In the May 23 installment, I noted that a reader had several medieval Russian silver coins in his collection that had a black corrosion layer on the surface and were quite brittle. The coins had come from a burial environment.
The black corrosion product, silver sulfide, results from exposure to sulfur in the burial environment or after excavation, or both. The brittleness is quite common with silver coins that have been buried and results from selective corrosion of the copper originally alloyed with the silver.
Once the copper is corroded, the coin becomes brittle. Conservators and conservation scientists refer to this phenomenon as “microstructurally induced embrittlement.”
Now, how does the owner best preserve these coins? Reducing risks to the coins is key to their preservation. First, and foremost, the collector should not attempt to remove the corrosion layer. These coins are delicate and brittle. Cleaning would put the coins at too much risk of damage.
Proper care and handling is the best approach. The brittleness of the coins makes them susceptible to physical damage. The coins should be handled carefully, always over a padded work surface.
As with any metal coin, token or medal, gloves should be worn to prevent the acids and salts in our hands from corroding the coins. The reader should place the coins in a good quality coin holder and ensure that they are stored so that they are not likely to be bumped or jostled. He may wish to place the coins in their holders in a separate box cushioned with archival-quality foam such as Volara or a low-density Ethafoam.
Archival supply houses such as University Products (at www. universityproducts.com), Gaylord Brothers (www.gaylord.com) and Carr McLean (online at www.carrmclean.ca) carry a number of small archival boxes that could accommodate these coins. They also carry a variety of safe cushioning materials.
Regular readers will remember that although silver is a noble metal and resistant to corrosion, sulfur is one of its worst enemies.
Sulfur is ever present and hard to avoid. It can be found in foods such as eggs and onions, natural paints, fabrics (e.g., wool felt) and dyes, vulcanized rubber, cigarette smoke, paper and cardboard, and in wooden cabinets.
Sulfur is also an industrial by-product and can occur naturally. These coins should be stored away from any sources of sulfur.
If this is not possible, the reader should consider using a sulphur scavenger such as activated charcoal (e.g., 3M Silver Protector Strips). Placing the coins in a Corrosion Intercept bag or box would also provide good protection. Corrosion Intercept is unique in that it changes color from copper to black when it is time to change the bag or box. Unfortunately, activated charcoal and any of the other sulphur scavengers have no indicator telling one when they are spent.
Monitoring the condition of the coins is advisable. For effective monitoring, I recommend that the reader take good photographs of the coins and write a condition report for each one, to serve as a baseline.
If the coins’ condition begins to change, then steps should be taken to diagnose why this is happening. One’s collection is often the best indicator of an unhealthy storage environment.
Finally, it would be ideal if these coins were stored at a low relative humidity. Although this is less of an issue with silver due to its nobility, I consider it best to err on the side of caution.
Susan L. Maltby, Toronto, is a private conservation consultant, with an interest in numismatic preservation.