The first U.S. Patent was issued to Samuel Hopkins on July 31, 1790 for his "Discovery in the making of Pot ash and Pearl ash by a new Apparatus and Process."
According to "The First Patent" by Henry M. Paynter,
"Potash is an impure form of potassium carbonate, mixed with other potassium salts. Until the 1860s it was derived solely from the ashes of hardwood trees and certain other plants. Pearl ash is a less impure form of potassium carbonate. Its name comes from its pearly color. Potash was a leading industrial alkali from antiquity until the close of the nineteenth century, when it was finally abandoned for most uses in favor of soda (sodium carbonate). . . .
Hopkins's key advance lay in burning the raw ashes in a furnace before they were dissolved in water. This second burning resulted in much greater carbonate formation, apparently because the free carbon in raw ashes (which partly accounts for the black color) was more completely oxidized and because of exposure to concentrated carbon dioxide gas from the fire. Hopkins also ncreased yields by mixing the insoluble residue from one batch with the raw ashes of the next, instead of simply discarding it. For a five-year license for a furnace using his process, Hopkins required a down payment of $50, or a half-ton of potash, and another $150, or a ton and a half of potash, over the next five years, payable to his agents in various cities.
For more on the history of patenting in the U.S. from Kenneth W. Dobyns, click here.