From the Conservator's Bench: Cleaning Medals and Coins
Do you have a medal or coin collection that you would like to preserve and display? Have you wondered about the best way to care for them? Is it better to clean coins, or leave them as is?
Proper storage is always the key to preservation, especially of metal objects like coins and peace medals. Objects like these are made of alloys, or mixtures of metals. Peace medals tend to be made of silver and copper alloys. Medals and coins should be stored individually in polyethylene bags, acid-free envelopes or boxes, or PVC-free Mylar® flips (slotted plastic coin-sized pockets or sheets). Never use bubble wrap for padding; instead, choose an archival foam, like Volara®, Zotefoam® or a thin polyethylene foam. Shadow boxes made of archival materials are a good option for display, but never use an adhesive to secure the medal or coin while framing. Storing each coin separately prevents the accelerated corrosion process that occurs when different metals are in contact (galvanic corrosion). Water vapor also increases corrosion, so metal collections should be stored in dry conditions (lower than 40% relative humidity is ideal).
Air quality also affects metals, especially silver and copper. When sulfur particles interact with silver, a film of silver sulfide forms on the surface. Silver sulfide is black, but thin layers can look yellow, iridescent, or even be splotchy. Anti-tarnish strips and silvercloth are passive pollutant scavengers that reduce the quantity of sulfides and other harmful ions interacting with collection objects, thereby slowing the rate of oxidation and corrosion.
If you notice powdery or a shiny, wart-like, green corrosion, contact an objects conservator for assessment and treatment. It might be a pervasive form of chloride corrosion often called "bronze disease." Chloride ions bond with copper to form cuprous chloride. Cuprous chloride interacts with water vapor and forms hydrochloric acid, which then forms more cuprous chloride and restarts the cycle. A coin can disintegrate very quickly to green powder if this process isn't disrupted.
Peace medals, coins, and other metal objects should always be handled with nitrile (the best) or cotton gloves (without PVC grip dots), or a lint-free cotton cloth to prevent oils and salts from skin transferring to the metal. Though you won't see it now, fingerprints (a mixture of natural oils and salty perspiration) that aren't removed will someday become dark and permanently etched into the metal. If you want to remove dust, you can occasionally use a soft artist's brush, being careful to avoid scratching the object with the metal ferrule (bristle holder).
Polishing of peace medals and rare coins is generally discouraged. The natural patina of oxidation and oils that have accumulated over time is often valued by collectors. (But beware of relying too much on patina to authenticate coins and medals — accelerated 'patinas' can be very convincing if someone knows what they are doing). Additionally, polishing too often will decrease the sharpness of the relief and reduce legibility of inscriptions. Commercial polishes and dips should especially be avoided. If polishing residue or other gunk is caught in the crevices of a coin, you can use an artist's brush and distilled water to gently dislodge and remove the accretions. I like to use red sable brushes, size 1 or 2, bright or filbert shape for this. Be sure to allow the metal to fully dry before storing. You can gently blot the surface with a lint-free cotton cloth to speed drying but be careful not to rub.
Finally, it is always a good idea to photograph both sides of the coins in your collection. It can reduce the need for handling when you want to share images, and it serves as documentation of your collection for insurance purposes.
If you have questions about artistic techniques or preservation, you can ask Victoria Book Lupia, our chief conservator. Send questions to: GMConservation@utulsa.edu.