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White lead, a powder when it is purchased for ceramics, is used to create a very shiny glaze. It has no color of its own, when fired. Now, clay pots, unless something extraordinary has occurred, such as the below-mentioned disposal of toxic waste, is toxin-free. It is simply ordinary sedimentary rock that has been decomposed by the action of water. In sandy areas, it is decomposed sand, or silica. Particle size is what separates clay from the rock, sand or subsoil that contains it. It is usually washed out of the material that it exists in. This is a simple but ugly process using hoses, pressurized water and settling ponds. Firing clay pots, which brings them up to about 1200 degrees Fahr, will also destroy "contaminants" of some sorts.