Metals appear to be durable and lasting but like everything else in our collection they will deteriorate, rust, green, and discolor over time. Comparing a bronze statue or a silver teapot to a photograph, 18th century letter or a Victorian handkerchief, it’s easy to see the metal piece as the strongest and the most durable. This may be true, but metals erode and will eventually turn to dust like everything else, it just might take a little bit longer. In the meantime, we want to do all we can to ensure their longest life and to protect them from the ravages of age. In truth, the metals in our collection may be the most valuable in terms of resale cost.
Metals begin their natural life as rock-like ores that are mined. The raw stock of metal ores is smelted into compounds, or alloys, through various processes. Working at high temperatures, artists and craftsmen shape and mold metals into artworks and objects that are both practical and aesthetic. Most metals found in our homes are alloys, which mean they are a mixture of several metals.
Almost immediately after artists have completed their work, metals begin to revert back to their natural state through a process called corrosion, oxidation, or rust.
As with other parts of our collection, environment plays a key role in lengthening the life of metal objects. The optimum environment for metals can be challenging for most homeowners. In the case of metals, humidity is the greatest enemy followed by interior pollution. Most of us our aware that moisture is extremely damaging to metals. Metals will corrode on contact with water. Relative humidity should remain between 40% and 45% with 50% being the extreme maximum. If your metal objects are already showing evidence of deterioration, keep them in an environment with 35% humidity until a conservator can look at it and advise how to care for it. Early signs of corrosion may show up as watery or powdery areas on the object.
Temperature in and of itself does not affect metals. Temperature is important only in its relationship to relative humidity. Temperature stability is very important. A sudden drop in temperature can cause condensation in a small, enclosed area. Metal contracts and expands with increases and decreases in temperature, which will also stress an object and accelerate deterioration.
The hidden culprit that damages metals is indoor pollutants, including dust.
Most people are aware of air pollution outside our homes caused by automobiles and industrial waste, but they are unaware of how many pollutants within our homes can be damaging to our collection, especially metal which is highly sensitive to moisture, different gasses, acids, and alkali. Dyed fabrics, wooden cabinets, plastics, glues, and products with formaldehyde can all emit harmful gasses, corrosive acids and damaging alkalis that will accelerate the deterioration of different metals. Cigarette smoke causes silver and copper alloys to discolor or corrode. Rubber products can off-gas pollutants causing discoloration. Salts, oils, acids and other chemicals damage metals. Even certain aggressive metal polishes can accelerate deterioration. Mishandling objects causing breakage or scratching can create sensitive areas subject to quicker corrosion. If possible, keep metal objects covered and free from dust. Hutches with glass doors or glass frames for smaller objects provide a great place to display beautiful family heirlooms while
One of the best ways to stop the deterioration of metal objects is to examine them. Take your grandmother’s silver tea set out of the closet every now and take a look at it. Examine your mother’s pewter candelabra. Check the backside of your uncle’s bronze star. Don’t forget that gold piece of your great grandmother’s. Look for tarnishing, rust, discoloration, and flaking. Remedy the situation before it becomes too extreme. Prevention is much less costly than repairing a damaged object. You don’t want to open the drawer containing your ancestor’s Civil War Campaign medal to find it has crumbled back to its original ore state looking simply like a pile of dirt.
If you have archaeological or ethnographic objects in your care, please contact a museum or professional conservator or an archaeologist. The care of ancient artifacts is specialized and needs different procedures than is used for more modern objects.