The Plate of Brass is an inscribed brass artifact which has been attributed to the landing of Francis Drake on the California coast in 1579 A.D. In the present study the chemical abundances of the major and trace constituents in the Plate of Brass were quantitatively measured. These values were compared with measurements on both medieval and modern brasses and with the levels of impurities expected in brass-making processes from medieval times to the present. This study indicates that the Plate of Brass was made during the eighteenth to twentieth centuries and suggests that the most probable period was the early part of the twentieth or the latter part of the nineteenth century. See also AATA 15-810.