Pottery Glaze Safety

Introduction

The safety of pottery manufactured by credible large scale pottery manufacturers particularly in Europe and the USA is a priority. However many smaller scale studio potteries have a lesser understanding of the leaching of toxic components from glazes. Why? Because it is difficult to understand and even more difficult to test accurately without specialist equipment.

However everyone involved with pottery manufacture needs to understand about glaze toxicity in the 'sue everyone' culture of today.

Changes to legislation in the USA in the early 90's provided the springboard for worldwide change.

A case of lead poisoning in the USA caused by pottery which did not conform to any legislation worldwide (YES NON CONFORMING WORLDWIDE!) led to big changes. The result of the political intervention that followed was to effectively ban lead glazed ware (under proposition 65) in California and many other US states.Whether this was a justified scientific solution to the original problem is still debated by many in the industry. However the result is here to stay.

Lead in Glaze

Lets start with some facts about lead :

• Lead in large quantities is known to be toxic. Indeed the ancient Egyptians used it for homicidal purposes.
   
• Lead in small quantities is known to be harmful. It can seriously affect the learning ability of small children and cause other harmful effects in adults.
   
• Lead glazed pottery leaches lead when subject to strong acids. However this may be as low as parts per billion; less than might be found in drinking water!
   
• Lead leaching from the glaze surface is not directly related to the lead content of glaze. Many other factors such as firing have an equal or greater influence.
   
• Lead glazed pottery has been in existence for thousands of years

Clearly the picture for lead glazes is not good. However the technical understanding gained over many years of industrial research and manufacture allowed them to be used with relative safety. Nonetheless the pottery industry has moved forward.

 Environmental as well as political pressures has ensured that millions of pounds have been spent to research and develop unleaded glazes and colours by major manufacturers. Unleaded glazes and colours have now become the norm and are available for most types of pottery body.

But unleaded glazes are not without issues. Lets consider why:

Unleaded Glaze

Unleaded glazes are mainly glass, sometimes with a crystalline phase and are considered as long-lasting and indestructible. This is not strictly true as all glass leaches to some extent when it comes into contact with acid foodstuffs even water. In the case of acids,  contact with the glaze surface over a period of time can cause a much greater leaching effect. The intermittent use of alkali dish-washing agents can also dull the glaze surface leaving it more prone to acid attack. Early unleaded glazes were particularly prone to this type of attack by strong dishwashing detergents.

Some unleaded glazes contain elements such as Barium, Zinc and Cadmium which are also considered toxic when released in large quantities from the surface of the glaze. Indeed some countries legislate for this by imposing limits on the release of cadmium and zinc elements and other heavy metals in their metal release legislation or guidelines. The safe handling of barium carbonate and cadmium compounds in the manufacture of glazes is also a concern for glaze producers.

Coloured Glazes

Copper green glaze

Coloured glazes have long been known to give greater problems than white glazes in terms of toxic metal release. This is due to the often overlooked fact that the choice of pigment greatly influences the ease at which acids can attack the glaze surface. For example it is well established that the combination of copper and lead in a glaze gives significantly greater lead leaching than from the lead glaze alone.

Other colouring elements such as cobalt, manganese can act in similar ways even in unleaded glaze. Therefore it is important to know the effect of pigments and intrinsic durability of each glaze you make.

Toxic Metal Testing

Toxic metal testing of pottery intended for food contact is carried out by specialist testing organisations who are accredited to carry out standard tests such as EU 2005/31/EC or ASTMC738 in the USA. The most recent European limits for this test is specified in 2005/31/EC. To reduce cost and avoid testing of clearly unacceptable glazes quick tests can be used to screen out poorly durable or lead glazes. Cutting a lemon in half and placing it overnight on the glaze surface is one such quick test. Another is using a quick lead test such as lead inspector.

However it is recommended that all glazes produced for food contact use are tested by certified test laboratories to ensure compliance.

Summary

To produce safe ceramics that comply with current legislation it is important to understand the formulation and firing of glazes. It is  more complex than most people think and testing by a recognised testing laboratory is the only true way to ensure compliance. The problems of safety of  lead glazes are well known, and these glazes are being phased out and replaced with unleaded equivalents. However unleaded and coloured glazes are not totally free of issues and reference to the health and safety glaze documents of suppliers is strongly recommended.

Happy Potting

The Potters Friend

More information and other technical articles on pottery and ceramics can be found at my website The Potters Friend.

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