Oxidation is a chemical reaction that can occur when a material comes into contact with oxygen.

A very simple demonstration of oxidation can be given by cutting an apple in half. The inside of the apple, now in contact with the air, quickly turns brown. This is oxidation.

In watchmaking, there are two main types of oxidation: surface oxidation and the more troublesome oxidation that goes deeper into the material.

Metals such as aluminium, brass, copper, german silver and silver typically have surface oxidation. This oxidation is sometimes unsightly but can be easily removed by surface cleaning. It can be considered as a self-protective layer. It is not very annoying because it does not penetrate the material.

Ferrous metals such as steel (when it is not a stainless alloy) or cast iron are exposed to in-depth oxidation. This oxidation on ferrous metals is also known as rust. To avoid it, ferrous metal components must never be handled with bare fingers. If they are, they can be cleaned in a benzine cup and then properly dried to prevent oxidation.

Ferrous metals can be protected from oxidation, and therefore rust, by a layer of grease or oil, taking care to ensure that the component is properly cleaned before being greased or oiled.