Untreated bronze is sometimes anodized to make it stronger and more resistant to wear. In order to anodize a metal, an electrical current is passed through it in such a way that the anodized metal forms the anode in the electrical circuit. This attracts oxygen atoms to its surface, forming a protective oxidation layer. Though oxygenated metal can be weaker than metal that has not reacted with oxygen, as in the case of a rusty piece of iron, some metals, including aluminum, are made stronger when they are oxygenated through anodization. With a protective layer of atoms on the outside of it, a piece of anodized bronze is able to withstand exposure to temperature, water, and radiation better than it would without this layer.
Bronze can be made up of a number of different metals, though copper is the primary component. The most common type of bronze is copper and tin, a combination that forms a hard metal that is prone to breaking and fragmentation. Combinations of copper and tin are not put through the anodization process because neither copper nor tin form a protective layer when anodized. Instead, bronze made from copper and between 5% and 11% aluminum is usually made into anodized bronze because aluminum can be anodized.
Anodized bronze is more durable than non-anodized copper-aluminum bronze is. Anodizing this type of bronze helps to keep it from cracking, breaking, or chipping, especially when it is exposed to the elements for a long period of time. It may also be anodized in order to produce an aesthetically pleasing finish on the piece of bronze. The color can vary between a light, gold-orange to a deeper orange with reddish hues.
It is also possible for the term to refer to a specific color of paint. An anodized bronze paint will be a metallic color with orange, gold, and red pigment. In many cases, these paints are designed to be used on metallic surfaces and may offer these surfaces some protection from the elements while coloring them so that they look like anodized bronze. A layer of this paint is not at all the same as a layer of anodized metal, however, and any protective qualities the paint bestows upon the surface are the result of its sealing properties and not a chemical change in the surface of the metal.