Caring for Brass & Bronze, Part II

Article written for MCHS newsletter, Vol. 23, No.1, 2010

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The saga of the piano pedals continues. I have to admit that after the last newsletter article, Collections Connection – Caring for Brass and Bronze (Part 1), in which the main causes of damage to metal objects were identified and basic cleaning procedures were described, I still harbored hopes that a “good metal fairy” would appear in the form of a magic elixir. While no such miracle occurred, we were able to mitigate the effects of the corrosion on the pedals, removing the damage physically if not visually. Below are some suggestions for how to best deal with corrosion and tarnish on brass and bronze objects while doing the least harm.

Though the rapid development of small spots of light green powder, like those found on the piano pedals, is a good indication of harmful corrosion, some corrosion is actually considered safe. Typically developing on the stable surfaces of uncoated objects that are clean and dry, “safe” corrosion can be any of a wide variety of colors, including red, black, brown, green or blue. “Safe” corrosion is often an important part of metal art objects, adding expressive detail and patina. Purposely applied patinas and dense corrosion that does not progress usually serves as a protective coating.

Unfortunately for my family, our piano pedals were experiencing what looked like moderately active corrosion. Varnished or lacquered brass usually does not corrode unless the coating has failed. The protective coating on our piano pedals may have been worn down by our cat, Sven, who has occasional bouts of “inappropriate urination” in various locations throughout our house (i.e. not in his litter box). One such location is near the piano pedals. As this sometimes went unnoticed or was not taken care of immediately, the acid in the urine may have caused the coating to break down, allowing moisture and contaminants on the unprotected surface. The corrosion had, thankfully, not proceeded to an advanced state, which can cause significant damage. Indicated by bright or “waxy” whitish-green powder forming in small areas or cavities, advanced active corrosion often results in pitting and aggressive metal loss and usually requires the services of a trained conservator.

If you have brass or bronze that needs attention and choose to clean the piece yourself, make sure the object is clean and dry before you begin and that you are willing to take the risk. If the metal you are working with is part of a larger piece, such as a piano, carefully protect the rest of the object with lightweight plastic. If the damage to your brass or bronze object appears minor, a mild abrasive may be all you need. Light polishing with a jeweler’s cloth containing rouge (an extremely fine polishing compound) is recommended as brass and bronze are relatively soft metals and too much metal can be lost through aggressive cleaning. On the piano pedals, the jeweler’s cloth easily removed the worst of the corrosion but not enough to ensure that the active corrosion had been stopped.

If light polishing does not do the trick, you may have to resort to more aggressive measures. One recommended treatment involves the creation of a homemade slurry or runny paste consisting of equal portions of fine calcium carbonate (chalk or “whiting”), ethanol (denatured alcohol, ethyl alcohol) and distilled water. Calcium carbonate can be purchased online or at various builders’ supply companies. Apply the paste to one small area at a time using cotton balls, clean cotton rags or Q-tips. Rinse well with distilled water and dry carefully. To speed up the drying process, add a little ethanol to the rinse water or give the object a final wipe with ethanol. The calcium carbonate paste seemed to do the trick on the piano pedals, though they no longer look as good they did before being damaged. The paste, which requires a little patience and elbow grease, turned out to be an excellent polish. This was especially evident in areas on the pedals that had not been damaged.

If the calcium carbonate paste does not do the trick and you may want to try something more abrasive. Autosol® metal polish is recommended because it contains less ammonia than most other commercial brands of metal polish and has the added benefit of leaving a protective wax coating. Autosol® can be purchased online or at select hardware or auto parts stores. Buff the polish on with a clean rag and rinse with mineral spirits. For both this procedure and the calcium carbonate paste, be sure to work in a well-ventilated area and wear protective clothing. While Autosol® has not yet been used on the piano pedals, we are thinking of purchasing some in the near future. If this momentous event occurs, the results will be included in a latter issue of the newsletter.

By Ann Marie Johnson
Copyright 2010, Morrison County Historical Society

Sources:

“Appendix O. Curatorial Care of Metal Objects.” Museum Handbook: Part 1 – Museum Collections. Washington, D.C.: National Park Service, 1990.

“Caring For Your Treasures: Metal Objects – How To Protect Your Metal Objects.” American Institute for Conservation of Historic and Artistic Works (http://www.conservation-us.org).

Deck, Clara, Senior Conservator. “The Care and Preservation of Historical Brass and Bronze.” The Henry Ford (http://www.thehenryford.org).

Long, Jane S. and Richard W. Caring for Your Family Treasures. New York: Harry N. Abrams, Inc., 2000.

The National Trust Manual of Housekeeping. Oxford, UK: Butterworth-Heinemann. 2006.

Williams, Don and Louisa Jaggar. Saving Stuff: How to Care for and Preserve Your Collectibles, Heirlooms, and Other Prized Possessions. New York: Simon & Schuster, 2005.