About Brass Oxidation

Brass is an alloy composed of copper and zinc. In its newly cast state, brass takes on a shiny golden hue that can be polished to a brilliant glow. Without polishing or a protective lacquer coating, brass can tarnish through a process known as oxidation. This oxidation, which affects primarily the surface of the metal, serves to slow down the corrosion process and gives the brass a patina, which may be either desirable or frustrating, depending on your perspective.

History

Brass is an alloy that many scholars trace back to prehistoric times. Because of it high luster brass was frequently used for mirrors in numerous ancient civilizations. Many cultures used brass for coinage and for decorations. Depending on the proportion of copper to zinc, brass can be slower to tarnish, or oxidize, than other metals making it desirable for more contemporary uses such as in the manufacture of musical instruments, plumbing and some electrical applications.

Function

Oxidation of brass occurs when the copper molecules come into contact with the oxygen. The chemical reaction that occurs results in a dull tarnish that range from greenish-yellow to coppery red, depending on the proportion of copper to zinc in the brass. The oxidized layer, or patina, provides a protective coating around the underlying layers of brass. For decorative applications this patina is considered by many to be a desirable feature, giving the brass object an antique, Old World charm.

Types

There are two types of brass oxidation: naturally occurring oxidation and man-made oxidation. Brass oxidizes naturally when exposed to the elements. In vintage homes brass fixtures such as door knobs and knockers take on subtle changes in color as the oxidation process continues. Occasionally, home restorers wish to recreate the patina of oxidized brass without waiting for several decades. To achieve the desired patina they expose the brass fixture to one of many commercially available oxidizing solutions.

Benefits

Oxidation is a natural means of slowing the corrosion process of brass. In applications that are functional rather than decorative, such as in fine clockworks, it is advisable to allow the oxidation to remain rather that trying to polish it off. In these cases removing the existing oxidation will only increase the rapidity of the metal's deterioration. In decorative applications, the natural oxidation, or patination, of the brass fitting or fixture is considered by many to add a desirable luster to the object.

Prevention/Solution

When highly polished, brass takes on a shiny, gold-like appearance that many people find appealing. This shine can be maintained by coating the brass with a thin layer of clear polyurethane or lacquer which prevents the oxygen in the air from coming into contact with the copper molecules in the brass. A number of commercial manufacturers now offer brass that has been coated with an ultra-thin layer of palladium, nickel or some other stable metal which prevents oxidation and insures the underlying brass's brilliant luster.

Resources

• Patination Formulas
• More Patination Formulas