This article tries to cover the steps involved in the coin minting process so that readers understand and appreciate the various steps and stages involved in the minting process. The process described is what is followed in current modern mints and does not discuss the older method of minting which although similar has its own intricacies.
Although the article focuses on coin manufacturing, the process is essentially same to manufacture Medals [Army/others], Silver coins with Jwellers, Metallic Keychains with shop name etc
The coin minting process begins with the process of creating a design. First a sketch is created on a paper often by an artist. The sketch may or may not have all the routine details that are typically present on the coin. From here Sculptors and engravers take over. They convert the 2-D design into a 3-D design. Often the material employed plaster or such materials that are no different from the ones that are used for creating a work of art. Typically the size of the plaster model is 10” to 16”. The design is always in relief [or positive] just like the one that appears on the coin. This plaster sculpture after finishing is then coated with epoxy resin. This acts as a hardener as well as a preservative. This is coated epoxy plaster sculpture is called “galvano”. This completes the design process.
There are 2 different models, one for the obverse and one for the reverse.
The next step is the Die making process.
As the Galvano is quite large, the first step is to reduce to the desired coin size.
The galvano of 10” to 15” dia is placed in a Transfer Reducing Machine on one side. The other side has a steel rod [circular bar] of the diameter of coin that would be minted. The machine based on fulcrum principal will carve out the steel rod with the design. This is also called reducing stage of die production. This steel piece is then passed through a tempering or annealing process. This hardens the steel. At the end of this we have a “Master Hub”. Typically there is only one single Master Hub for the entire set of coins that are minted for the particular design. It is to be noted that the master hub has same relief as the galvano, i.e. the image is like what is seen on the coin.
The next step is to create master die from the Master Hub by a process called hubbing. The Master Hub is placed in a Hydraulic hubbing press at one end. The other end has a steel rod of same diameter as of the coin and typically 4 inches long for grip. The two are then pressed together under pressure. This result in the master die having the impression of the Master Hub. However the image is Incuse [Mirror Image, Negative image]. As the Master die itself is of steel, a single press would not result in sufficient depth of design on the Master die. It would require the above step to be repeated several times as required to have the incuse of required depth. At every step, the Master Die is annealed so that it retains the required strength. If the entire process is attempted in one press by putting more pressure, it can weaken the Master Die and it could develop small cracks that would wear it out sooner.
Typically depending on the number of coins to be minted, the Master Dies created are 2 to 4. At times it can even be a single Master Die.
The next step is to create what are called as Working Hubs. The process followed to create the working hubs is same as that of the Master Die; except that one end of the hubbing press has Master Die. The design on Working Hubs will be in relief [Image as on Coin, positive].
The number of working hubs created is more than the Master Die, typically 4 to 10.
The next step is to create working dies. These are the dies that are actually used to strike the coins in the coin press. The process followed for created is same as that of Master Di; except that one end of the hubbing press has the Working Hubs. Just like the Master Die, the design on the Working Dies is of Incuse [Mirror image, Negative].
The number of working dies created is as per die wear and the number coins being stuck. On an average a working die can strike around 500,000 coins before wearing and needing replacement.
If there are multiple mints that are minting the coins, typically the Original plaster sculpture does not have mint marks or date on it. These are added on the working dies typically by hand punches. This is where some of errors like date doubling, double mint marks, and errors happen.
However if the number of coins being stuck are quite high, the mint marks and the dates are added to the Master Die.
At times if every mint is doing their own design [which is not often, as it’s expensive and results in small difference in design], the mint mark and date are also added to the plaster sculpture itself.
These correct sized blanks then undergo annealing and cleaning process. After this the blanks are fed into an upsetting machine. This machine creates the raised edges around the blank.
There are 2 purposes for having raised edges around the coin. During the striking process the edges help the metal flow into the die design rather than flow off. Plus the edges allow the coins to be stacked properly.
As a final step, these blanks are now passed through a furnace and given a chemical bath. This gives the coins a distinctive color or tone based on the metal used.
After this entire process is completed the blanks are now called as Planchets that are ready to be stuck.
The process actually produces the coins that are put in circulation. The striking chamber has 2 working dies. The reverse working die is placed in what is called as “Anvil die” and is stationary. The “Hammer die” has the Obverse working die. The striking chamber has a funnel through with coins are fed at the top. At the bottom of the funnel has feeder tubes at the end of which are 2 feeder fingers. These place the planchet on the anvil and eject the stuck coins. The fingers keep sliding over a smooth steel surface and keep moving forward and backward.
Once a coin is placed [it’s actually slide] on the anvil, a collar [metal ring] holds the coin in its position. The hammer now comes down with pressure and strikes the planchet with the desired force. This strike at high pressure causes the metal to flow and get the desired imprint of the coin. The collar also prevents the planchet to expand beyond the desired diameter of the coin. As a special feature, some collar will have reeds so that the planchets stuck will have a reeded edge. After the strike, the hammer retreats, the anvil movers slightly upwards vertically on the cam shaft. The feeding fingers then slide this coin out of the striking chamber, move still further and deposit another planchet. The anvil retracts back to its original position, the fingers slide back, and the hammer comes down again strikes another coin. This process continues.
Current machines are capable of minting around 120 coins per minute. There are some machines that can strike coins at even faster rates. There are striking machines that use one pair of dies or 2 pairs or even 4 pairs. IE in one strike, 4 hammer dies will strike 4 coins; the feeding fingers feed 4 planchets.
Once the coins are stuck, these are then passed through quality check. Typically whenever a new die is fitted in the striking chamber, the initial run is visually inspected to rule out any errors on dies.
The coins that come out of the striking chamber move on a conveyor belt into the counting and bagging area. The coins are then counted by machines and bagged accordingly. The bagged coins are also weighed to verify that the coins are of correct quantity and weight. As a quality control, the bagged coins are put into gunny bags with the official of the mint signing the bag. The bags are then dispatched to the central bank for further distribution to public.
The manufacturing of the Proof Coins is similar to the process described above. There are some additional steps:
Extremely high grade die blanks are used to create dies for Proof coins.
Die Selection & Preparation:
The dies are carefully chosen from the multiple working dies that are created; more so for lower denomination coins. Each die is then hand polished to give it perfect mirror finish. At times 3-4 rounds of polishing with different diamond paste is done so that the coin struck with this gets a highly mirror finish. The mirror finish was earlier followed, and today the finish is Frost finish, here after polishing the die, the entire die is covered with soft tape, only the key design is then exposed. The exposed area is sand blasted, so that when the coin is struck, the background gives a mirror finish and the design gives a frost finish.
Blanks Selection & prepration:
The blanks used for striking proof are polished so that they give a smooth and good finish.
The proof coins are hand struck, at times more than 3-4 times so as to give a sharp edge to the design. The pair of dies used is fresh pair of dies and hence the wear is not shown on the coins. After every 20-30 strikes, the dies are cleaned. Further the dies are continuously inspected for wear, minor imperfections are removed by polishing the die again as required.
Each coin is manually inspected for imperfections before packaging in the desired package and shipping.
Typically the rate of coin production for proof is less than 500 per day compared to the around 100 coins per minute in a fully automated circulation coin strike.
Typically each die lasts for around 500,000 strikes. The capacity of the press with a single die set is more than 120 coins per minute. So even if only one set of dies are used to strike a coin and the press is operated for just a day, we have around 60,000 coins. The number of commemorative coin collectors in India are estimated to be around 15,000 to 20,000 serious collectors and around 15,000 to 20,000 occasional collectors. Logically it should then follow that every collector should be able to lay his hands on any of the coins that he / she wish to collect. But we all know at some point in time the serious collectors has to buy them from dealers.