Mint-made errors
Mint-made errors occur when coins are made incorrectly at the mint, including anything that happens to the coin up until the completion of the minting process. Mint error coins can be the result of deterioration of the minting equipment, accidents or malfunctions during the minting process, or interventions by mint personnel. Coins are inspected during production and errors are typically caught. However, some are inadvertently released into circulation. Modern production methods eliminate many errors, and automated counters are effective at removing error coins. Damage occurring later may sometimes resemble true mint errors. Error coins may be of value to collectors depending on the rarity and condition. Some coin collectors specialize in error coins.
Errors can be the result of defective planchets, defective dies or the result of mistakes made during striking. The planchet, die, and striking classification system happens to correspond with the mintmarks of the three largest U.S. mints, Philadelphia, Denver, and San Francisco. Some errors have multiple causes and not all errors fall neatly within the categories. For example, design elements may be missing from coins because die crevices were filled with grease a problem with the die but the error occurs when the coin is struck. Labels used to identify specific categories of errors may describe the cause of the error, the appearance of the coin or other factors. Some errors are known by multiple names, e.g. filled die errors are also known as missing design element errors and as strike throughs.
Some errors, such as an off-center strike, are unique. Other errors, such as those resulting from a specific die crack, form a variety, i.e., a group of coins with distinctive details or characteristics. Uniqueness does not necessarily make an error coin valuable. Although no other coin may be the same as a coin with a particular off-center strike, off-center strikes of varying degrees are not extremely rare. Accidental error coins are perhaps the most numerous, although in modern minting they are rare, making them potentially valuable to collectors. Intentional intervention by mint personnel does not typically involve a deliberate attempt to create an error, but usually involves an action intended to improve quality that miscarries.Planchet preparation errors
Mints purchase long strips of metal which are fed through blanking machines that punch out disks known as blank planchets on which coins are struck. This determines the size and shape of eventual coins.Blank planchet
The punched disks are first known as "Type-1" blanks. After an upending mill adds uniform rounded rims, the disks are called "Type-2" blanks. Occasionally, Type-1 and Type-2 blanks aren't further processed, ‘escape’ the mint facility and enter circulation. Type-2 blanks may also be considered striking errors as they are prepared correctly, but are released without having been struck.Clipped planchet
A misfeed can occur when the metal strip is fed through the blanking machine. The punches sometimes overlap the leading edge of the metal producing a straight clip. Sometimes, the punches strike an area of the strip which overlaps the hole left by the previous strike producing a curved clip. On such curved-clip coins, often the rim opposite the clip shows a distinctive distortion and loss of detail called Blakesley Effect. Sometimes punches strike the irregular trailing edge of the metal strip producing irregular clips.Improper planchet thickness
Coins are sometimes struck on planchets that are either too thin or too thick producing underweight or overweight coins. This error can be due to incorrect equipment settings causing the metal strip to be rolled to the wrong thickness or due to the use of a metal strip intended for another coin denomination, such as a U.S. quarter planchet cut from a metal roll intended for dimes.Lamination flaw
A lamination flaw is a planchet defect that results from metal impurities or internal stresses. Lamination flaws cause discoloration, uneven surfaces, peeling, and splitting.Split planchet
A split planchet coin error occurs when, during the preparation of the planchet strip, impurities such as gas, dirt, or grease become trapped under the surface of the metal blank, creating a weakness or lamination defect. This weak area of the metal may flake, peel, or split because the adhesion is poor.
Split planchet errors are normally restricted to planchets composed of a solid alloy, such as U.S. cents and nickels, and the Australian fifty-cent coin. Split planchet errors should not be confused with "separation errors", which only affect clad and plated coins. Separation errors are bonding errors, not alloy errors.
A split can occur either before or after the coin is struck. The descriptive terms split before strike and split after strike are used to distinguish the respective types. A "split before strike" will show design on both sides of the coin, have coarse to fine striations, and will usually be weakly struck. A "split after strike" will show a normal strike on one side, but will have a rough, design-free surface on the other side and will always weigh less than a normal planchet.Cladding flaw
Many modern coins are made of layers of different metals known as clads. These cladding layers sometimes peel, fold, or completely separate.Hub and die errors
Mints use hubs bearing raised images similar to the images that appear on a coin to imprint indented images onto the ends of steel rods. Those rods become the dies which strike planchets making them into coins. Hub and die errors can occur at the time the dies are made, when the dies are installed into presses, and from die deterioration during use. Modern coins can still be released with hub and die errors when the defects are too small to be seen with the naked eye. Sometimes, dies are used despite producing obvious flaws, such as the 1955 U.S. Lincoln cent.Fundamental die-setting error
A fundamental die-setting error occurs when the die is not set as the producers intended. For example, in April 2013 the Central Bank of Ireland issued a silver €10 commemorative coin in honor of James Joyce that misquoted a famous line from his masterwork Ulysses despite being warned on at least two occasions by the Department of Finance over difficulties with copyright and design.Missing design elements
Missing mintmarks, dates, and other design elements occur as the result of errors in the die or at the time of striking. A design element that is missing from the die when it is made is a fundamental error. Missing design elements that occur because dies are tilted and do not strike the planchet face-on are known as misaligned dies. A design element may be missing because foreign matter, such as grease, plugs the cavity into which the planchet's metal would normally flow under the striking pressure. This error is also known as a filled die or a strike through. Although this does involve a die, it is typically thought of as a striking error, but is included here for completeness. This type of error led to some specimens of the 1999 release of the US 50 State quarters design for Delaware to be missing the final E from the state's motto, thus accidentally declaring Delaware "THE FIRST STAT."Doubled die
A doubled die occurs when a die receives an additional, misaligned impression from the hub. Overdate coins such as the 1942/1 U.S. Mercury dime and 1918/7 U.S. buffalo nickel are also doubled dies. They are both listed by CONECA as class III doubled dies. Class III means the die was hubbed with different "designs". They are not repunched dates, since the dates were punched onto the hub. Die deterioration may also appear as doubling.Die defects: cracks, breaks, and chips
can crack during use producing jagged, raised lines on the surface of subsequently struck coins. In U.S. coinage, many Morgan dollar coins show slight die cracks. Dies with cracks, especially those with cracks near the edge, sometimes break. The broken piece may be retained in position or fall away. Die cracks and retained die breaks can be difficult to distinguish. Retained die breaks cross the coin's face from rim to rim with the area to one side of the break being slightly higher than the other. Coins struck after the break falls away have a raised, rounded, unstruck area along the edge. These coins are known to collectors as cuds. Sometimes, an area of a die will chip out of the center. These so-called die chips also appear as raised, rounded, unstruck areas on subsequently struck coins.Die clash
A die clash occurs when the obverse and reverse dies are damaged upon striking each other without a planchet between them. Due to the tremendous pressure used, parts of the image of one die may be impressed on the other. Planchets subsequently struck by the clashed dies receive the distorted image. A well-known example is the "Bugs Bunny" Franklin half dollar of 1955, where part of the eagle's wing from the reverse gives Franklin the image of protruding teeth.MAD clash
A MAD clash occurs when an obverse and reverse die strike each other while misaligned in relation to each other. Additional misalignment errors are discussed below.Die setting errors
Historically, some design elements near the outer perimeter of a die were added by individual iron punches because technology made it difficult to press both the central and perimeter design elements at the same time. Also, some dies were made without mintmarks or dates to permit their use at different mints. A die technician added missing elements by positioning a puncheon, a small steel rod with the mirror image of a letter or number on it, and striking the puncheon with a hammer pressing the image into the die. If the image is not strong enough, the technician will punch it a second time. Puncheons placed in a different position between strikes will produce a doubled image which is called a repunch. Dual punches occur when punching is repeated in a second location. Sometimes technicians use a puncheon with the wrong or incorrectly sized letter or number. A well known example of a small mint mark is 1945-S "Micro S" U.S. Mercury dime, when the mint used an old puncheon intended for Philippines coins. A much rarer example is the 1892-O "Micro O" U.S. Barber half dollar, which may have come about from the brief use of a mintmark puncheon intended for the quarter. The same error occurred with the 1905-O U.S. Barber dime. Numerous examples are known of this mistake. Modern techniques have eliminated the need to add design elements by punching as they are an integral part of the design and included in each step of the hub and die making processes.