The Minting Process

Why study the minting process?

In the study of mint errors and varieties, and even with the “regular” segment of numismatics, a basic knowledge of the minting process is vital. If one does not understand how an error or variety occurred, they will not be able to determine if it is genuine. Therefore, knowledge of the minting process is vital to fully understand mint errors and varieties.

The P-D-S System

There are four basic areas of production in The United States Mint, and it is within these four areas that all errors and varieties occur. First is the design area, where the coin is designed and a model is engraved. Next is the die making process, where the design is transferred to a die steel to strike the coin. The production of planchets is the third area of production, where coin blanks are produced and transformed into planchets for striking. The last area of the process is the striking process, where the planchets are struck and made into coins. The last three areas; planchet, die, and striking are where errors occur. This is where the P-D-S system comes from, which makes remembering these areas of production easier. Also, remember the three primary minting facilities for the twentieth century; Philadelphia, Denver, and San Francisco. Die varieties can occur during the design and engraving area, and within the die making process.

Error versus variety

There is some disagreement as to whether a certain coin is an error or a variety. Generally speaking, most specialists consider an error as an occurrence which does not repeat exactly, and a variety is an occurrence which does repeat exactly. An off center strike therefore would be considered an error. True, some do look very similar, but generally speaking, each off center strike will be different. However, a doubled die will repeat exactly with each strike, and is considered to be a variety.

The design process

Sculptors and engravers are employed by the mint to design coins and medals, and to sculpt and engrave other designs into workable subjects for coining. These highly trained specialists take a design from a drawing, painting, or other two-dimensional object and transform the design onto a plaster model (approximately 15″ diameter) that will eventually be transferred to a coin or medal. The designs are always in relief, or positive, just as it will appear on the finished coin. This plaster sculpture, after slight changes and improvements, will be coated with epoxy resins to act as a preservative and a hardener. The epoxy coated plaster sculpture is now called a galvano, and is forwarded to the die making area of the mint.

The die making process

Since the galvano is usually many times larger than the actual coin to be produced (usually 15″ in diameter), this design must be reduced. To accomplish this, the galvano is placed onto a Janvier Transfer Reducing Machine. This machine will trace the design on the galvano, and using the principle of the fulcrum, will carve the design onto the end of a piece of steel bar the actual size of the coin to be produced. This is called the reducing stage of die production. This finished piece of steel is called the master hub (sometimes called hob). When the master hub has been produced, it will be heated to extreme temperatures, then quenched (cooled) quickly in a vat of oil. This heating and cooling process, called tempering or annealing will harden the steel even further.

The master hub will have the design in the same, relief configuration as on the galvano, and as it will appear on the finished coin. It will them be placed into a hydraulic hubbing press, opposite a piece of die steel which is about 4 inches long. When each is seated into the press, hydraulic force will bring the two together, transferring the image from the master hub onto the end of the die steel. When complete, this will be called the master die, with the design incused, (mirror image) of the finished coin.

The above operation, known as hubbing, will usually take several impressions to bring the design to the depth specifications required. After each hubbing, the die will be annealed (as described above) so that the steel will become even more hardened. If the die was to receive the image deep enough in the first hubbing, stress on the die steel would result, and very likely create cracks, or at best, would weaken the die. Strength and durability are stringent requirements.

After the master die is produced, the master die will then be placed into the hubbing press to create the working hub in the same manor. Several working hubs will be produced. These working hubs will then produce working dies, again in the same manor as described above. The working dies will then be placed into the coining presses to strike coins and medals.

All dies are made in the Philadelphia Mint. Until 1987, the dies were produced without mintmarks. The mintmarks were added by hand with punches, and then shipped to the branch minting facilities for production. However, beginning in 1987, the mintmark was added to the master die, and later (about 1990) added to the original plaster sculpture. Subsequently, the mintmark will be transferred to the master hub, and on down the die production chain. Beginning in 1996, the Denver Mint will produce dies for the Denver Mint and the San Francisco production facility.

Remember that the plaster sculptings, the galvano, master hubs and working hubs have the image of the coin in a positive, or just as the finished coin will appear. Master dies and working dies will have the image of the coin in negative, or a mirror image of the finished coin.

Planchet making

Cent planchets are primarily now made outside the mint, as is the metal for the other denominations. However, the process is much the same as when the Mint produced their own planchets and metal. Raw metal, after being melted, will be rolled into long sheets until it is the proper thickness for the intended coin. These long sheets are then coiled for storage, shipping, and eventual use. The coils will be fed into a blanking press, which is nothing more than a series of punches which will punch blanks out of the metal coils. Notice that the word blank was used instead of planchet. Technically speaking, a blank is a disc of metal which has not been prepared for striking, and a planchet is a blank which has been prepared for striking.

Once the blanks are produced, they will pass through what is known as a riddler, which will remove the imperfect blanks. This is done by passing the blanks over a three tier vibrating screen, with holes in the first tier slightly larger than the intended blank. The blanks of proper diameter or smaller will drop through, and blanks too large will be maintained on the upper tier and carried to a scrap bin. The second tier screen has holes slightly smaller than the proper size blank. Blanks which are too small will pass through these holes, and those will be carried away to a scrap bin. Those blanks remaining on the second tier are presumed to be of accurate size, and will be forwarded to the next process.

At this point, the blanks must be annealed and cleaned. The annealing process will soften the blanks to improve striking and reduce wear on the dies. After annealing, they are passed through a wash, to be cleaned. Once cleaned, they are fed through a dryer to remove any water or soap which might cause spots. At this point, the blanks are ready for the next process.

After the blanks have passed through the annealing and wash process, they are passed on to the upset mill. The upset mill is a machine with two primary components; a stationary die with “V” shaped grooves, and a rotating die in the center also with grooves. Blanks are fed into one end of the upset mill, fitting into the grooves. As the center die rotates, the blanks pass around between the outer stationary die and the inner rotating die. As the blanks go through the mill, the spacing between the two dies is gradually reduced, forcing the metal on the edge of the blank to be forced around the edges of the blank. When the disc of metal exits the upset mill, it will have raised metal all around the edge, and will be considered a planchet.

The purpose of the raised edges on planchets before striking is that it greatly helps in coin production. This helps to force the metal, during striking, toward the center of the coin, and then into the crevices of the die. Additionally, these edges or rim, help the coins stack neatly after they are struck.

The blanks are called “type I” blanks, and planchets are called “type II” planchets. There are times when these will be referred to as “type I” planchets and “type II” planchets. This will probably be the norm rather than the exception.
To accomplish this, the planchets are fed into a furnace. This furnace is much like a long dryer, with the planchets being fed in one end and tumble as they travel through the furnace. When the planchets exit the furnace, they are washed in a chemical bath and tumble dried, slowly. By the end of this process, most planchets made of nickel will have a yellowish tint prior to striking.


The striking process begins with the planchets being brought to the coining presses via overhead conveyors. These conveyers, with small bins, will deposit the planchets into a hopper above the coining presses. From this point, the planchets will be fed by gravity through feeder tubes down to the coining chamber (the area of the coining press where the striking takes place).
The planchets drop from the feeder tubes into feeder fingers. These feeder fingers are finger like metal slides, which will both eject a coin from the coining chamber, and deposit another planchet into the coining chamber. Feeder fingers have a slot in the end to push out the coin, and a hole a couple of inches back which will hold the planchet to be deposited into the chamber. These fingers will slide back and forth over a smooth steel surface.

The coining chamber consist primarily of the anvil die (lower die), the hammer die, the collar, and the feeder fingers. The anvil die, which is usually the reverse die, will basically only move when coins are being ejected and planchets are received. The hammer die, usually the obverse, will actually come down and strike the planchet. The collar is a metal ring which will retain the metal during the strike, preventing the metal from expanding outside the desired diameter. The collar will also serve as a third die, creating the reeding on coins with a reeded edge.
The planchet is deposited on the anvil die and within the collar by the feed fingers. As the feed fingers retract, the hammer die will come down and strike the planchet. As the hammer die retracts, the anvil die (riding on a cam) will rise, the feeder fingers will push the struck coin out of the chamber, continue forward, and deposit another planchet onto the anvil die. The feed fingers will retract, and the striking process begins again. Over the past few years, an additional set of riddlers have been installed after the coining operation to catch errors. This has greatly reduced the number of error coins escaping the Mint, and fewer for us to collect.

The coining presses being used today are the Bliss presses, capable of strikes up to 120 per minute, and the newer Schuler presses, capable of 720 strikes per minute. The Bliss presses are equipped with single dies, dual dies, and quad dies. All the Schuler presses to date have been equipped with single dies. Single dies means that there is one die pair in the press; dual dies means two die pair; and quad dies means there are four die pair in the press.

Counting and bagging

Once the coins are ejected from the coining press, they will fall down a chute, and will be carried to the counting room on conveyors. In the counting room, the coins will be counted, packaged and weighed. Some coins are bagged in the well known Mint sewn bags, and others (usually cents) will also be shipped in large totes to Federal Reserve banks. These totes will usually contain 1500 pounds of cents, within a heavy plastic liner.

The minting process as described above is abbreviated, and some minor changes have taken place over the past few years, but the basics are here for easy understanding.


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