Traditionally an artist will first make a large model for the coin in clay or wax of which a plaster model will be cast. With a so called pantograph the plaster model will be ´felt´ and engraved into the stamp in miniature.
Nowadays most designs are being created on computers. In principle the design can be milled into the steel in one go with a computer guided milling machine. For more complicated designs however it is better to mill some layers ito the large templates first. Just as with the cast models the templates are being engraved into the steel with the pantograph. This has been the case with the background pattern of the border text 'ARS PECUNIAE MAGISTRA' on our coins.
In this background the 3D model can be milled layer after layer.
The stamp is a negative – so left will be right and the deeper the relief, the higher the coin. The trick is of course to create a realistic 3D effect in a 0.3 mm depth. This effect depends on the design but alsof for an important part on the engraver. He or she has to estimate how deep which part of the design will ultimately be placed.
After this computer guided milling work the image has been put into the steel. In the image however all the routes of the mill are still visible and the engraver has to eradicate them by hand with all kinds of precision instruments. Here the master’s hand is needed, as the engraver has to interpret every arch and depth correctly. At the same time he adds detail.
Master engraver Lei Lennaerts of Venrooy Goud en Zilverindustrie has made the mint side of the coin. This will become the standard ‘back side’ of our coin – each time the other side will be designed by a different artist. In the cartouche between the press’s flywheels there is room for the coin’s unique series and piece number.
Next the steel stamp has to be ‘hardened’. That is: heated to a temperature of app. 1200 degrees Celcius and then quickly cooled down in oil. This changes the internal chrystal structure and makes the steel proverbially ‘hard as steel’.
As this process makes the steel very brittle at the same time, it has to be ‘let out’ afterwards. By heating the stamp anew to some lower temperature and then letting it cool down slowly. Only then the stamp is hard as well as durable and can be used to mint coins.
More information about engraving see van Lei Lennaerts’ website.