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Making blanks

Making blanks for coins in homogeneous alloys, such as cupro-nickel and nickel-brass

The appropriate metals required for the alloy are added to a charging furnace where they are heated to temperatures of up to 850°C. A sample is taken from the melt and immediately analysed on an x-ray fluorescence spectrometer to ensure that the composition is within the permitted tolerance. The melt is then poured from the charging furnace into a holding furnace which sits just below. At the base of the holding furnace are a pair of water-cooled graphite dies. The metal is drawn out between these graphite dies in the form of a continuous strip which is wound into large coils weighing up to three tons.

Casting impurities are then removed from the surface of the strip in the scalping machine, where rotating blades shear off half a millimetre from the upper and lower faces. In this manner, the dull and grubby appearance of the strip is transformed to one that is clean and shiny.

The strip, usually cast around 16mm thick, then has to be rolled down to the correct thickness (4). Most of the work is done by the tandem mill, or heavy roller, which in two passes reduces the thickness of the strip to around 3mm. The process is completed on a finishing mill which in several passes reduces the strip to coin gauge.

Blank discs of metal of the appropriate size are then punched from the strip in the blanking press (5). The blanking plate, incorporating around 15-20 blank punches, goes up and down 4-5 times a second so a single blanking machine can produce up to 6000 blanks a minute.

Having become work hardened, the blanks then have to be annealed, or heat treated, in order to soften them (6). To achieve this, they are passed along a conveyor belt through an annealing furnace where they are heated at temperatures of up to 950°C.

The blanks have then to be cleaned in a pickling bath to remove any blemishes from the surface (7). Ball bearings are added to the blanks and, together, they are swirled around in the pickling bath in a solution of sulphuric acid, the abrasive action of the ball bearings and the solution serving to clean the surface of the blanks.

After a final wash and dry, the blanks are ready for striking.

Making blanks for plated-steel coins

Given that the Royal Mint is not equipped to make its own steel, large coils of the metal are bought in and blanks punched out in the normal way. The blanks are then sent to the plating lines where they can be plated with a thin layer of copper, nickel or brass. Each plating line is made up of a series of baths, consisting of cleaning stations, plating baths and rinses. The blanks are loaded into rotating barrels which pass from one bath to the next: in the cleaning stations the surfaces of the blanks are cleaned to ensure a good adhesion with the plate; in the plating baths the required metal is electroplated onto the surface of the blanks; and in the rinses the acids or alkalis used in the plating baths are washed away. After plating, the blanks are cleaned in a pickling bath in the manner described above.

Royal Mint Production Process