An assay is a type of metal analysis used to measure the purity of a certain metal. Typically used to ensure that coins or bars produced by the refiner to reach the purity standards of the manufacturer. Once meeting the standards, i.e. .999 pure or .9999 pure, the metals can be stamped or poured into silver or gold bars and coins. Mints will send their products in individually sealed plastic assay cards to certify the assay is still accurate even after changing hands and sellers.