A Primer on How Coins are Made

Courtesy Peter Jones

In ancient times, engravers carved a dies of bronze and iron, hardened it, then used a hammer to strike the die on a planchet to make a coin. Engravers started using die punches around 800 CE to create uniform letters and save time making dies. By the mid 1400s their use was commonplace. By this time they also used punches with multiple letters or devices like someone's head (called a matrix or die punch), instead of re-engraving the same die multiple times. Hammered die coins were quick to produce. But the screw-press (first used in the London Mint in 1561 by Eloye Metrelle, a French moneyer) created much better looking coins. But it was more time consuming and they reverted to hammer-struck coins until Peter Blondeau (another French moneyer) restarted screw press coins in 1656 for Oliver Cromwell. By 1663, all British coins were struck with a screw press also called 'machine made,' as opposed to 'hammered'.

When the first US Mint started in 1793, they used a screw press to strike coins, and a Castaing edging machine (invented in 1679) to create edges with letting and devices before striking. Later in the 1790s, the US Mint used punches of heads, eagles, etc., so they did not have to engrave the same die repeatedly. In 1836 the Second Philadelphia Mint opened with a new steam press with a fixed collar. As coin production increased, it became too tedious to create each die individually and they used a system of:

WORKING HUB (relief) -> WORKING DIE (incuse) -> COIN (relief)

Initially the US Mint used the reducing lathe (Janvier lathe) of outside medal makers, then got their own machine in 1906.

Error coin collectors are well versed in the minting processing use a system called PDS: planchet, die, and striking. Each error e.g., a planchet clip, is unique and is not repeated. They involve the planchet or the striking process. Varieties are a result of die errors so are not unique e.g., 1955 double die cent.

Planchets

First planchets are punched out of correct thickness webs of metal. With increasing production in the 1990s, it became impractical to smelt metal in the middle of a city, so the mint then outsourced the production of planchets. Sometimes the planchet can be ripped along the edge with a curved, jagged, or straight clip. They may be the wrong metal (e.g. copper cent planchet struck with a dime design), or have inclusions, laminations and imperfections. Planchets are then put into an upsetting machine which raised the rims, making them into a 'Type 2 Planchet'. The 1836 steam press used a fixed collar to hold the planchet in place during striking, so edge designs had to be paced with a Castaing machine using edge dies for reeding, designs or lettering. In 1907, they started using segmented collar dies on the Saint Garden double eagle, allowing all-in-one striking of the obverse, reverse, and edge. Thus, planchets before 1907 required edging with designs before being struck by the steam press.

Dies

Dies are engraved, punched, or squeezed into softened steel, then hardened. You soften steel by heating it until red hot, keeping it there for a few minutes to an hour depending on the size, then cooling it slowly by burying it in sand or ashes. You harden steel by heating it until red hot then cooling it quickly (quenching) with water, oil or brine. A US quarter working hub produces 50-200 working dies, each of which can strike 100,000 to 200,000 coins (5 million to 40 million coins). But to produce 500,000,000 quarters, you need to add to the working hun and die system: MASTER HUB (in relief) -> MASTER DIE (incuse) -> WORKING HUB (relief) -> WORKING DIE (incuse) -> COIN

The master hub was made from a plaster or clay model, later a metallic galvano, using a reducing lathe. By 1991 all US dies for circulating coins had the mintmark included in the die, rather than punching a mint mark into each die. Also they had single squeeze from the hub not two. Thus, misaligned double squeezes of hubs into a working die (e.g., 1955 double die cent) ceased to exist.

Striking process

The striking process allows for the greatest number of errors (name of the error is in parentheses)

Planchet may not be restricted by the collar (broad strike)

Planchet may be half out of the striking area (10% to 90% off-center strike)

Planchet may stick to the die giving the next planchet a reverse on one side and a mirror reverse on the other side (brockage)

If that same planchet keeps sticking to one die, it may become cup shaped over time (die cap)

Same planchet may remain in the die but move (double strike)

Wrong planchet may be struck (planchet error), e.g., 1943 bronze cent was supposed to be steel planchet

Planchet may be poorly struck, caused by die trial, too thin planchet, insufficient pressure, or grease or tape in the way, etc.

Planchet may be struck with an inclusion between (inclusion), creating a divot in the coin, or (filled die error)

As die wears, strike may become mushy, but this is not considered a mint error

Die cracks may cause line of raised metal on the coin (die crack)

A bit of metal that falls off a die (cud on edge, or die break in center of coin)

As the coin is pushed off the die, (machine doubling) can cause appearance of double struck numbers or letters

As dies age and flow lines become too coarse, they may polish the surfaces, this made the 3-legged buffalo error

If no planchet comes between the dies, the dies imprint each other creating (die clash) errors e.g., bugs bunny Franklin half