These Roman silver Antoninianii were struck using hand-held dies and hammers.
The earliest Roman coins and some Biblical issues were cast (molten metal poured into molds and allowed to harden). Ancient Roman coins were cast until about 211 B.C., when the method of production changed to hand-held dies and hammers. The design for one side of the coin (usually the obverse) was engraved into a metal disc or die which fit into an anvil, and the design for the reverse side was carved into the base of a metal punch. Though some dies were made of iron, engraving in iron was difficult, so dies made of bronze were far more prevalent.
The planchet is positioned on the anvil die with the punch positioned over it.
A planchet, or coin blank, was made by pouring molten metal into a mold, then allowing it to cool and harden. The blank was reheated to soften it for striking, then was placed on the anvil die with the punch positioned over it. With one or more sharp blows from a hammer, a coin was created...with varying degrees of quality...
If the temperature of the coin blank was right, if the dies were properly aligned, if the planchet was positioned in the center of the dies, and if the hammer found its mark with precision, the images would transfer cleanly and squarely from the dies to the coin blank. This hand-held technique often resulted in weak or off-center strikes, as well as misaligned strikes if more than one hammer blow was issued. Thus, nearly every ancient coin is slightly different from any other!
Mechanical coin presses were devised in the early 16th century and took two distinct forms. The roller press featurured two horizontal steel cylinders between which a strip of metal of uniform thickness would pass. Each cylinder featured engraved obverse or reverse designs, equally spaced around the roller circumference. Perfect alignment between the two cylinders was essential as the planchet metal moved between them.
Teamwork was required to operate the large screw presses which replaced the roller presses in the mints of the early 1800s. Four men spun two swinging, weighted arms in order to power the press. The coiner placed the blank in the correct position for striking and then removed it by hand – he had to be fast! Loss of fingers was an occupational hazard. Because coins made this way had no reeded or grained edges, clipping (shaving small pieces of metal from the coins) still occurred.
Steam presses, using lever power rather than hand power, were introduced at the Philadelphia Mint.
Hand-operated roller and screw presses were replaced during the Industrial Age with steam-powered coin presses, developed by Matthew Boulton and James Watt. These new presses used a "toggle joint" mechanism, yielding far greater pressure upon the planchets (coin blanks) than the screw press operated by humans or horses. The coin blank was now automatically placed in the center of the die where a collar impressed graining on the coin's edges. Not only did this invention enable mints to produce thousands of coins, but it cut down on the centuries-old problem of clipping. Used throughout most of the 19th century, these steam presses corresponded with the era of steamships, steam-driven locomotives, and even early steam-driven automobiles.
The Modern Age
The modern minting process requires the refinement of metal to specific proportions, rolling it into strips, and cutting coin blanks or planchets of precise size from the metal strips. The rolling process requires a process of heatings and coolings known as annealing which brings the metal stock to the consistency needed for shaping and stamping. Once the coin blanks are cut from the metal strips, they are polished and "pickled" in a centrifugal finishing machine that tumbles the planchets with stainless steel balls and special chemicals to burnish and brighten the surfaces in preparation for coining.
Coins are still struck from dies, which today are cylindrical pieces of hardened steel bearing the recessed design, date and other inscriptions upon one end. To make the dies, an artist first creates a large plaster model of the coin, from which a metal model called a galvano is made. The approximately 8-inch diameter galvano is attached to one spindle of a reducing lathe, while a steel cylinder the diameter of the actual coin – called the master hub – is attached to another spindle. As a spinning stylus reads the large galvano image, a spinning cutter device engraves the design onto the steel master hub. A number of "working hubs" with raised images and inscriptions impress blank steel cylinders with recessed designs – creating the "working dies" that actually strike the coins.
This planchet, when it leaves the coin press, will have become a Washington quarter.
The dies are mounted in an electric or hydraulic coin press. A coin blank or planchet is inserted upon the lower die, and the upper die is then forced against the planchet with many tons of pressure. As the coin blank is squeezed by the press, the metal "flows" outward to the collar or retaining die – which may have vertical grooves known as reeding – though some coins such as Presidential and Native American dollars have edge inscriptions. The metal also flows into the recessed design on the dies, creating a "raised" image on the coin.
Finished coins are mechanically sorted and counted, frequently inspected, and conveyed to bagging and rolling machines. Special collector issues, such as Proof coins, are hand fed into the coin press and hand removed, avoiding any contact with other coins, and specially packaged to protect their exceptional Proof finish.