Definition

• Formulation.  Mixing of metals to form the exact alloy for a coinage composition. The procedure to prepare metal to be rolled into strip, blanked and struck into coins consists
  of three steps: learning the exact composition of metal to be used, weighing the metal and adding to it the proper metal to meet the standards of the coinage alloy. The makeup of the melt must be the exact composition of the intended coins. This is done by weight to make the alloy the correct percent of each metal.
  Metal arrives at mints in many forms: melted coins, salvaged metal, virgin metal, bullion ingots from various suppliers. Precious metal is tested (assayed) to ascertain its composition and purity, then weighted and mixed with other metal before being melted together. Coinage laws spell out the standard of this composition. It is seldom that a pure metal is ever used for coins (aluminum being an exception, but even this is often alloyed with manganese). Pure metal seldom has the wearing qualities required for a circulating coin.
  Precious metals are refined to 0.999+ purity before they are used in coinage alloys. Unrefined billion or native materials such as placer gold are not used for coinage in that form regardless of assay results. Trace amounts of impurities can adversely affect the working characteristics of gold and silver.
  Typically, gold and silver is alloyed with copper. Nickel (being very hard to strike in pure state) is also alloyed with copper to form copper nickel. For a bronze composition, copper is alloyed with zinc (formerly with tin and zinc). Thus most all coinage compositions are alloys. Formulation is the mixing of the metal – in pure state or already alloyed – to reach the required combination of metal components.
  As an example, when Great Britain was on the sterling standard of .925 silver, the formula was 37 fortieths of silver to copper. Nine parts of copper had to be added to 111 parts of silver to make the proper coin composition of .925.
  Salvaged metal can be in many forms. A large quantity is skeleton scrap
  from blanking (long strips cabbaged into a size for melting in furnaces). This is ideal since it is already of proper formula. But scrap or salvaged metal can be in sheet, bar, tube, wire, pellet or any fabricated form.
  Brass shell casings from World War II were salvaged for American bronze
  coinage metal from 1944 through 1947. This was melted and pure copper added to create normal bronze for cents.  Likewise coinage metal in the years following that war. Cannons have provided salvaged metal after previous wars. (The Paris Mint made note of salvaged Russian iron canons make into several of their coining presses.)
  Silver has been scraped from tableware, receptacles and silver objects of all kinds.
  Coinage silver, as another example, was made from melted firemen's horns from the Diligent Fire Engine Company #10 at the Philadelphia Mint in 1871 according to numismatic writer Walter Breen. Thus the source or physical form of metal is immaterial.
  All scrap metal must be tested for purity, the function of assaying for precious metals, refined and then formulated into the standard composition required for coinage strip to be used by a mint for coins. Such a strict metal standard is generally not a requirement for medals, typically struck in bronze or silver. Thus medals can be struck in bronze of most any formulation (see bronze chart) or silver of prescribed fineness from commercial metal suppliers.
  After formulation (in a mint's melt shop or the Melting and Refining Department of a U.S. Mint), the alloyed mixture, called a charge, is melted in furnaces, agitated into one homogeneous mass. It is treated with flux, then poured into ingot molds and rapidly  cooled. The bullion at this point may have physical imperfections, however a chemical imperfection – an inhomogenous mixture – is serious, as it may continue to exist throughout rolling, blanking and even in struck pieces. See silver  (Segregation).
  After formulation (in a mint's melt shop), the alloyed mixture, called a charge, is melted in furnaces, agitated into one homogeneous mass. It is treated with flux, then poured into ingot molds and allowed to cool. The bullion at this point may have physical imperfections, however a chemical imperfection – an inhomogenous mixture perhaps – is serious, as it may continue to exist throughout rolling, blanking and even in struck pieces.
  Incorrectly formulated bullion alloy is called bullion blunder. This occurs when an incorrect amount of metal was added during the makeup of the melt. Other than a possible discoloration of alloy, this anomaly is usually not evident but may only be determined by laboratory testing. Also the several metals may not have blended correctly to form a homogeneous alloy.  See inhomogeneity.
  Reference:                                                                                                                               
  NC8 {1988} Breen.

  excerpted with permission from

  An Encyclopedia of Coin and Medal Technology

  For Artists, Makers, Collectors and Curators

  COMPILED AND WRITTEN BY D. WAYNE JOHNSON

  Roger W. Burdette, Editor

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