You might have seen it happen many times and ask why pennies turn brown. There are those that guess that pennies get rusty and thus producing a brownish color. That wild guess might not be too far off. In fact, the formation of rust on iron is quite related to why pennies turn brown.
Properties of Copper
Pennies are made of a copper alloy. This might bring up a side question of why choose copper when minting pennies. The answer relies on the very properties of copper and why is it a wise choice when minting coins. Understanding the properties of copper will also explain why pennies turn brown.
Looking Into Copper Oxidation
It is common knowledge to us that iron rusts when you expose it to air or water, which is a process known as oxidation. Oxidation, in layman’s terms, is the process whereby oxygen is added to a compound. It usually occurs with a loss of electrons and reduction. This same process also occurs with a lot of ordinary things like rust forming on iron.
When iron oxidizes, rust forms and corrodes the iron. When you cut an apple in half and expose its insides to air, you will notice that parts of the inside will turn brown. All these phenomena are simply the results of oxidation. This is also the same explanation why pennies turn brown in the long run. However, the oxidation process for pennies will be a lot different than the oxidation you’ll observe on apples or iron.
A Non-Corrosive Oxidation
The primary difference between pennies, which are made of copper alloy, from iron is that you won’t see your pennies corrode. Copper does not generate any tarnish nor does it emit something. Oxidized copper will turn green in color and form patina. Patina is a coating of different compounds that serve as a protection to copper rather than corrode the metal.
Why Pennies Turn Brown and Not Green
All this discussion on copper oxidation and the formation of patina on the copper still doesn’t bring to light why pennies turn brown, since patina is green. The brown tarnish on pennies is a combination of oxidation of copper and copper’s inherent characteristic of attracting minerals.
In simple terms, aside from oxidizing copper attracts minerals from the dirt and everything around it. This happens when people handle the coins. In the long run, copper alloy pennies turn brown eventually through this process. Because of the properties of this metal, copper pennies change to a brownish color over time.