Deterioration is caused by physical damage and chemical activity—usually in combination. For many materials, physical damage can create conditions that are favourable for chemical activity.
For example, as aluminium corrodes, an aluminium oxide layer forms on the surface which protects the rest of the metal from corrosion. If this layer is scratched or broken in any way, un-oxidised aluminium will be exposed and it will corrode. Fortunately the corrosion produces a new oxide layer which protects the rest of the metal.
Iron objects are often coated to protect them from contact with moisture and oxygen. If they are not protected they rust. Rust is iron oxide; but unlike aluminium oxide it does not protect the underlying metal from further corrosion. If a coating applied to an iron object is scratched or broken in any way, the object rusts. At first, the rust is localised, but it spreads gradually over the whole object, destroying it totally.
Chemical activity often accelerates physical damage, or leaves objects more susceptible to physical damage. For example, pressure-sensitive adhesives—as used to make sticky tapes—age and become less sticky. The adhesives harden and no longer hold things together. This also happens to adhesives such as rubber cement. Collages and other items which include a lot of adhesives can fall apart once the adhesives have aged.
Paper that was once flexible and easy to use can become brittle, to the point where it crumbles away to fragments when handled roughly. This extreme vulnerability to physical damage is a result of chemical deterioration. Acids within the paper attack the paper’s fibres, making them shorter and much less flexible.