by Nirvan Garg

Ceramics are hard, corrosion-resistant materials made by subjecting non-metallic, inorganic substances (such as clay) to high temperatures. First known to be used around 24,000 BCE, ceramics are widely used today in households, as utensils for storing, cooking or eating food. The most common ceramics include earthenware (in most cases, terracotta), porcelain and brick. However, on their own, these ceramics can be visually unappealing, rough, and porous, allowing water to be absorbed and seep through. To combat this, ceramics are glazed. This can be done in various patterns and colours, making the ceramics attractive and waterproof, and giving them a smooth texture. 

Most glazes, however, are toxic. Pigments are used as glazes, and most pigments, especially those from prior the 1990s, contained various toxic compounds. Various colours have their own toxic chemicals associated with them. The colour white was often called ‘lead white’ as the main ingredient in the pigment was the salt lead carbonate [found in nature as the mineral hydrocerussite, the hydrated form of the mineral cerussite: (C2H2O8Pb3)[1]]; lead is a heavy metal and neurotoxin which accumulates in the brain and causes insomnia, nausea and blindness, among other diseases. In addition, lead is also used in tinted glass, crystal glass, solder, jewellery, and the lead-acid used in automobile batteries.

Green coloured glazes are made of copper(II) arsenate {the anhydrous form of which in nature is the mineral lammerite (As2Cu3O8)[2]; the hydrated form – copper(II) arsenate tetra-hydrate – is found in the mineral rollandite [Cu3(AsO4)2•4H2O][3]}, a variant of which – the pigment ‘Paris green’ – was famously used by the impressionist Claude Monet. It has historically been used as an insecticide, rodenticide, and a murder weapon. Arsenic poisoning causes severe discomfort in the gastrointestinal tract, diarrhoea, nausea, headaches and skin discolouration, similar to symptoms produced by other heavy metals. 

A surprising addition to the list of toxic elements used in ceramic glazes is uranium. Used to make various colours of glazes, including red, orange and yellow, blue and black, the most common compound used is uranium trioxide (UO3)[4], which itself is coloured an orange-yellow, and is produced during uranium enrichment. Blue and black glazes are made from uranium dioxide (UO2)[5], which occurs naturally in the mineral uraninite and is used in nuclear reactors. Uranium is a radioactive heavy metal that releases alpha particles (helium nuclei, ionising radiation) as it decays, which can cause respiratory diseases (e.g. fibrosis and lung cancer), damage the kidneys, cause cell necrosis, initiate cancers, damage (mutate) DNA, and cause the usual symptoms of radiation poisoning: nausea, skin damage, hair loss, etc. These extreme symptoms, however, are caused only by high levels of exposure, but prolonged exposure to small doses (like regularly eating/drinking out of uranium ceramics) can also cause damage. A lethal dose is more than several grams [6]. Uranium was also added in crystal glass, to give the glass tints of yellow, red and primarily green. It also made the glass fluorescent and glow green under UV light.

Various other toxic elements are also used in ceramic glazes: cadmium (cadmium yellow) and mercury (vermillion) are a few examples. Cadmium is a carcinogen and causes osteoporosis; mercury is a neurotoxin which can cause necrosis of cells in the brain. Mercury poisoning is popularly called ‘mad hatter’s disease’, after the fact that hat-makers used to undergo mental and behavioural changes (‘go mad’) due to the use of mercury in hat-making.

The use of toxic elements such as those listed above have two potential targets to expose: the potters and people who manufacture and use the glazes, and the people who eat and drink out of the glazed ceramics.

The former has a much higher exposure dose: the glazes are in raw, unfired, corrosive powder form and exposure is through the skin and lungs. Exposure to uranium is much more dangerous when it is through inhalation, as absorption of uranium through the gastrointestinal tract is relatively low, with most of it being removed in faeces. In contrast, the acute dose for uranium compounds via ingestion is more than several grams: via inhalation, it is around one gram [6].

Skin contact with arsenic can cause skin lesions and skin cancer. Potters (and other people who use glazes) working with arsenic glazes will be exposed. Inhalation can cause lung cancer. Lead is easily absorbed through the skin and respiratory tract and damages the blood, bone marrow and central nervous system.

Besides the boycotting and replacement of these toxic glazes, simple measures can be taken to ensure minimum damage: wearing appropriate respirators, eye protection and rubber gloves while handling these materials will lower the exposure potters and glaze manufacturers face.

The people who eat and drink out of ceramics glazed with toxic compounds are exposed because, even though the glaze is fired and cannot simply waft through the air to their nostrils, acidic foods and drinks leach the compounds out of the glazes. This is very common in crystal wine glasses, as wine is acidic, and leaches the lead out into the drink.

The problem does not stop at the toxic substances leaching, people ingesting them and being exposed. When the ceramics are washed, or some leftover food/drink is discarded, the toxic substances will be washed down the drain, contaminating the local water supply. 

A simple test can be carried out to determine if a glazed ceramic is safe to eat/drink out of: a lemon is cut and rubbed across the glazed surface, and some lemon juice is evenly squeezed on to the surface. The ceramic is left overnight and then washed; any discolouration/fading indicates leaching and that the ceramic is not safe to eat in.

The reason elements like these are extremely dangerous is that there is no ‘safe dose.’ In other words, exposure to any amount of these elements is harmful. Fortunately, as public awareness and knowledge increase, toxic glazes are being substituted with less hazardous, newer, modern alternatives. Examples include the substitution of mercury and strontium with iron additives in red glazes, and the substitution of lead with titanium dioxide (which is far less toxic, but listed as a possible carcinogen) in white glazes. However, toxic glazes are still very much around and are not something that should be taken lightly.