Lead in pottery
What should a pottery consumer know about lead?
It is not, nor can it be, used in 'high temperature' glazes.
Firing temperature for the glaze is key. It is more important to ask about lead when there is some indication that the pottery is fired at a temperature below about cone five. This includes 'Bone China'.
Bone China is one of the more interesting cases. The clay body is unorthodox in that it uses the Phosphorus (and Calcium) from cattle bones. (This is a poor use of the more uncommon fertilizer element Phosphorus.) This mix is first fired to a high temperature without a glaze to achieve translucency. Later it is fired at a lower temperature with a glaze that has been traditionally a lead fluxed glaze. (Lead is one of the less common elements too, but without the fertilizer value). This is a waste of good Phosphorus.
Can lead be used safely?
If it is pre-melted in low percentages with plenty of silica and a small amount of Alumina, it can even pass California's standards for solubility. The problem is that, where economics rule, percentages of lead grow.
Are commercially produced pots safe?
It depends. Since lead makes every thing cheaper and easier, there is always the temptation for mass production facilities to use a little more than is safe. Further, lead can be used in very high proportions - over 50% - with ever increasing benefits to the bottom line.
The FDA randomly checks domestic producers of tableware in the United States. However, it does not have enough inspectors to test either imports or exports. So since the profit margin on commercial ceramics is low, and lead cuts firing costs, I would not trust imports in any country that does not inspect them. This even applies to imports from the United State coming into a foreign country!
What are the other potential sources of lead?
School ceramics, hobby ceramics, and primitive low fire tourist ware.
Why is lead used in these situations?
Again, it is often about cost savings, but also more colors, and less skill required to get a pleasing finish.
What does lead do in a Glaze?
Lead has the same number of 'hands and feet' as a silicon atom, so it grabs onto all the same things that silicon would, i.e., it surrounds itself with 4 Oxygen atoms. It is almost a silicon impersonator. This is why you can use so much of it. But Lead is very big and it is very heavy, (as atoms go), so it does not fit as well as silicon, (with oxygen). Plus, lead can not hang on nearly as tight as silicon. These properties combine to progressively lower the melting temperature, while progressively increasing the toxicity from solubility. The worst low fire glazes actually have more lead than silica. With these glazes the kiln barely has a dull red glow to be hot enough to melt them.
A good visual might be a gym full of square dancers, where the percentage of lead was represented by the percentage of extremely over-weight 'couch potato' dancers. Speed up the music and tip the gym up on a 30 degree angle - to simulate the stress of kiln firing - and watch how the largest of the dancers contribute to the flow/collapse downwards of the others dancers. This is analogous to lead acting as a flux, where the smaller dancer represent silica.
To avoid lead altogether, go with the high-temperature glazes (e.g., porcelain and stoneware). These are the ones that rely most heavily on the seven common and safe oxides.