Saltpeter ( Potassium nitrate ) is used in hoodoo for breaking jinxes, cleansing, and turning luck around. 

It's used in making self-lighting incense powders, and in rites of Purification and Protection. It is often mixed with two other minerals to make a "Three-Ingredient” Bath or Sprinkle.  It can be used with sulphur and charcoal in spells when gunpowder is not available

One popular combination uses a teaspoon of saltpeter, a cup of Salt, and a tablespoon of Powdered Blueing in a bath; wash downward only, to remove all evil influences. The same mixture can be used as dry granules to sprinkle around the house for Spiritual Cleaning.

In the folk magick of the American South, Saltpeter is used in cleansing baths and rituals.