In 1973, Congress authorized silver-clad collector versions of the Bicentennial coins; in April 1975, the Mint began to strike them. The coins were issued in both proof and uncirculated quality. Copper-nickel clad Bicentennial coins were placed in both the 1975 and 1976 proof sets, while their silver clad counterparts were sold in three-coin sets. Since 1992, the Mint has struck Kennedy half dollars in 90% silver for inclusion in special silver proof sets. 1964 proofs were struck in Philadelphia, and since 1968, proof coins have been struck in San Francisco only. In 1998, some silver proof pieces were struck to a matte finish for inclusion in a set along with a Robert F. Kennedy commemorative silver dollar. From 2005–2010, uncirculated pieces included in mint sets received a matte finish, which differentiates them from the pieces sold in bags and rolls.