Based on an analysis of United States coinage, the 40% silver composition is characteristic of a specific era, primarily from the mid-1960s to the mid-1970s. This period marked a transition away from the 90% silver content that had been standard in many circulating coins. The change was a response to the rising price of silver, which made the metallic value of the coins exceed their face value.
The most prominent example of a 40% silver coin is the Kennedy Half Dollar, minted from 1965 to 1970. These coins were produced with a clad composition, featuring outer layers of 80% silver and 20% copper bonded to an inner core that was approximately 21% silver and 79% copper. This layered structure resulted in an overall metallic content of 40% silver.
In addition to the Kennedy Half Dollar, certain Eisenhower Dollars also contain 40% silver. Specifically, the uncirculated and proof versions minted from 1971 through 1976 for collector sets were struck in this composition. It is important to note that the standard Eisenhower Dollars intended for general circulation during these years contained no silver.
Furthermore, special Bicentennial commemorative coins minted in 1976 were also produced with 40% silver content for collector proof and uncirculated sets. These included the Washington Quarter, the Kennedy Half Dollar, and the Eisenhower Dollar, all bearing the dual date of 1776-1976. As with the Eisenhower Dollars, the versions of these Bicentennial coins that were released for general circulation were made of a copper-nickel alloy and contained no precious metal.
