Assaying is a method of chemical analysis for determining the presence and amount of noble metals (such as gold or platinum) in ores or alloys, used since the 2nd millennium BC. The sample is fused with a flux containing lead (II) oxide. This produces a lead button containing the noble metals, which is heated in oxygen to oxidize the lead and other impurities, leaving a bead of the noble metals which is weighed and separated chemically.