Craft store selling traditional terra cotta ware in Mexico 2020

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. That glaze does not craze and thus seals the otherwise porous surface against bacterial growth. 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 opportunities to use this at the table and show it off to their friends.

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Make pottery in Mexico. What should you take?

These bars show a clay from St. Ignacio, Sinaloa, fired at cones 2, 4, 5, 6 and 8. Although it appears this would make a nice red and even metallic stoneware, all have very high porosity. LOI is high, impurity particles gas and cause glaze blisters and pinholes at stoneware temperatures. Better to do what Mexican potters do, focus on low temperatures where glazes fire with fewer defects.
Here are some suggestions:
-Plaster is available everywhere there so make yourself a plaster table.
-If you can bring a 3D printer you can make molds and really impress them.
-They know how to make burner systems, get their help to make a simple kiln.
Bring the following (for materials bring the SDS to show to security or customs officials):
-Tools needed to characterize their clays.
-A propeller mixer to make slurries and grind clay (or purchase a kitchen blender to do blender mixing).
-A 30 mesh Tyler sieve will remove pebbles and organics from a slurry, 50 mesh will remove grit, 80 mesh will remove sand, 200 mesh will produce porcelain smoothness. You'll need to make a vibrating sieve.
-Some bentonite, 1-2% additions can greatly improve plasticity of short clays.
-250g of barium carbonate (in case of soluble salts in their clay).
-250g of Veegum and CMC gum to mix brushing glazes.
-Supplies to make variations of G1916Q base clear glaze and L3685Z3 white engobe. Bring Zircopax to opacify and stains to color the glaze.
-If you need to build a kiln: Sodium silicate to make kiln mortar (likely available locally).
-Tissue transfers, brushes, underglaze colors for decorating white engobed ware.
-Cone 06, 04 cones and supplies to make L4543 clay to make shelves, posts.
Interacting with locals to find supplies and expertise will force you to learn Spanish better. You will find that Latin and South American countries have a much deeper ceramic tradition than we do, so you will discover many new things.

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

A lead-glazed handled cup sold in markets in Mexico

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.

Can terra cotta ware survive an open flame without cracking?

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

Glossary	Terra Cotta A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.
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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