Tim Powers Here is my complete natural clay processing guide I put together.
The natural clay that I use is dug up from my property here at my home. I add water and 2 cups of white vinegar to dissolve calcium and leave it I buckets for several years to grow bacteria, mixing it every few months. When needed I mix it and run it through a screen to remove rocks and organic material. Then I place it on a plaster bat for several days to remove the excess water. Then I wedge it and store it to use as needed. When I fire it, it’s first bisque fired to cone 06. (1830 degrees). Then decorated with glaze and fired again to cone 5 (2185 degrees). It begins to melt around cone 7. It’s a beautiful terra cotta color when bisque fired and an antique copper color when glaze fired. To increase plasticity in this Clay add:
Adding about 5 oz of Epsom salts (magnesium sulfate) per 100 lbs of dry clay formula (approximately 0.3% of the formula) can increase plasticity on another level. Epsom salts increase the attraction of clay platelets in the moist clay state and cause the clay to become flocculated. Clay platelets are then drawn together, similar to the way in which the north and south poles of a magnet are attracted and bind together. The overall effect is a tight, plastic clay body with good throwing properties. Each clay body will require a slightly different percentage of Epsom salts due to the individual materials in the formula. Too high a percentage of Epsom salts can cause salt migration to the drying clay surface, which can result in blistering and carbon being trapped in the clay. Adding the Epsom salts to the water and then adding that mixture to the clay mixing operation can help ensure that the salts are dispersed evenly throughout the batch.
Mold growth in the moist clay can increase plasticity since it increases the binding action or attraction of the clay platelets. When mixing a 100-lb batch of clay formula, add 1/2 cup of beer, coffee or apple cider vinegar, or 3 oz of yeast, to the water before adding the water to the clay. All of these agents will start mold growth in the clay/water mixture. Moist clay kept in a warm, dark place for several days can also be the perfect medium to grow mold.