|Monthly Tech-Tip |
This piece was made in Puebla, Mexico by a potter using traditional techniques. 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).
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.
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.
-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, that will remove pebbles and organics from a slurry (40 or 50 mesh will remove more).
-Some bentonite, 1-2% additions can greatly improve plasticity of short clays.
-100 grams of barium carbonate (in case of soluble salts in their clay).
-100g 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.
-Tissue transfers, brushes, underglaze colors for decorating white engobed ware.
-Cone 06, 04 cones and supplies to make L4543 clay to make shelves, posts.
The act of interacting with locals to find all the other supplies and expertise you need will force you to learn Spanish better. After you develop a good procedure to make higher quality ware than they can, donate it all to a local potter.