How Coins are Made

The various coining facilities of the United States Mint are factories, whose products happen to be coinage of the realm.

Like any metal product, coins don't "just happen." A number of intricate steps must be taken, from the preparation of the raw metal used in the coins to the striking of the coins. And before the coins can be struck, dies must be produced.

For a new coin, the design process begins with design renderings by artists for the Mint, who are members of the Mint engraving staff or contracted through the U.S. Mint's Artistic Infusion Program. Once a design for a coin is selected from all the provided sketches, a model is created in the Engraving Department at the Philadelphia Mint.

While some engravers may chose to engrave a model using traditional methods, working in modeling clay, much of the modeling and engraving is now accomplished digitally, using scanners, computers and a computer-controlled milling machine.

In the 21st century, the United States Mint has employed several techniques in designing and model making: the traditional approach, with its origins in the 19th century and earlier, and practiced by a skilled engraver and sculptor; and a modern approach harnessing the power of scanners, computers and lasers in which computer design skills are prized.

Some of the first official comments on the changes in the Engraving Department were made in testimony given before a House subcommittee July 19, 2006, by the deputy director of the Mint. He said: "We have introduced new technologies to improve our design capabilities. The old coin design method – a drawing by hand turned into a clay model followed by a plaster model to be traced and cut into steel – is being replaced with a digital design process – a computer drawing scanned into an engraving machine."

According to the deputy director, this scanning technique was used in making the hubs for the 2006 American Buffalo gold bullion coins, whose designs are based on the designs found on the 1913 Indian Head, Bison on Mound 5-cent coins. The original plasters for the earlier coin by James Earle Fraser were taken from Mint storage and digitally reproduced; new equipment, controlled by computer, cut a hub by laser.

The Engraving Department also used a computer system in designing the first Presidential dollar. The computer system enables Mint staff to design coins using such computer programs as Adobe Photoshop and Illustrator. Artists can create individual design element "layers." If a particular device needs to be revised, the changes can be made to that layer without the need to revise the entire design.

Whether a model is completely computer generated or modeled in clay and then scanned, the final sculpture is assembled and fine tuned by computer. Then, the master hub is cut by a computer numerical controlled (CNC) milling machine. As of 2008, the U.S. Mint had mothballed its Janvier engraving lathe, which had been installed at the Philadelphia Mint in 1907 and had been used ever since for engraving master hubs.

In recognition of the changes in the 21st century design techniques introduced, Mint officials have indicated the traditional terms for its staff coin and medal designers – engraver and sculptor-engraver – are becoming as obsolete as silver coins in circulation. The new name for coin and medal designers is "medallic artist." Mint medallic artists hired since 2005 and 2006 reportedly have backgrounds in computer design rather than traditional sculpturing and engraving.

Although the designing and hub production for such issues as the Presidential dollars and American Buffalo gold coins were largely done by computer, most of the coins discussed and priced in Coin World’s Coin Values were struck from dies produced in the traditional manner, as least as practiced since the late 19th century and early 20th century. Under this traditional process, a sculptor-engraver following a sketch made a model in plastilene, a modeling clay substance. The model was three to 12 times the size of the actual coin, depending on the denomination.

A hard model was then generated, either in metal by an electroplating method (the model made from this method is called a galvano) or, during the last decades of the 20th century, in epoxy.

The galvano or epoxy model was then mounted on a Janvier transfer engraving machine. This machine used a stylus to trace the exact details of the model, then reduced them through a ratio bar. At the opposite end of the machine, a needle-like carbide cutting tool cut the design into soft tool steel, producing a positive replica that Mint officials call a "reduction hub" and collectors call a "master hub."

Making dies and hubs

The master hub, no matter the method used to cut it, has the exact dimensions of the coin. It is then tempered and the steel hardened. The steel bar on which the master hub was engraved is trimmed and the shaft is turned to a specific shape. The master hub then is placed in a hydraulic press and slowly pressed into a blank piece of soft die steel called a "die blank," creating a negative replica called a master die.

Die blanks start as a cylindrical piece of steel with a cone-shaped face. A robotic arm loads each die blank into a polishing machine where the face of the blank is polished to a mirror-like finish. The cone shape of the face of the die blank facilitates the process of creating the design details. Design details are first formed in the center, where the metal of the die blank is the highest and thus comes into contact with the hub first.

The master hub is used to form a master die. (Using a single or master hub and die ensures that subsequent work hubs and dies are identical. Dies were made virtually by hand in the early years of the U.S. Mint, with inscriptions, dates and Mint marks punched into the work dies individually, resulting in dies that can be identified individually by the placement of the individual design elements.)

Multiple work hubs (the Mint's term) or working hubs (the hobby's term) are made from the master die in the hydraulic press in the same way as the original, master die was impressed. Work dies (those used to strike coins) are made from the work hubs in the same way. (A single master die can make multiple work hubs, each of which can be used to make multiple work dies.)

The U.S. Mint used a multiple hubbing process to make master dies, work hubs and work dies for much of its existence. The metal of the hub or die hardened before the design could be fully formed; the uncompleted die or hub had be heated to soften the metal, cooled and reinstalled in the hubbing press. Beginning in fiscal year 1986, the Philadelphia Mint began making master dies and work hubs using a single-squeeze hubbing process. The Denver Mint began making work dies using a single-squeeze press in 1996, upon the opening of a new die shop there. The Philadelphia Mint began using a single-squeeze operation for most dies in 1997. Half dollar and other larger denomination dies continued to be produced throughout 1998 on older equipment requiring multiple impressions of hub into die to fully form the image. The two Mint facilities began producing half dollar dies in a single hubbing operation in 1999. The dies for the 1999 Anthony dollars were made on the old multiple-hubbing press and required multiple hubbings.

The final hub or die needs to be tempered and hardened, and the shafts must be shaped on a lathe to permit their use in the presses.

The introduction of one design element to the die production process requires additional explanation. Mint marks have been added at various stages of the die-production process, depending on the era. Traditionally, beginning in the 1830s, Mint engravers placed the Mint mark by hand on each of the thousands of working dies. That is no longer the case. Mint marks on commemorative coins and Proof coins are placed at the initial modeling stage and have been since the mid- to late 1980s. For the circulating coins, beginning in 1990, the Mint began placing the Mint mark on the master die for the cent and 5-cent coin. The dime, quarter dollar and half dollar followed in 1991. Most recently, the Mint mark (if any) on a circulating coin is placed on the initial model.

All of these changes lessen the possibility of producing Mint mark errors and varieties.

Blanks and planchets

Modern U.S. coins have their beginnings in the private sector, where a number of companies process the raw metals from which coins are made, and produce some coinage blanks and planchets and all coils of strip metal the Mint purchases.

In preparing the raw metals used in coining, the coinage metals are assayed, mixed to the proper proportions, melted and formed into slabs that are heated and rolled to the correct thickness. For clad coinage, bonding operations are required to bond the two outer layers to the inner layer. The strip is then coiled and shipped to the Mint for blanking. The Mint once did all of its own metal processing, including melting, assaying and mixing different metals together to create alloys. The Mint produced its own strip metal as late as Fiscal Year 1982 at the Philadelphia Mint, but the operations were closed in Fiscal 1983.

Once coinage strip is rolled to the proper thickness, blanks are punched from it (both at the Mint and at the private suppliers, depending on the denomination). Blanks and planchets are the unstruck, circular pieces of metal that become coins when struck between the dies. Blanks and planchets represent the same product at different stages of production, although sometimes the terms "blank" and "planchet" are used interchangeably. Blanks are unfinished planchets that haven’t been through all of the processing steps necessary before they can be struck into coins. Once a blank has been through all of the processing steps, it becomes a planchet and is ready to be struck.

Blanks are produced on blanking presses, which are simply punch presses similar to those found in any machine shop. They have a bank of punches (or rams) that travel downward, just barely penetrating the strip of coinage metal. The blanks are partially sheared, partially torn from the strip each time the punches make their downward cycle and pass through the strip. At this stage, the blanks have rough edges where they were torn from the strip. Most of the rough edges (or burrs) are removed during succeeding operations. The blanks at this point are slightly larger than the finished coins.

From this point onward, through the counting and bagging operations that represent the final steps before the Mint ships the coins to the Federal Reserve or other recipient, all the steps are automated. The Mint uses conveyor belts for moving blanks, planchets and coins throughout most steps of the production process, except for Native American and Presidential dollars. (The description of the processes that immediately follow are those used for the blanks intended for the cent through half dollar denominations, and were for the Sacagawea dollars. Because Native American and Presidential dollars have lettered edges, the blanks for these dollar coins undergo slightly different processes on the way to being struck; those processes are discussed a few paragraphs later.)

Once punched from the strip, the blanks for every circulating coin but Native American and Presidential dollars must next be softened by being heated to controlled temperatures in a process called annealing. The blanks are heated in a rotating tubular furnace to about 1,500 degrees Fahrenheit, changing their crystal structure to a softer state. The annealing process prolongs the life of the coining dies by ensuring well-struck coins with lower striking pressures.

The annealing creates a grayish coloration on the planchets through oxidation, which must be removed. Following the annealing, the blanks are cooled in a "quench tank." From the tank, they are moved into a huge cylindrical tube called the whirlaway. The whirlaway is tilted at a 45-degree angle; blanks travel upward along the whirlaway toward the washer, while the excess liquid picked up in the quench tank is removed. The blanks are placed into washing machines similar to home washers, where they go through a series of cycles that soak and clean the blanks, according to the Mint. The agitation in the washing machines removes the gray oxides, tarnish, discoloration or contamination imparted during annealing. (Blanks for Native American and Presidential dollars are burnished in an extra step.)

After the blanks are removed from the washing machines, they are placed in a tube for drying, then moved to the final step that turns them into planchets. This next step is to give most blanks a slightly raised rim (or proto-rim). This is done in an upsetting mill.

The upsetting mill consists of a rotating wheel with V-shaped grooves on its edge. The grooved edge of the wheel fits into a curved section (or shoe) that has corresponding grooves. The distance between the wheel and the shoe gets progressively narrower so that, as the blank is rolled along the groove, it is compressed and a raised proto-rim is formed on both sides of the blank. This raised rim serves several purposes. It sizes and shapes the blank to lower the stress on the dies in the coining press and facilitates the formation of the rim on the coin. (Their status changes from blank to planchet with the addition of the proto-rim.)

The planchets are ready to be struck into coins.

For Native American and Presidential dollars, the annealing and upsetting steps occur at different points in the process than for blanks for the other coins. These dollar blanks receive a "hard upset"—that is, they are run through the upset mill to form the raised proto-rim without having been first softened by annealing. Mint officials indicate the hard upset for these dollar blanks is done before annealing so that the diameter of the blank does not increase during the upsetting step, which would affect application of the edge lettering after striking.

After annealing and being washed in a detergent solution and dried, the Native American and Presidential dollar coin planchets are burnished with steel pellets held inside a solution containing a brightening agent with anti-tarnishing properties. After drying, the brilliant planchets are ready to be struck on the coinage presses.

Striking the coins

Coining presses are designed for any denomination of coin. Dies and collars can be removed and new ones for a different denomination installed. Striking pressures are adjustable for the various denominations and metals. A circular piece of hardened steel forms the collar, which acts as the wall of the coining chamber. The dies impress the various designs and devices on the obverse and reverse for the coin while the collar forms the edge of the coin, flat and smooth on cents, 5-cent coins and Sacagawea/Presidential/Native American dollars, and reeded on the dime, quarter dollar and half dollar. The collar is mounted on springs that allow slight vertical movement. (Forming Native American and Presidential dollar edges requires a separate operation, described later.)

The principal coining press used by the U.S. Mint for striking circulating coinage is made by Schuler AG, a German firm. Each Schuler press uses a single pair of dies, mounted so that the face of each die is perpendicular to the floor. Planchets are fed between the dies by a gravity-fed mechanism and stand on edge during striking. Each press can strike about 750 coins per minute.

While dies are traditionally designated as obverse or reverse, they are also assigned the technical designations of hammer die or anvil die. The anvil die is the fixed die while the hammer die is the one that moves and thus generates the force that raises the design elements on the coins during striking. Either side can be the anvil die or hammer die.

Newly struck coins fall into a trap. Frequently, while a press is in operation, the press attendant will pick up a finished coin for inspection from the trap, to catch some of the remaining varieties and errors that are still produced. The inspector examines the coin under a magnifier to search for any defects made in the die during operation. If the coin passes inspection, the press operator pulls the trap’s lever, which dumps the coins onto a conveyor belt for transportation to the counting and bagging operations, except for the Native American and Presidential dollars.

When those dollars are struck, they have a plain edge. Finished, however, they bear edge inscriptions. The Mint adds these in a separate step following the striking of the coins.

The struck Native American and Presidential dollars, still with plain edges, are moved along a conveyor belt to an edge-lettering station. (The 2007 Presidential dollars were moved from the presses to the edge-inscription station in bins. Tens of thousands of George Washington dollars were released without lettered edges when the bins were moved directly from the presses to the counting-bagging station.)

At the edge-lettering equipment, the coins are vacuumed into an open cylinder that centrifugally forces each struck coin into an edge-lettering channel. On the inner side of the channel is a steel wheel spinning counterclockwise that contacts the plain edge of the struck coin. On the other side is a block of tooling steel that contains the raised edge lettering, formed by laser, in a groove that resembles a semicircle. The struck coins—traveling at the rate of 1,000 coins per minute—pass along the edge lettering segment where they are impressed with the edge inscriptions. The newly edge-lettered coins discharge into a cash box and then are transported to the counting-bagging operation.

Throughout the minting process, computers track such statistics as the productivity of each press operator, any repairs to a coining press, quantities of coins struck per press, plus installation, movement and destruction of the dies.

Once the coins reach the final station, they are counted automatically by machines, and are bagged or placed into shipping bins. The Mint now uses large "ballistic" bags rather than the traditional, smaller canvas bags used for more than a century. Counters atop the mechanism that dumps coins into the ballistic bags remove most out-of-specification coins (errors).

The ballistic bags are sealed and loaded onto pallets for shipment to the Federal Reserve Banks or the banks' contracted private money-handling firms for distribution into commerce.