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. Scuptures or items are usually cast by the lost wax process. A wax image is created, encased in a mold which is heated to an extreme temperature. 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 inside the bronze 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.
A recent 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 bronze and dark, you must make a personal decision. Cleaning will sometimes affect the value of an item. However, if you decide you want to clean the item, consult an expert.
Tarnish should be judged with three criteria. Is it dark, is it black, or is it green? Then grade the intensity of the tarnish.
If the bronze is dark, and you must have it lightened, hand rubbing is probably the best solution, and the Blitz 105 cloth would be best. Simply rub the cloth over the metal lightly until the color is restored. Wash the item, and re-wax.
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. 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 should be consulted.
If the bronze is in 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.
A useful site is by Richard Rist largeart which explains how to clean an outside sculpture that you want to have remain a metallic-bronze. Essentially, washing and re-waxing periodically is required. Waxing will rarely last longer than 3 months outdoors.
There are a number of online household cleaning solutions. These essentially amount to creating a home made acid-paste.
The first is known as the vinegar, salt and flour paste incubation: This creates a form of dilute hydrochloric acid soak. The same thing worked when you were a child, and cleaned pennies by swirling them around in a solution of salt crystals, salt water, and vinegar. The effect will probably produce a reddish raw look when used.Then the piece will need to be polished. The pores of the metal will be open and will no longer match the copper-tin alloy beneath, since more of the copper will have been rinsed away.
Citric acid base cleaning involves the use of lemon juice and baking soda. The baking soda tends to be gritty, thus making a mild acidic polish. The mix should be used quickly, lest the citric acid and sodium bicarbonate neutralize one another.
Though usually reserved for brass, the Blitz 105 cloth is precisely the kind of cloth that will have mild, balanced ingredients that will clean this kind of bronze. After the polishing, a tarnish resistant ingredient will be left behind.
Today's technology is confused by marketing terms. There are many new alloys of copper, tin, zinc, aluminum, and trace metals. One of these is light weight, aluminum bronze. On this and other modern alloys, consult a metallurgical, or other metal specialist on how to clean and preserve these items.
Is your bronze lacquer coated, polymer coated, or varnished? If so, you should not see tarnish. If you do see tarnish, then you have a decision to make. You can clean only that portion which is tarnished, but if you do, it's likely that the polished section will no longer match the pristine surface still under the original coating. If that's acceptable, recoat the item. Otherwise, a laborious process of stripping the entire item lies ahead of you, cleaning, and then recoating.
