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Is Mexican Terra-cotta pottery lead-glazed? Yes. Does it leach? Yes.

This piece was bought in Sinaloa in 2020 (made in Puebla). By breaking it and refining shards I estimate the firing temperature around 1800F. This lead test procedure involves leaving white vinegar in the piece overnight, pouring some of that into a test tube, dipping a cotton swab into a reagent solution and then stirring the vinegar with it. The darkening of the color indicates the concentration of lead in the leachate. It has turned black! Yet a typical fritted lead bisilicate PbO:2SiO2 glaze (having 10-15% clay to suspend it) does not leach lead (when melted well). The very thin glaze application suggests potters were trying to save money. Frits are expensive so it seems likely they are using raw white or red lead powders. But they are not mixing enough silica to produce a stable lead silicate chemistry.

Yet this pottery is a tradition in Mexican culture (and elsewhere) and is used for food and liquid surfaces everywhere. There are manufacturers trying to make stoneware that retains the traditional terra cotta appearance, but people prefer this.

A lead-glazed handled cup sold in markets in Mexico

Related Pictures

What temperature is Mexican terra cotta pottery fired to?

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Two halves of a terra cotta bowl, one refired

This piece was made in Puebla, Mexico by a potter using traditional techniques and local clays. I have sawed it in half and refired one half at 1850F (Orton cone 04). That half (the right one) has not shrunk, which means it has not exceeded the temperature of the original firing (the piece was likely glazed in the dry state, not bisque fired). Notice how the refire has darkened the body color. This is probably happening because the lead glaze is maturing the surface of the clay (without the glaze it would take a much higher temperature to darken it to this extent, and, that would also entail shrinkage).

Ferro Frit 3602 melt flow over many temperatures

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A 9 gram ball of frit melts and flows downward more and more as temperature increases.

This demonstrates the amazing melt behaviour of lead-as-a-flux for ceramic glazes. Not only does it melt early, but it softens slowly over a 300F range of temperatures before it goes off the end of the runway on this GLFL test. Then, when fired 200F hotter than that, it remains a stable, clear and uncrazed glass. Beginning around 1750F, this becomes a transparent glaze, by itself.

Craft store selling traditional terra cotta ware in Mexico 2020

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Lead glaze terra cotta for sale in Mexico

This ware is used all over the city by street-side restaurants and food vendors. And routinely used in the house. It is all lead-glazed. They all know to handle it with care to minimize breakage. Surfaces and edges are rough, it is poorly finished but most people value the tradition enough to not even notice. Of course factory-made ware is much stronger and more functional, and cheaper. But at meals and occasions many seek opportunties to use this at the table.

Vitreous red body color with a clear glaze at cone 6. Impossible.

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Terra cotta bowls behind stoneware ones emulating their appearance

The two back pieces are traditional Mexican terra cotta ware. Lead glazed, porous, not durable at all. The front two pieces are stoneware, produced by two Mexican manufacturers to emulate the look of traditional ware. Both tried. Both failed. It is pretty well impossible to clear-glaze a mid temperature red burning stoneware and get a red color. The glaze fluxes the clay surface and turns the color to the more vitrified version of the body, which is always brown. The terra cotta is fired so far below the temperature at which the clay vitrifies to brown that it maintains the red body color. To get the color in the stoneware plate on the left they brushed on a red-stained engobe (you can see the brush strokes). This worked. And the glaze has fired crystal-clear (likely because they bisque fired it very high). But there is a problem. Although not crazed when purchased, it crazed badly after several months. The bowl on the right also has a beautiful transparent glaze and it has maintained crisp edges on the decoration (and with no micro-bubble clouding). But this is as dark as they could make the clay without having it turn brown under that clear glaze.

Vinegar dissolved the inside glaze and crazed the outside one!

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A terra cotta clear-glazed vessel with dissolved lead powder left behind on the surface

This is a piece of traditional Mexican lead-glazed terra cotta. It is fired at 1800-1850F. Upon leaving white vinegar in it overnight the entire inside surface was altered to the point it appeared the glaze had dissolved away! Then that vinegar migrated outward through the porous walls, came through the craze lines in the outer glaze (which were not visible before), and on evaporation left the lead behind in the crazing pattern! The water-logging of the piece expanded it thus crazing the outside glaze. Fritted lead glazes don't do this.

Lead bisilicate frit data sheet claims high resistance to leaching

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A lead bilicate data sheet

Notice it is specifically "formulated for use in pottery glazes". And that "Lead bisilicate is extremely resistant to leaching by dilute acids, including gastric juices, which reduces its toxicity and offers the maximum safety of any of the lead products." In many countries, the use of lead glazes is still considered normal and safe. There is zero use of lead in pottery glazes in North America. Is it possible that we have tarred all lead products with the same brush? Could we at least be using it on non-functional decorated surfaces of low-fire stoneware? This would drastically reduce the energy consumption of kilns.

Can terra cotta ware survive an open flame without cracking?

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It can if the flame is not too big! This is a roadside stand in Mexico in 2016. Each of these "cazuelas" (casseroles) have a small flame under them to keep the food inside warm. The pedestal is unglazed. The ware is thick and heavy. The casseroles are hand decorated with under-glaze slip colors with a very thin layer of lead glaze painted over (producing a terra sigilatta type appearance, but with brush stroke texture). These have been made and used here for hundreds of years. How can they not crack over an open flame? The flame is small and it is applied in the center of the bottom so that stresses are distributed radially and symmetrically. The ware is fired at what potters in Canada or the US would consider bisque temperature. It is porous, open and able to absorb the stresses. Some of them do have small cracks, but these seem to relieve stresses and prevent more. They know these pieces are not strong, so they treat them with care in heating, handling and washing. They know the glaze will leach lead or even dissolve if they put acidic liquids in them - so they don't do that.

Links

Hazards Lead Toxicology
Glossary Lead in Ceramic Glazes
Lead is a melter in ceramic glazes and performs exceptionally well and must be misused to be toxic. It is also now environmentally pervasive. It is toxic and cumulative at any level of exposure.

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