To what extent do the fumes from clay corrode the kiln?

The two elements in clay that are especially corrosive are fluorine and sulfur. Avoid clays with large amounts of these elements. Your clay supplier should be able to tell you how much of each is in your clay. Clays that are especially troublesome are the varieties that potters dig up themselves. If your clay contains these impurities, you should use a KilnVent, a downdraft type, to remove the fumes from the kiln. Make sure you have good airflow through the KilnVent. Keep the vent running until the kiln reaches room temperature. If you turn the vent off too soon, the gases will condense on the vent motor, causing excessive corrosion.