Tarnish

Tarnish is a thin layer of corrosion that forms on many different metals. It occurs when the outermost layer of a metal undergoes a chemical reaction. This process can affect metals such as copper, brass, aluminum, magnesium, and neodymium. Tarnish often appears as a dull coating. It might look like a gray or black film over the surface.

While it looks like damage, tarnish is actually a surface phenomenon. This means the reaction is limited to the very top of the metal.

The way tarnish works is quite different from rust. Tarnish is considered self-limiting. This means it does not spread deep into the object like rust does. Only the top few layers of the metal react with their environment. Once that thin layer forms, it actually seals the metal. It protects the underlying layers from further chemical reactions. When this happens to metal used outdoors, it is called a chemical patina.

One famous example is verdigris. This is a blue-green or green form of copper(II) carbonate that forms on copper.

Chemistry explains exactly how these changes happen. Tarnish is the result of a chemical change between a metal and a nonmetal compound. Common causes include oxygen and sulfur dioxide. When a metal reacts with oxygen, it undergoes a process called oxidation. This creates a metal oxide. In other cases, the reaction may create a metal sulfide. The metal oxide can also interact with other substances in the environment. For instance, it might react with water to create a hydroxide. It can also react with carbon dioxide to form a carbonate.

Different metals require different conditions to tarnish. Oxygen in the air is a common factor, but it is not the only cause. For example, silver may tarnish with oxygen over a long period of time. However, silver specifically needs hydrogen sulfide to tarnish. Because the chemical components vary, the resulting appearance of the tarnish can also vary. Some metals might develop a colorful appearance through these complex chemical interactions.

Whether tarnish is good or bad often depends on the object. In some cases, a chemical patina is very useful. It is advantageous for copper roofing and for bronze or brass statues. These layers protect the metal from the elements. However, tarnish can also be undesirable. People often find it unattractive on silverware. In other cases, like the toning on coins, it is simply a matter of taste or convention.

For coin collectors, certain types of toning can even increase a coin's value.

If people want to remove tarnish, they have several options depending on the severity. Heavy tarnish can be removed mechanically. This involves using tools like a file or abrasive materials. Examples include steel wool, sandpaper, emery paper, and heavy polishing compounds. Lighter tarnish can be handled more easily. People might use lighter polishing compounds or chemicals like baking soda. Museums take extra care with delicate items. They often use very gentle abrasives, such as calcium carbonate, to clean tarnished silver. This prevents scratches and avoids leaving unwanted residues.

There is also a non-destructive way to clean metal called electrochemical reversal. This is a clever method used for items like silverware. To do this, you place the object on a piece of aluminum foil. Then, you rest it in a pot of boiling water. You must add a small amount of salt or baking soda to the water. This process uses chemistry to reverse the tarnish without damaging the metal surface. Understanding these reactions helps us both preserve beautiful art and maintain functional metal tools.