How Coins are Made
The initial steps of coin production in which a design is brought from concept to engraving have undergone a huge transformation in recent years. Driven by technological advances, most coin design and engraving is now performed using sophisticated design software, scanners, and computer controlled milling machinery.
Of course most of the coins you are likely to collect were designed through very different means. Throughout much of the 19th and 20th centuries, engravers worked from artist sketches to produce models in clay. In order for the engraver to achieve the desired level of detail, these models would be much larger than the final coin. From the model, the engraver would progress through a series of castings refining the image and adding inscriptions. Finally a transfer engraving machine would reduce the image to its final size.
Regardless of whether the design is rendered using modern technologies or traditional engraving techniques, the end result is a device called the master hub. The master hub is an exact rendition of the target coin in hardened steel. The master hub will be used to create a master die, in which the coin design is in negative relief. From the master die any number of working hubs and then working dies will be fabricated.
It is from the working dies that coins will actually be struck. There are actually two working dies, an obverse and a reverse, for each coin. The die pairs are mounted in a coin press.
Of interest to collectors regarding the evolution of coin design methods is the relationship of methodology to errors and varieties. For example, in the United States the practice of adding mint marks to coins has changed over the years. In the earliest days mint marks were added manually to each individual die. Over time, the Mint began incorporating mint marks in the master die and today the marks are part of the original design. As the process for adding the marks changed over time, so have the types and frequency of mint mark variations and errors.
In the United States, metal for coin production is procured from private contractors. For the circulating coinage (except for cents) strips of metal about 13 inches wide, 1,500 feet long, and the thickness of a coin are supplied. These strips are feed through a metal punching machine called a blanking press which stamps out blank disks the size of the target coin called a blank. For cents, the Mint buys metal already stamped into blanks.
Modern United States circulating coins are made using clad metal, in which a core of one type of metal is sandwiched between two layers of another metal. Since 1965, dimes, quarters, half dollars and dollars have been made from a copper core between two layers of copper-nickel. Cents are made from a zinc core clad with copper.
Once punched, blanks go through a series of processing steps to prepare them to be struck into a coin. The most important and universal preparation is a process called annealing. In annealing, the blank is heated in a furnace to about 1,500 degrees Fahrenheit to soften the metal and make in more workable. Other steps and the order in which they are performed vary by the different coins in production, but in general the blanks need to have their rough edges smoothed, misshapen blanks need to be culled from the batch, and blanks for most coins will be put through a machine called an upsetting mill which gives it a raised rim called the proto-rim. Once a blank has been through all the necessary preparations for minting it is called a planchet.
Having been properly prepared, planchets are fed into the coining press. The planchets first fall by gravity into a tube attached to the press and then are fed one at a time into the coining chamber where the obverse and reverse dies have been set. At this point the dies are also referred to as the hammer and anvil dies based on their respective roles in the striking process; the anvil die is stationary, and the hammer die makes the actual strike. A modern coin press is capable of striking upwards of 800 coins per minute.
The finished coins are ejected from the press and continue along the process. They are inspected for quality, counted, and finally bagged for distribution to banks where they will be placed into circulation.