In metallurgy, the separation of gold and silver by chemical or electrochemical means. Gold and silver are often extracted together from the same ores or recovered as by-products from the extraction of other metals. A solid mixture of the two, known as bullion or doré, can be parted by boiling in nitric acid. The silver is dissolved as silver nitrate, leaving a gold residue that is filtered off and washed; silver is precipitated out of solution by adding ferrous sulfate. This is the traditional method used in assaying the content of gold and silver samples.