Process & Production
In contrast to the modern computerised processes, the essence of coin making is entirely reliant on the skills of master craftsmen. The master engravers at St Paul's Mint combine their artistic flair in interpreting designs. The first step in the creation of a master die is three-dimensional modelling of the design in plaster. The plaster is scanned using a 3D scanner and refined using the latest 3D modelling packages. The artwork is reduced and cut into steel which is called a master die. This master die is then hardened and used to produce a positive tool called a hob. A working die is produced by pressing the master die design into another piece of soft steel using very high forces in a hydraulic press. The working dies are then machined to suit the coining press, and then hardened. Dies are textured and proof polished and finally, a plating process is used to deposit Chrome Nitride on the surface of the die to reduce wear in the coining process and to extend the life of the dies.
If the run is of a low volume of below 3,000 pieces, then the design can be cut directly into working dies which are finished in the same manner as a hobbed die.
Blanks are cut from coils of metal alloy, which usually consists of a mixture of base metals, the most common of which is copper. The composition of these alloys is carefully controlled. Blanks are usually round in shape but can be any shape - this is controlled by the punch and die. The coil of material is fed through the blanking machine where blanks are produced and taken away on a conveyer to keep the blanks surface as free from scratches as possible.
The majority of blanks produced have their edge rimmed. This is a process where the edge of the blank has been raised by rolling the blank through a specially shaped groove. The raised metal assists in the coining operation by partially forming the rim so that dies do not have to displace as much metal.
The rimming machine can also apply edge lettering or security marking to the edge of the coin blanks.
During the preparation of the blanks the action of the rolling and rimming operations creates a change in the condition of the metal. This effect is called Work Hardening. The blanks need to be softened slightly before they can be struck with the die. This softening process is carried out in a furnace by heating the blanks to around 850 Celsius and then cooling them again. After annealing, the blanks are then transferred to a burnishing unit.
Burnishing is used to make the surface of the blanks brighter, remove any discolouration and in some cases apply a small amount of lubricant to assist in coining. This is carried out in a Burnishing Machine that tumbles the blanks amongst a mixture of small balls and ceramic media combined with special chemicals that polish the surface. After burnishing, the blanks are dried using hot air. The annealed, burnished and dried blanks are stored ready for coining.
The prepared blanks can be either pre-plated with gold or silver, depending on the customer’s requirements, or can be plated after striking. The advantage of using a pre-plated blank is to maintain the frosting and polished texture on the dies.
Blanks are placed manually on the Coining press into carbide collars. The collar locates the blank prior to striking and controls the finished size and shape of the edge of the coin. The coining action is such that the blank is struck simultaneously with two dies and multiple times to produce the obverse and reverse designs. Once the coin is struck it is ejected and visually checked by the operator. The coin is placed on acid free tissue paper on a tray for further inspection and packing.
Inspection and packing
Packing is the last phase before a coin is ready to be sold. This is a manual process during which the packers do a final inspection of the coins as well as the actual packing.