the cobalt carbonate sample contained arsenic and 5.6 parts per million of manganese as trace materials.

The Potter’s Position

The potter alleged he was unable to earn a living and could not work as a result of the ceramics supply company’s negligence due to insufficient labeling of their product, cobalt carbonate, which also contained manganese and arsenic. He had stopped making pottery, which he believed was the cause of his health problems. The potter wanted compensation for economic and pecuniary harm, including, but not limited to, past and future medical bills and expenses and past and future lost wages and income. It was further claimed he suffered and will continue to suffer pain, emotional and psychological harm and distress and loss of enjoyment of life. He believed his symptoms were directly related to the cobalt carbonate contained in his slips and glazes. The claim for damages was in excess of 5 million dollars.

The Ceramics Supply Company’s Position

The ceramics supply company’s position was that the safe use of any ceramic raw material was in part the responsibility of the user to inform himself of the health and safety procedures needed when working in the field of ceramics. Based on the potter’s education and training he should not have been eating, smoking or drinking in his studio. Most significantly, they claimed the potter did not have cobalt, manganese or arsenic poisoning since his extensive medical examination did not indicate such a diagnosis. However, the ceramics supply company was not required to prove what illness or illnesses the potter was suffering from since the onset of his symptoms. Additionally, several physicians and toxicology experts in environmental medicine testified that the potter’s history of symptoms did not indicate poisoning form any heavy metal, with one physician speculating that the symptoms could be caused by the array of drugs he was taking for unrelated illnesses.

Evidence was also presented by the ceramics supply company’s lawyers that demonstrated cobalt, manganese and arsenic poisoning was not found in the population of potters. Previously, this information had been based on anecdotal reports accumulated over fifty years by individual potters and informal surveys taken at the National Council on Education for the Ceramic Arts (NCECA) annual meetings. A statistically accurate study of potters and their use of raw materials was sponsored at the 2000 NCECA meeting. The Potters Health & Safety Questionnaire was issued to all participants.3 The questionnaire was also listed on the Internet and was available at ceramic supply companies throughout the United States. The results of the survey indicated the four most serious health problems potters encounter in their studios are back pain, carpal tunnel syndrome (repetitive motion), cuts (from glaze shards) and burns (from reaching into hot kilns). While a small percentage of potters reported illnesses attributed to raw materials, a search of the National Library of Medicine data banks and other medical libraries did not reveal any diagnosed cases of potters contracting cobalt, manganese or arsenic poisoning.

Duke University Medical Center, Department of Community & Family Medicine, Division of Occupational & Environmental Medicine had reported that the original cobalt carbonate sample was reanalyzed using a test of greater accuracy and that it did not contain arsenic. Furthermore, it was determined that the hair sample taken from the potter, which was reported as having high concentrations of cobalt, was within normal limits for the general population.

The most significant finding, of which potters should take note, occurred at Duke University’s Medical Center for Occupational & Environmental Medicine. The potter’s actual glaze and slip formulas were tested for respirable and ingestible concentrations of cobalt. The exposure assessment was conducted under controlled conditions duplicating the poor ventilation found in the potter’s studio. The exposure to cobalt was tested by a modeling technique where each activity was carried out duplicating the actions of the potter in his studio. The glaze and slip formulas were mixed and applied in the same “spatter” technique as used by the potter. Respirable exposure and ingestion levels were calculated for an average adult male based on the mixing operations in the potter’s studio. Test results determined, even allowing for a combination of incidental additive ingestion and inhalation in cleaning, mixing, and glazing activities, daily absorption of cobalt would be in the range of 170-945 micrograms per day. The levels of cobalt the potter was exposed to in his studio were the same as in the general population in the United States. This was reflected in testing hair, blood, and urine, values which fell within the normal range. A risk assessment for cobalt found no adverse effects in humans at exposures of 3400 micrograms per day.4 The assessment of the potter’s studio by Duke University’s Medical Center for Occupational & Environmental Medicine is believed to be the first documented case of cobalt material toxicity testing in a private ceramics studio.

Aside from the environment, the general population is exposed to cobalt from many occupational and consumer sources. The most significant source of cobalt is found in food and drinking water. Food ingestion alone can average 5-100 microns per day. Drinking water levels can average from 2 micrograms per liter to 107 micrograms per liter per day. Background cobalt air levels can range as high as 0.61 micrograms.5 In addition cobalt is found in vitamin B12. A typical multivitamin contains 4 micrograms of cobalt in the form of B12.6. Cobalt is a necessary element, with background exposures from vitamin preparations, water and food in the range of 30-37 micrograms per day.

The Verdict

The trial was held in the potter’s hometown and lasted six days. After reviewing the testimony of the ceramics supply company’s expert witnesses, the potter’s expert witnesses and the potter, twelve jurors took less than three hours to reach a verdict in favor of the ceramics supply company.

The Impact

The court of law can be an imprecise instrument for solving ceramics related toxicology questions. It is simply the wrong tool for determining the health and safety factors associated with the potter’s use of ceramic raw materials. This case represents several related issues concerning the current realities in ceramics. First and primary is the potter’s past and current health and the debilitating symptoms that he is experiencing. An inaccurate assessment by himself and others has caused a delay in the correct medical treatment of his illness. If nothing else, the medical evidence submitted at trial should eliminate cobalt, manganese or arsenic poisoning as the possible causes for his symptoms. Hopefully, the information presented will direct him to the correct diagnosis and treatment.

The economic impact of the trial on all parties was extensive. Preparation and research for the trial and payment to expert witnesses, in addition to legal and court fees, were substantial. The cost to the ceramic supply company, which is essentially a small business, caused a revision in its selling practices to prevent future litigation. The adjustments will not appreciably increase the safety margins to workers or customers but will increase the costs to customers. Even though the ceramics supply company defended itself successfully many companies will eliminate raw materials and services which might cause legal problems. Such preemptive tactics will not serve potter’s health interests and will not protect ceramics supply companies from litigation. The economic balance between safe handling and selling of ceramic materials and reducing legal exposure to lawsuits depends in part on access to expert technical advice, which many ceramics companies cannot afford. Defending against this type of litigation for the ceramics company, no matter what the outcome, was a battle that need not have taken place.

A legitimate array of physical symptoms by the potter, which led to a hair sample test for cobalt, caused the potter to seek legal action. When the symptoms did not match cobalt poisoning, manganese and arsenic poisoning were claimed to be the causes of the potter’s health issues. While the cobalt carbonate used by the potter did contain 5.6 parts per million of manganese (trace amounts) it did not possess arsenic. Unfortunately, what was needed at the onset of contemplated legal action was not present, namely knowledge of ceramic materials and their toxicological effects on potters. Without this understanding, a course of action was taken, giving an economic incentive for the potter and his lawyers to move proceedings along even when the medical facts did not line up with the physical symptoms of the potter. Once the legal mechanism was underway the cost in time and money for both the defendant and plaintiff started to accrue.

No one is arguing for less stringent safety procedures in selling raw materials, accurate labeling, or potters adhering to their own studio safety measures. Anything that will promote safety in ceramics should be considered, even though raw materials used by potters have had a good safety record. One step in learning about raw materials can take place when potters request from their ceramic supplier a Material Safety Data Sheet (MSDS) for every raw material they purchase. It contains useful information such as the chemical name, chemical family, hazard class, formula, occupational exposure limits, emergency first aid procedures, waste disposal, respiratory protection, and precautions for handling and storage. Each category of information will help the potter establish safety guidelines for raw material handling procedures within their studio. In order to reach a wider audience of potters courses in material safety should be part of every college ceramics and craft center curriculum.

Raw materials can be handled safely with the proper barrier methods of protection such as National Institute for Occupational Safety & Health (NIOSH) approved respirators, kiln viewing goggles, rubber or latex gloves, and high temperature kiln gloves, to name a few items that should be utilized consistently by every potter in their studio. Raw materials, as used by potters in their studios, have a remarkable safety record considering the varied safety procedures used and not used by potters over the years. There is no room in the ceramics studio for ignorance or misconceptions about raw materials that we as potters come into contact with on a daily basis. When cobalt was placed on trial, it highlighted several areas of future concern for the ceramics community. Ignorance of the toxicological effects of raw materials caused a false diagnosis, which initiated a rush to litigation. The trial caused an economic loss for the potter and the ceramic supply company without an acceptable solution for either party. A lack of knowledge almost perpetuated an incorrect assessment of cobalt carbonate for the pottery community.

Article © Jeff Zamek.


  1. Hamer, Frank, The Potter’s Dictionary of Materials and Techniques, Fourth Edition, A&C Black Publishers Limited, pages 64 & 65.
  2. Myerson, S. Ross, MD, MPH, Myerson Occupational & Environmental Medicine, P.C.
  3. Zamek, Jeff. Safety in the Ceramics Studio. Iola, Wisconsin: Krause Publication, 2002, page 141.
  4. Stopford, Woodhall, MD, MSPH, Assistant Clinical Professor at Duke University Medical Center.
  5. Kriss JP, Carnes WH, Gross RT. Hypothyroidism And Thyroid Hyperplasia In Patients Treated With Cobalt. JAMA, 157(2): 117-121, 1955.
  6. Barceloux, D. Cobalt. Clinical Toxicology 37(2), 301-216, 1999.

Jeff Zamek received B.F.A. and M.F.A. degrees in ceramics from Alfred University, College of Ceramics, NY. He taught ceramics at Simon's Rock College, Great Barrington, MA. and Keane College, Elizabeth, NJ. In 1980 he started his own ceramics consulting firm and has contributed articles to Ceramics Monthly, Pottery Making Illustrated, Clay Times, Studio Potter, and Craft Horizons. His books, “What Every Potter Should Know” and “Safety in the Ceramics Studio” are available from Jeff Zamek/Ceramics Consulting Services, 6 Glendale Woods Dr., Southampton, MA 01073, website: http://www.fixpots.com.


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