The third type of leavening, pearlash, was the precursor to modern baking powder. Pearlash was a purified form of potash. It was first used as a leavening agent by Native Americans and was the subject of the first patent in the United States, issued in April 1790. Its preparation was time-consuming, but could be accomplished by a housewife with a cast-iron kettle: it involved soaking fireplace ashes in water to make lye, and then boiling the lye to remove water and obtain "salts".