What About Lead in Pottery?
On a Barnes and Noble discussion today with Paula Wolfert about her new book “Mediterranean Clay Pot Cooking”, Judith asked: Question: I understand that lead in ceramic pots is cause for concern, but why was/is lead used in the first place? I’ve seen some references that it occurs more in bright colors, but why?
My answer was:
Lead is one of the fluxes that help clays and glazes melt into a glass-like substance. Historically lead compounds were used when the potters had no other fluxes available. They’ve never been added to the clays, but used to be frequently used in glazes. Today, for pottery made in, or imported into, the US, it is almost never present. Today we use other minerals such as calcium, soda and phosphorus as components with other minerals.
By the way, the greatest problem is when these pots are used with acid foods, like citrus, tomatoes or foods with high vinegar content, and then over a period of time.
From a consumer standpoint, Paula covers this topic very well in Mediterranean Clay Pot Cooking on page xvii-xviii.
Glazes are essentially ground rocks…ground silica, calcium carbonate (limestone), clay and other minerals. In simplest terms, there are 3 key components in any glaze, the glass former-usually silica, the stiffener – usually alumina from clay which gives the melted glass stiffness to keep it from running off the pot; and, a flux which causes the mixture to melt at a temperature to which we’re going to fire the pot. Fluxes today are usually minerals which contain sodium, calcium, potash, Lithium, talc or strontium.
These core components are combined with other minerals that affect the gloss, the opacity, the firing temperature and how the materials and colorants work together.
Coloration usually comes from the addition of various metal oxides such as iron, copper, cobalt and others. Traces of materials such as tin oxide will vary the colors as will the method of firing, thickness of the glaze application and clay body on which the glaze is applied.
Lead used to be used because it melted at low temperatures and would smooth low fire glazes out more quickly and had the effect of brightening the glaze colors. It was mostly used on low fire (1800 degF) clays in bright colored glazes.
In high-fired pottery, (2387 degF) lead has little or no effect so there is no real reason to use it. Indeed, it’s fluidity at high temperature could cause glaze defects and running off the pot.
Today, even in low-fired pottery lead is almost never used in glazes. It’s actually hard to find the materials.
If you have old pots which you’d like to use, but aren’t sure of, lead testing kits from the hardware store are quite accurate. Another way of testing glazes is to leave them overnight with a slice of lemon covering part of the color. If the glaze isn’t stable, you’ll see a lighter patch where the lemon was in the morning where it leached out the colorant. In general, the only foods that could cause this leaching are acidic…citrus juice, high vinegar foods, tomatoes, etc. And, unless these foods are in the pot for a long time, there is little leaching possible.
This discussion is necessarily limited in scope. If you want to dig much more deeply this paper by John Hesselberth and Ron Roy is a good start. They literally ‘wrote the book’ on glaze stability.
If you have any additionsl questions, please post them as a comment to this blog, or contact us at our email.
What About Lead in Pottery?