Why do pennies turn brown
To understand the answer to this question properly, the first thing to know is that pennies today are made out of zinc but have a layer of a copper alloy over them which makes them so shiny when they are still new. The reason as to why the copper pennies turn brown is of course chemical in nature and the reaction involved is somewhat similar to the formation of rust that accumulates over iron. Rust formation over iron is a result of exposure to oxygen and the chemical reaction is known as oxidation; although the same oxidation is the primary reason which is responsible for turning the pennies brown, copper oxidation is different in nature. Oxidation is usually associated with corrosion, but in this case, the pennies do not flake. After the oxidation, your copper pennies will start to turn green actually, because of the formation of “patina”, which is an additional layer that also serves to protect the metal. This is only the first stage and the copper pennies only turn brown later when the patina mixes with the minerals that copper attracts to itself naturally. The minerals come from everywhere, especially from our hands, which are often dirty when we handle them; once the minerals start to mix with the patina, the coin starts to turn brown over time.
Before the onset of the Second World War, the American coins were ninety five percent copper, while zinc and tin was only five percent. Presently, the composition of the pennies have been reversed completely as Zinc now constitutes roughly ninety seven percent of the penny while copper is used only as an outside coating. The change occurred primarily because of the war, but it was changed permanently when the Congress had no other choice but to pass a bill that made zinc the primary metal base for pennies because copper started to become too costly for primary use during the eighties.
As the composition of the penny has changed many a times since 1837, the exact color (it could also be orange or reddish) that the coin would take up as it becomes older, is largely dependent on the composition of pennies at the time of its manufacturing and also on the materials that the penny was exposed to.