Off-metals are one of the most interesting types of error coins. They occur when the planchet of one coin is fed into the press of another coin type, and struck. The result is a coin struck on the incorrect planchet.
A good example of this error type would be the nickel on cent planchet. In this instance, the planchet of a copper Lincoln cent planchet was fed into a coining press that was set-up to strike Jefferson nickels and was struck. The resulting coin has the design of a nickel but the planchet of a cent. Pretty simple to understand, as the only thing “wrong” on the coin is the disc of metal that the coin is struck on is incorrect. Otherwise, the minting process is all correct for the coin.
But how did the cent planchet end up in the wrong coining press? There are a few scenarios that are most likely.
The first happens when planchet bins are used to transport planchets around the mint. These bins are sometimes used interchangeably for different coin denominations. If the bin was used to transport cent planchets, and then was switched to transporting nickel planchets, it’s possible that a few cent planchets might have been accidentally left in the bin. When the nickel planchets are then put into the bin, the cent planchets are then accidentally fed along with the nickel planchets into the nickel coining press, to be struck into “nickels on cents."
Another possibility is that if a press is switched from striking one coin denomination to striking another denomination, it’s possible that a few planchets might be left in the striking chamber area, later to be struck when the press is started back up to strike the new denomination of coin. This is pretty unlikely, but theoretically possible.
Last, there is also the chance that a bin of, for example, cent planchets, might be fed into a press striking nickels. The mint workers should catch this fairly quickly, but perhaps not before some coins made it into bags along with other nickels. Certain dates of off-metals are far more common than others, and perhaps a scenario such as this is the reason.
Off-metals are close cousins to the “wrong planchet” error, with the only difference being that an off-metal is the incorrect metal (e.g., nickel on cent planchet), whereas a wrong planchet error is the same metal (e.g. quarter on dime planchet.) However, often times these terms are used interchangeably by both collectors and coin dealers.
Most of the time, off-metals are collected by collectors wanting one of each type for a particular coin series. An example of this would be collecting a Kennedy half struck on a 1c, 5c, 10c, 25c, and SBA $1. Such a set is usually fairly achievable, since collectors can pick and choose between the different dates and conditions, making it fairly budget friendly (as much as collecting off-metals can be called budget friendly, since they tend to be somewhat expensive due to their rarity.)
Another way to collect, and one which is far more difficult if not impossible to complete, is to try for a complete date and mint collection of one series on another series. For example, a collection of Lincoln cents on dime planchets from 1959-2008 (Memorial design) would be possible if done by date, and mostly possible if done by mint as well, although some mintmarks in the 2000’s would probably be impossible since none exist to my knowledge. Of course getting proofs would be impossible, as only a handful of proofs are known for cents on dime planchets.
Collecting a 20th century “type set” of off-metals would represent an incredible challenge, but would be largely possible for someone on a nearly unlimited budget, since there is an off-metal known for most 20th century coin designs, with the exception of gold coins, and the Morgan dollar design (there’s one peace dollar known!) A more achievable 20th century set would be to just get one of each denomination: $1, 50c, 25c, 10c, 5c, and 1c.
Other collectors are creative, and prefer to make their own unique off-metal collection following other rules, and in some cases “no rules” except to collect “what they like.” However you collect, enjoy the process and take the time to learn about your coins, since learning about them is half the fun.