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a once common glaze ingredient

The use of toxic ceramic materials in the potters studio, such as lithium carbonate, barium carbonate and lead frits is still widespread. The toxicity of lead can be hard to detect, except in cases of acute lead poisoning. Lead is an accumulative poison and will collect in the body over a period of years, even decades, before noticeable symptoms occur or a toxic level is reached.

In ancient Rome and in Britain until the 19th C, lead pipes were used for household water, leading to a slow poisoning of the population. Lead was an ingredient in ointments and was used in rouges and skin creams at the court of Versailles. It was used in ancient pottery as far back as Egypt of 4,000 - 3,000 BC, but the earliest documented lead glaze was found near Seleucia, of ancient Mesopotamia, where the earliest Maiolica techniques also originated. The recipe included

Glass 60
Lead 10
Copper 15
Salpetre 0.5
Lime 0.5

Lead glazed platter by B. LeachLead is still used in glazes today, but we have become much more aware of its dangers, not only in glazes, but also the environment in general, and the use of lead in petrol in particular. The use of lead in glazes, particularly low-fired glazes such as raku, can be a hazard not only for the potter, but also the consumer or buyer of such wares. The use of raw lead in the form of lead oxide, red lead, white lead (lead carbonate), galena (lead sulphide) and lead monosilicate is particularly dangerous, as these types of lead can find their way into the body and be dissolved by stomach acids and then be deposited in various internal organs where they accumulate.

Fired glazes can leach lead if the glaze has not been fired high enough (at least cone 03) to bind the lead with other glaze components, forming lead bisilicate. Any acidic foods or liquids subsequently used with such glazes, e.g. fruit juices, will dissolve unbound lead particles from the glaze surface. In extreme cases this could create a lethal dosis of lead.

One solution to this problem is not to use lead, even fritted, in tableware at all, a practice which I generally recommend. Fritted lead does supposedly render leaching negligible, but leachability can increase through the combination with other incompatible glaze ingredients, particularly oxides and carbonates, especially cobalt and copper. In any case, leaded frits are usually used as a temperature lowering flux and can often be substituted with other non-leaded frits.

In those cases where leaded frits seem necessary, it is recommended to fire to at least cone 03, to ensure sufficient reaction of lead and glaze ingredients. They should be fired only in oxidation, as reduction discourages the binding of lead to silica, alumina and other ingredients. In some countries, it may be mandatory to have tableware glazed with leaded frits to be tested in a professional laboratory!

Inorganic Lead Toxicology
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