Mule Die Clash (MDC)

A mule die clash occurs when two dies that were never intended to be paired together clash against each other without a planchet between them. Error-Ref defines a mule clash simply as clash marks "from dies that should never have been paired." The resulting clash marks are distinctive because they do not match the normal obverse-to-reverse design relationship for the coin type. Instead, the ghost impressions come from a different design -- usually a die of another denomination.

How Does It Happen?

To understand mule die clashes, it helps to know how dies are handled at the Mint. Working dies are made in a die shop and sent to the coining floors, where they are stored and installed in presses as needed. In normal production, a press receives an obverse and reverse die made for the same coin.

A mule die clash needs two dies of different coins to meet face to face:

1. A mismatched pair: A die from one denomination or design type ends up opposite a die from another. When that happens in a working press and coins are struck, the result is a mule.
2. An empty strike: The two dies come together with no planchet between them and leave clash marks on each other.

How the dies came to meet is not always clear. Error-Ref lists six 19th-century mule clashes and says it is widely suspected that all of them were the result of mischief by Mint employees. For the one modern example it records, Error-Ref says it is unclear whether the two dies were even installed in a press at the time, or whether the clash was accidental or intentional.

There is a critical distinction between a mule die clash and a mule coin. A mule coin is struck with the wrong die pair and carries the full design from both mismatched dies. In a mule die clash, the coins themselves carry their own correct obverse and reverse designs, plus ghost impressions from a die that should never have touched theirs.

How to Identify a Mule Die Clash

Identifying a mule die clash requires recognizing that the clash marks do not belong to the coin's own design:

• Wrong design elements: The ghost impressions visible in the fields are from a different coin type entirely. On a Lincoln cent, you would expect Memorial columns or Shield lines as clash marks. Lettering or devices that belong to no Lincoln cent die point to a foreign die.
• Size mismatch indicators: If the clash came from a die of a different diameter, the scale of the clash marks is wrong. Design elements from a larger die appear broader and extend further than expected, while those from a smaller die appear compressed and centralized.
• Unfamiliar design geometry: The transferred elements do not match any feature from the coin's own reverse (or obverse). A collector familiar with the typical clash patterns for a given series will recognize that the marks are foreign.
• Correct primary design: The coin's own obverse and reverse designs are normal and properly paired. The mule clash marks are superimposed on an otherwise correctly paired coin.

Required Expertise

Mule die clash identification demands thorough knowledge of U.S. coin designs across multiple denominations. The collector must:

1. Recognize the clash marks as inconsistent with the coin's own reverse/obverse
2. Positively identify which foreign die produced the marks
3. Confirm the size and design geometry match a specific other denomination or type
4. Rule out other explanations such as post-strike damage, environmental effects, or contact marks

Notable Examples

19th-Century Mule Clashes

Error-Ref says mule clashes were long considered a 19th-century phenomenon and lists six from that century:

• 1857 Flying Eagle cent: obverse clashed with a $20 double eagle obverse.
• 1857 Flying Eagle cent: reverse clashed with a Seated Liberty quarter reverse.
• 1857 Flying Eagle cent: obverse clashed with a Seated Liberty half dollar obverse.
• 1857 Seated Liberty quarter: reverse clashed with a Flying Eagle cent reverse.
• 1864 two-cent piece: reverse clashed with an Indian Head cent obverse.
• 1870 Shield nickel: obverse clashed with an Indian Head cent obverse.

1999 Lincoln Cent Mule Clash

The one modern example Error-Ref records was found in 2018 in cents taken from pocket change. Near the left edge of the reverse of a 1999 cent, just inside the rim, an arc of sunken, reversed letters reads "oF AMERIC". The letters are doubled, showing two slightly offset clashes, and their position suggests that one die, probably the hammer die, was tilted. Error-Ref places it in the 1990-2000 period that also produced dozens of "radical" misaligned die clashes on Lincoln cents, and suggests the same conditions may have contributed.

Cent and Dime Mules

The Lincoln cent (19.05 mm) and the Roosevelt dime (17.91 mm) are closer in diameter than any other pair of circulating U.S. coins, and real cent/dime mule coins are known: Error-Ref illustrates a 1993-D and a 1999 cent struck with a dime reverse die, and a 1995 dime-planchet mule with a cent obverse. Those are mule coins, not mule clashes, but they show that this pairing has happened in working presses.

Collecting Tips

• Very few are known: Error-Ref's list of recorded mule clashes runs to seven. Most die clash collectors will never encounter one.
• Expert authentication is mandatory: Given how few are known and the high risk of misattribution, mule die clashes must be authenticated by a recognized authority. Submit to PCGS or NGC, and consider additional verification by a die clash specialist.
• Know your designs: Building the expertise to identify mule die clashes requires intimate familiarity with the designs of all circulating U.S. denominations. Study the standard typical clash patterns for each series so that foreign design elements stand out immediately.
• Research first: Before concluding that a clash mark is from a foreign die, exhaust all possibilities that it could be an unusual feature of the coin's own opposing die. Some design elements clash in unexpected ways under atypical conditions.
• Documentation: If you discover what you suspect to be a mule die clash, photograph it extensively under multiple lighting angles. Provide comparison images showing the suspected source design from the foreign denomination. Thorough documentation is essential for expert evaluation.
• Study known examples: Review published mule die clash attributions in numismatic literature and specialized references. Understanding what confirmed examples look like is the best preparation for identifying new ones.