Ammonia Alum Crystals are the hydrated form of potassium aluminum sulfate and have the chemical formula KAl(SO4)212H2O. Any substance having the empirical formula AB(SO4)212H2O are referred to as a alum. Alum occasionally appears in crystals, despite being frequently sold as a powder. Potassium alum, a fine white powder, are available for purchase with other seasonings for cooking or pickling. Ammonia Alum Crystals are also offered as a big crystal known as a deodorant rock for usage under the arms.