Is There Lead in Clay Cooking Pots

CLAY POTS ARE MADE OF, WAIT FOR IT, CLAY!

Clay is generally formed from the way-prehistoric decomposition of rocks, most notably granite.  (Way-prehistoric means 50 million, give or take, years ago.) Therefore it contains the various volcanic minerals that made up the granite.  Since lead was basically never in that volcanic source material, there is rarely a clay that contains lead.

PRIMARY CLAY and SECONDARY CLAY

Some clays are found where the rocks from which they are formed were originally deposited. These are primary clays.  If they are transported by wind or water, they are secondary clays.  Most clays used in making pottery are secondary clays. The clays that were not transported (primary clays)are generally light in color like porcelain or kaolin.

In being transported, the clays pick up other minerals, most typically silica (sand) and iron, and organic materials. (The organic material burns out during the firing process).  These transported clays are known as secondary clays. EVERY earthenware clay is secondary.  It is the iron that gives many clays a reddish color either before or after firing.

The best earthenware clays for making flameproof pots were formed with micaceous material in them.  Deposits of these micaceous clays are relatively limited. For non-micaceous earthenware used for pottery, they are usually a blend of one or two earthenware clays and silica and a melting agent.

CLAY, LIKE EVERYTHING, IS MADE UP OF CHEMICALS

Clay is generally pure, as mined from the earth, with a certain chemical and physical structure.  Stoneware and Flameware is a combination of natural clays, put together to give certain atttributes such as firing temperature, fired color, and handling characteristics.  There may be additional ingredients such as feldspars (another type of rock), to help them melt at achievable temperatures.  Clays almost never contain lead because the processes of formation were different. The clay itself is made up a wide variety of naturally occurring chemicals. If a pot isn’t glazed, it almost certainly does not contain lead.

SO, WHERE DOES THE LEAD COME IN?

Historically, lead has been used in glazes, because it can act as an active flux, melting the glaze ingredients together at a lower temperature. Thus, it was used to make shinier glazes and glazes with brighter colors.  I say was, because no reputable maker of glazed ceramic pots today would, or needs to, use lead to achieve the melting.

WHY JUST IN GLAZES?

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.

Glaze 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 (2100 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.

TESTING OLDER POTTERY

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. These kits are rated to be 90% accurate.  If you want to be doubly sure, test a second spot somewhere else on the pot.

Another way of testing glazes is to leave them overnight with a slice of lemon covering part of the color. This does NOT test for lead but 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. If the glaze is stable, and doesn’t leach, no chemical in that glaze can be freed from the glass matrix that is the glaze.

There is a earthenware website that proposes testing using baking soda.  This WILL NOT test for lead or stable glazes. It will only indicate that somewhere a metal is coming off the pot and flavoring the food. It is NOT a test that indicates anything dangerous about the pot.

This discussion is necessarily limited in scope. If you want to dig much more deeply there was a good article from Ceramics Monthly and another, more technical article on Digitalfire.com.

Another recommendation I would make is, if you are seriously worried about lead in your clay pots, be prepared to do some homework.  Otherwise, buy your pots only from a known manufacturer who you can ask about their work.

MORE INFORMATION

From a consumer standpoint, Paula Wolfert covers this topic very well in Mediterranean Clay Pot Cooking on pages xvii-xviii.
If you have any additional questions, please post them as a comment to this blog, or contact us at our email.