How to Clean Bronze

Blitz frequently receives requests on how to clean bronze items. Bronze is a very old alloy, yet cleaning it can be complicated. First, be careful to differentiate brass and bronze. Brass is a copper-zinc alloy, usually has a smooth, yellowish surface, and is relatively easy to clean. Upon the first appearance of tarnish (darkening) use Blitz 105 cloth! Bronze is a hard, heavy alloy made of copper and tin. Sculptures or items are usually cast by the lost wax process. A wax image is created, encased in a mold which is heated very hot. The wax is burned away at these hot temperatures, and the empty cavity is filled with bronze. The casting is cleaned, and while warm a wax is applied. Since the bronze surface is porous, the wax eases into the surface and keeps air and moisture from penetrating and causing the copper to tarnish. If an item is kept indoors, bronze tarnishes very, very slowly. It might take many months to show any effect, or even years. A simple dusting and periodic washing with a mild cleaner is necessary. The more washing, the more wax will be removed, and eventually air will get to the surface. Rewax your inside item one or two times a year. There are many commercial products for this purpose. However, frequently used and frequently washed items will darken. Unused pieces might sit in a garage for years. Then, once rediscovered, cleaning is needed. An article (Chemical and Engineering News, 7 September 2009, page 53) explained that scientists examining a wide range of museum items found that there are three basic sculpture items: High zinc brass, Low zinc brass, and tin-bronze. Make sure that the item you have is brass or bronze first. That will involve showing it to an expert in antiques. If you have an item that is made of a bronze alloy and has an unpleasant dark appearance, you must make a personal decision. Cleaning will sometimes affect the value of an item. So carefully consider what you need, and consult experts in this field. However, if you decide you want to clean the item, first determine how much tarnish there is to remove from the item. Tarnish should be judged with three criteria. Is it dark? Is it black, or is it green? Then grade the intensity of the tarnish. Is it a faint discoloration, or is it deep and crusty? If the tarnish is rough and crusty, then you need to have an expert work with your item to judge how badly the object has deteriorated. If the bronze is a little dark, and you wish to have it lightened, hand rubbing is probably the best remedy, and Blitz item 105 would be best. Simply rub the cloth over the metal lightly until the color is restored. Use even hand pressure and be patient. Once the object has reached the color desired, wash the item, and re-wax it. If the bronze item is black, the copper has reacted with oxygen and sulfur. If you decide this must be removed, determine how deep the metal is affected. The corrosion could be extreme, and pits may have developed. (Again, seek an expert opinion.) The tarnish can be removed with great difficulty, and the surface will then be shiny again, but scarred. If so, then judgment will be needed if the surface should be smoothed. All this will take a great deal of time, patience, and possibly special tools and ingredients. An expert in metals should be consulted. If the bronze is various shades of green, greenish-blue, mottled with gray and earth tone blues, stop and consider what the next step should be. This is usually a highly desirable appearance, and much thought should be taken before cleaning. It is the natural state for a copper-tin alloy to turn an earth tone greenish-blue. This actually protects the metal. This will often be seen in garden sculptures.