Coining is a form of precision stamping in which a workpiece is subjected to a sufficiently high stress to induce plastic flow on the surface of the material. A beneficial feature is that in some metals, the plastic flow reduces surface grains size, work hardening the surface, while the material deeper in the part retains its toughness and ductility.
Coining is used to manufacture parts for all industries and is commonly used when high relief or very fine features are required.
Prior to the modern era, coin dies were manufactured individually by hand by artisans known as celators. In demanding times, such as the crisis of the Roman Empire in the 3rd century, dies were still used even when they became very worn or even when they cracked. The die that was on the hammer side, usually the reverse (back), tended to wear out first. The flans were usually hot prior to striking. On some Roman provincial coins, the tongs used to move the heated flan sometimes left permanent center indentations on the finished coins.
Medieval coin dies were created in mints by guild members known as engravers.
The vast majority of medieval coins were cold struck; the planchets were not heated. While medieval coin dies were largely made of iron, some dies have been discovered with a small region at the face of the die which is made of steel. As technology and the economy changed over the course of the Middle Ages, so did the techniques used to create coin dies. While most ancient coin dies used engraving very heavily, early medieval coinage was dominated by dies created mostly from punches, which displace the metal of the die instead of removing it. There is evidence of medieval die cutters using engraving tools to lay out designs, and to create detailed punches. However, engraving on the face of the die did not become commonplace until the early Renaissance.
Very detailed records exist for the venetian mint. In the late Middle Ages, the dies used to create tornesellos lasted as follows: "hammer" die, ~17,000 strikes; "anvil" die, 36,000 strikes. The mint made an average of 20,000 coins per day, so they were making one hammer die a day and one anvil die every other day! The "hammer" dies wore out quicker because they tended to be smaller and were hit directly with a hammer, leading to severe mushrooming on the tops.
The process of making dies to strike coins in today's mint has quite a few steps. First, an artist creates a large plaster model of the coin. The plaster model is then coated with rubber. The rubber mold is then used to make an epoxy galvano. All of this takes place on a scale of around eight inches. Next, a Janvier reducing lathe takes several days to reduce the image onto a steel master hub in a process that has not changed in over a hundred years. The master hub is then tempered to make it hard. A small number of master dies (incuse) are then made from the master hub. These are then used to make working hubs. The working hubs are then used to make working dies. With each step, the number goes up. The working dies are then used to strike coins. All dies are incuse, and all hubs look like the coin being struck (with the devices raised.)
The final step is that the dies are used to strike images onto the planchet so that it becomes a coin.
Mistakes can happen at any stage of this manufacturing process, and these mistakes are something that certain collectors look for. Coin errors that occur on the die are generally more desirable than errors made at the time of the strike. For example, a doubled die, where a date or another device appears twice slightly offset, is often a highly desired error. Strike errors are generally unique, whereas all coins struck with an error die will have the same characteristic. This makes them more easily collectible. The most famous doubled die in the past hundred years is the 1955 doubled die Lincoln cent. These trade for hundreds of dollars because the error can easily be seen by a casual observer. Many doubled die errors require at least a jeweler's loupe (if not a healthy imagination) to be seen. Doubling can occur at the hub stage as well. Some more recent errors are hub doubled. Most famously, there is a 1995 doubled die cent that is hub doubled.
Since coin production in the United States has exceeded 20 billion coins in some recent years, this means that a lot of dies must be manufactured as well.
On the edge of the US dime, quarter and half dollar, and many world coins there are ridges, similar to knurling, called reeds. Some older US coins, and many world coins have other designs on the edge of the coin. Sometimes these are simple designs like vines, more complex bar patterns or perhaps a phrase. These kinds of designs are imparted into the coin through a third die called a collar. The collar is the final size of the coin, and the planchet expands to fill the collar when struck. When the collar is missing, it results in a type of error called a broadstrike. A broadstruck coin is generally a bit flatter and quite a bit bigger around than the regular non-error coin of the same denomination.