The copper-nickel clad Washington quarter was first issued in 1965 and as part of the switch, the Denver mintmark was added in 1968, which did not reappear on any US coin denomination until 1968. During the early 1960s, the Federal government had been flooding the market with silver to keep the price down, and therefore keep US coins' intrinsic values from exceeding their face values. This was causing the level of silver in the US Reserves to reach dangerously low levels. Silver was estimated to only last another 3–5 years at the rate the Mint was manufacturing coins, so the US Congress authorized the Mint to research alternative materials for the silver denominations (dime, quarter dollar, half dollar, and dollar). The material chosen was a 75% copper/ 25% nickel cupronickel alloy (identical to that in the five-cent coin) clad to a core of "commercially pure" (99.5%) copper.