The outside film that can be found on coins actually has a name. The term that is used to describe it is “patina”. This coating if formed by many different chemicals such as carbonates, sulfides, and oxides. It can be formed onto the coin when the copper sulfate on the surface interacts with oxygen in the environment. Over time, these chemicals can affect the color and texture of a metal. The film is produced by oxidation over a long period of time and can also be caused from other chemical processes. Old age can also be the cause of this color changing. Other metals, such as bronze, can also be seen having this affect. Metals are not the only thing that can be seen with patina. Old leather and stone can have signs of patina after a lot of use and age. Wooden furniture can have these changes by age and exposure. When patina is found on furniture, it is seen as a sheer sheen that looks like a gloss. Patina is not necessarily a bad thing. It can be used as a protective covering over objects that could potentially be damaged by different types of weathering. Since the patina can be permanently molded onto the metal, it can become a part of the metal if not treated immediately. But, if treated properly, all (or most of) the patina film can be dissolved. 
Most people think of copper when looking at pennies. This is only true when looking at very old pennies. The pennies now are made of various metals such as tin, steel, nickel, and zinc. During the 1793 time period, the coin was completely copper. This slowly began to change over time. In 1837, the Mint began to produce the pennies out of bronze. Soon later, the pennies were made out of 12 percent nickel and 88 percent copper. These two elements combined had a whitish appearance that again was later changed. In 1943, many of the pennies that were being produced were made of steel in order to reserve copper for war effort. Pennies produced today contain 97.5 percent zinc and 2.5 percent copper. Although some metals do not dull out as soon as others. The surface layer of pennies is always copper. The other metals are what are used to make up the coin. Copper is a metal that dulls very fast and can gain layers of patina. Since the copper surrounds the penny, the atmosphere dulls out the penny faster than any other coin.
Copper turns dull when exposed to air. This exposure to air causes oxidation. Oxidation is a chemical reaction that occurs when an atom of an element loses one or more of its electrons to an atom of a different element. Oxidation can be a bad thing when fruit is involved (causes it to rot), but a good thing when making durable aluminum. Oxidation was used thousands of years ago to produce copper metal. The copper in the ore was reduced to copper metal and the carbon was oxidized to carbon dioxide. In the oxidation reaction, two molecules of copper oxide are formed when an oxygen molecule is combined with a copper atom. Rust occurs when oxidation is found in iron. When a layer of copper oxide is formed on the layer of a penny, it will not disintegrate in air. The reason for this is that the layer will prevent further corrosion. 
Cleaning pennies may or may not be a difficult process. The level of the damage can be causing factor of how much will change. If a penny is dipped into citric acid and then dried, it will cause a green coating. The cause of this is from malachite which is a salt of copper. The outside of a penny can be easy restored. The crust that may be found on old pennies is actually just copper oxide. The cause of this hard layer is formed by the bonding of oxygen and copper over a long period of time. Hot sauce is a great way to clean pennies because it contains vinegar. Vinegar is an acid solution which makes it excellent for getting this layer almost completely off the coin. The acid found in the vinegar is actually what causes the copper oxide to dissolve. When the copper oxide has dissolved, you will begin to see the shiny portion of the penny. After cleaning the penny, you can use the reserved vinegar and dissolved copper oxide as a home cleaner. A big part in the cleaning process is buffing out the coin. You must continue to brush the coin until you see a difference in the copper. When cleaning copper, you should avoid using abrasive cleaners that could scrap the foundation of the coin.