Sodium, potassium, magnesium sulfates can be found in many clays. These are soluble and often dealt with by the addition of barium carbonate to precipitate them. However, while the reaction that occurs produces insoluble chlorides of sodium, potassium, magnesium, these can have their own issues (eg. firing them generates of gases and fumes harmful to kiln refractories). Heavy clay industries can tolerate clays with higher sulfate contents, but other industries, such as tile, need lower contents).
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