Medal Making Techniques and ProcessesPosted: January 25, 2015
Victorian period. Central to this was the growing discrepancy between striking and casting as primary methods of medal production.
The striking process involves mechanically pressing two dies, engraved with a design in negative, on to a disc of softer metal held between them. This enabled the mass manufacture of medals in the Royal Mint and other commercial mints, in the manner of coinage. In contrast, casting occurs in a foundry and is achieved by pouring molten metal into a cast taken from an artist’s model.
In response to the dominance of the struck medal throughout the 1800s, the late nineteenth century saw a revival of the casting process, whereby small editions of single-sided medals and larger medallions were produced. The resulting, softly modelled portraits stress the creative input of the artist. While struck medals were often commemorative, these examples were frequently conceived as independent works of art without a particular message or purpose.
– The National Portrait Gallery, London
Sculpting of the plaster model.
For the conventional model sculpting method the sculptor works from the design starting in plasticine or clay on a sheet of glass to build up the image of the design. This is called bas-relief sculpture. A plaster is then cast from the plasticine and then using special hand tools the sculptor further works the plaster model until complete. The sculpting process takes around 1 to 2 weeks depending upon the complexity of the design. One plaster model is required for each side of a medal.
From the plaster model a silicone rubber mold is cast then an epoxy resin model is cast from the rubber mold. This epoxy resin model is a copy of the plaster model only in a more durable medium required for the next stage.
Alternately with the design in mind the digital sculpting method may be engaged by the sculptor. This does not necessarily make the process any faster but allows better accuracy of designs that use logos, text and a lot of straight lines such as buildings. It also allows easier editing for any changes that may be required.
The reducing machine is used when a conventional physical model is created by the sculptor. The CNC engraving machine is used when the sculptor decides on the digital model option. It is also possible to have the conventional physical model traced by a laser to capture a digital profile as data that can then be sent to the CNC engraving machine to cut the master tool from.
The reducing machine and the CNC engraving machine are both unique engraving machines used specifically by the minting industry. The epoxy resin model is traced by the reducing machine and cuts as it traces. The CNC machine works from the digital model data. Over a period of several days the design is cut into special tool steel, first with a roughing cut and then a finer finishing cut to capture the detail that the sculptor has skillfully sculpted into the model. The copied design cut into steel is known as the reduction punch or hob.
The reduction punch is checked by the hand engraver to ensure that there are no imperfections from the machine. This task is performed under a microscope and requires great skill in using specialist hand tools. Once the engraver is satisfied with the finish the reduction punch is heat treated so that the steel is appropriately hardened for the next process known as hobbing.
Master and Production Tooling.
The hardened reduction punch is then placed into a hydraulic press for the hobbing process. The reduction punch is pressed into another piece of tool steel at incredible pressures of up to 800 tonnes to create a negative impression of the design. This may take several attempts to achieve the full size and detail of the medal or coin design with an annealing (softening) of the steel between each pressing as it work hardens as pressure is applied. This negative is called the die. The die is used to strike the medals after it has been machined in a lathe and heat treated (hardened), otherwise known as the production die.
For coins this die becomes the master die for safety and security purposes and the hobbing process is then repeated two more times. The first time to produce an additional positive image similar to the reduction punch although of better quality. This is called the working hob. The second and final time to produce the production dies or working dies.
Pressing of Medals.
Two production dies are prepared for fitting into a hydraulic press, one for the obverse and the other for the reverse of the medal. Appropriate to the metal required for the finished medals or coins blanks are punched from sheets of the metal, copper, bronze alloys, silver or gold. The blanks are placed between the dies in the press and contained by a collar. Then squeezed at pressures of around 300 to 600 tonnes. Depending upon the height of the relief in the design the medal may have to be struck several times with intermediate anneals (heating to soften) to achieve the full detail of relief.
Finishing of Medals
The medals are then finished accordingly. Some may have a patina applied to give an antique appearance and also to protect it from oxygen within the atmosphere or a frosted, satin finish. Some may be electroplated with gold or silver some may have a proof type finish which is a polished appearance straight from the dies during the pressing process.
Occasionally medals may have fittings applied to hang from ribbons to be worn around the recipients’ neck or pinned to their lapel. The medals are then packed into their presentation boxes. Presentation boxes come in a variety of differing styles from soft-bodied leather style cases to custom made wooden boxes.
Coinage or Coin Production Research
This research is sourced from a Telegraph article and the Royal Mint website.
Process can be broken down into 3 different steps:
- Making the blanks
- Making the dies
- Striking the coins
- Making the blanks:
Appropriate alloy made in a furnace and extracted into continuous strip, cut to produce massive coils.
Strip is rolled through a massive mill until the alloy strip is the required thickness of the coin.
Blank disks are then punched from the strip in a blanketing press.
Rolling the alloy work hardens the metal so the blanks are now annealed at 950C.
Blanks are now cleaned of blemishes in a special machine.
2. Making the Dies
Once a design is chosen, a plaster model is prepared at several times the diameter of the intended coin. The plaster model is scanned producing an stl. or other 3D file.
This file is used to engrave the design into a piece of steel at the correct size of the coin. This is known as a reduction punch. The reduction punch is then used to produce the dies which will actually strike the coins.
3. Striking the Coins
The final stage sees the blanks are fed into a coining press containing a pair of dies and special edge die pieces to press the circumference of the coin. Applying a pressure of around 60 tonnes, the dies strike the blank disk and turn them into coins.
If I had had a CNC workshop last week like I should have then this would have been a great application for that process. Although we do not have a metal CNC machine in uni I could have produced model-board dies as a proof of concept or prototype so as to show a possible coining method for making a medal.
One method that I could try from this research is to model in plaster disk several times larger than the actual medal and then 3-D scan it to edit it on Rhino. I have a 3-D scanning workshop next Tuesday with Ingrid, it would be good to have a piece of plaster work to scan and experiment with. This scan could then be used to produce a 3-D print which can then be used to make a mould or maybe even be used in direct burn out? To do this I will need to do some more research and talk to Dallas.