Do coins burn a hole in your pocket? At some point in their manufacturing process, they certainly could. Hard to believe, but thermal processing plays multiple roles in the manufacture of the coins we use every day.
The earliest coins may have been made during the 600s B.C. in Turkey. Many historians believe they were also invented independently in ancient China and India. Early colonists in this country used any foreign coin they could get. The most commonly used was the large Spanish dollar called “pieces of eight”. To make change, the coin would be chopped into eight pie-shaped pieces called bits. Two bits were worth a quarter of a dollar – hence the term still in use today.
Coins today have government-approved designs stamped in them much as they did in ancient times. U.S. coins must all contain the following words: “In God We Trust,” “United States of America” and “E Pluribus Unum.”
Each year 14 to 28 billion coins are manufactured in one of three U.S. mints across the country. As many as 80 million coins are produced in a single day with up to 750 produced on a given stamping machine every minute.
The manufacturing process is both complicated and interesting. It begins with a coin design and the transfer of that design to a large clay model about eight times as large as a finished coin. This image is cast in plaster, which is able to be finished in fine detail. A reducing lathe traces the plaster model, machining the now coin-sized design into a soft piece of metal called the master hub, which is a positive model of the coin. It takes at least 24 hours to complete this detailed machining operation.
The master hub is then hardened and tempered to the optimal combination of mechanical properties. Pressing the master hub into a blank die creates a negative impression, resulting in the master die. The master die is then used to create working hubs, which in turn create working dies. The working dies are used in the coin presses to stamp the designs into the coins.
When making working dies, the mint has found that they can prolong the life of the hubs and dies by using a lower amount of pressure in the hubbing press. Each time the die is compressed by the hub, it is work hardened. It is necessary to anneal dies before they can be reused. Annealing furnace atmosphere must be closely monitored to avoid carburization or decarburization of the die. The dies are then cooled at a controlled rate, including an oil bath quench. If they are not stored properly or not allowed to fully cool, the die will not work harden properly and will wear more quickly.
Before coins can be stamped, bars of the proper chemical composition are heated and rolled into strips the thickness of a coin. The coils received at the mint are 1 foot wide and 1,500 feet long. A machine punches out smooth disks, called blanks, from the strips. Blanks are coin-sized but have no design. After annealing at over 700°C (1300°F) and quenching, blanks are washed to remove the discoloration. Blanks are then fed into an upsetting machine that puts a raised rim around the edge of each one.
The highest point of the final design is lower than the rim, which is harder than the rest of the coin. The rimmed blank is now called a planchet. When a coin is struck, the planchet is not heated, even though it would be softer and more malleable if it were. The extra time and expense would prove too great for the mint.
Thermal processing is used throughout the coin manufacturing process for both the coin material and the hubs and dies used in the stamping process. As you can see, well-controlled thermal processing sure makes cents.