|Monthly Tech-Tip |
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.
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 ware is used all over the city by street-side restaurants and food vendors. They all know to handle it with care to prevent breakage. It is all lead-glazed.
Adobe brick clay is simply the local clay that is used to make the adobe bricks. Almost always red burning but not always terracotta or earthenware. These clays are almost always suitable for pottery (with more processing). Here is an adobe brick clay from Sinaloa, Mexico. Locals have been making adobe bricks from this for 500 years. Adobe clays are generally just terra cotta bodies. But this one is not. These bars are fired from cone 2 to 7 (bottom to top) or 1125-1225C. They would be firing them to less than 800C, far lower. This clay actually qualifies as a stoneware but they use it as an earthenware. This is not as bad as it sounds. At 800C almost all clays are highly porous and pretty soft - that is actually an advantage for the method of construction used (e.g. watery mortar, easy cutting, good adherence of plaster, insulating value). These same bricks are used to build the kilns and they service well since temperatures never get high enough that they warp or crack.
Evaluating a clay's suitability for use in pottery
Would you like to be able to use your own found-clays in your production? Follow me as we evaluate a mystery clay sample provided by a potter who wants to do this.