Aging coins. Two US one cent coins of different age. The left was minted in 2015 (copper-plated zinc) and the right was minted in 1980 (brass, 95% copper and 5% zinc). When coins age their luster is reduced due to formation of tarnish and accumulation of the dirt. Tarnish consists of various oxides, sulfides, sulfates, etc. that form due to reactions with gases in the atmosphere as well as oils from human hands. Photo shot in 2016.