Generally, if a penny looks dirty, it is because of copper oxide. Copper oxide is what results when copper atoms (the penny) attract oxygen atoms. When they combine, they form a molecular reaction that appears to make the penny look dirty. Sometimes it really is a penny being dirty; since they change hands so frequently, they can become covered in anything. If a penny is rusted or needs to be cleaned, the best (natural) thing to use on it for cleaning would be lemon juice. Vitamin C contains ascorbic acid and this is what eats through rust and copper oxide on pennies. The acid breaks down the bonding between the copper and oxygen on the penny and dissolves most of it, or just washes away and makes the surface less sticky if it something else is covering the penny.
There are other juices that can be used to clean pennies, as long as they are high in acid content. Tomato juice (pure, with no additives), lime juice and vinegar are good at cleaning pennies as well. After these are things that are normally sweetened, like orange juice and cranberry juice, but if unsweetened types can be found they will clean pennies much better.Learn more about Cleaning Products