US Coins

Shell money is fragile

Collectors of shell money, like California clamshell money, shown, need to be aware that their collections are susceptible to damage if stored in an inappropriate environment. The calcium carbonate shell is susceptible to acids.

Image courtesy of Smithsonian Institution, Museum of American History.

Proper housing and storage for numismatic collections are key to their long-term preservation. This is true with traditional numismatic items like paper notes, coins, tokens and medals, and with nontraditional, or “exotic,” forms of currency.

Collectors of shell money (e.g., California clamshell money, wampum and cowrie shells) need to be aware that their collections are susceptible to damage if stored in an inappropriate storage environment. This is the first part of a two-part look at such collectibles.

Shells, from either a fresh water or marine environment, are essentially calcium carbonate. Calcium carbonate is sensitive to acids.

Shells stored in an acidic environment such as a cardboard box or plywood cabinet can develop what conservators refer to as “Byne’s Disease.” Shells with Byne’s Disease are covered in a white efflorescence resembling salt.

Byne’s disease was first identified by Loftus St. George Byne, who published his findings over 100 years ago in the Journal of Conchology. In his article, “The Corrosion of Shells in Cabinets,” he reported that he found shells kept in wooden cabinets developed a white crystalline deposit.

He referred to this condition as a “disease” as he felt it was the result of bacteria. Subsequent research showed bacteria were not to blame. It was not a disease; rather it is a symptom of an inappropriate storage environment.

Byne’s Disease — also referred to as “Bynesian Decay” — occurs when calcium carbonate reacts with acetic acid or formic acid vapors to form a calcium acetate or calcium formate salt (sometimes both). These salts crystallize on, and through, the shell’s outer surface causing irreversible damage. Shells that exhibit Byne’s Disease will, if not removed from their corrosive storage environment, eventually disintegrate.

Both acetic and formic acids are released from poor quality paper, cardboard, nonarchival adhesives (e.g., polyvinyl acetate emulsions), wood and wood composites (e.g., plywood and “chipboard”). The acids in these materials react with atmospheric moisture to create an acidic environment; essentially an acid rain. Higher temperature and higher relative humidity will speed up this reaction.

Avoid storing shells in wooden cabinets or cardboard boxes. Storage materials recommended for shell collections are similar to those recommended for the storage of other numismatic materials. These include: archival quality acid and lignin-free boxes and tissue; and safe plastics such as polyethylene, polypropylene, polyester and polystyrene. Powder-coated steel is the material museums have preferred for safe shelving and cabinetry for many years now.

Initially, it was a costly material. Thankfully, powder-coated steel cabinets are now readily available.

Susan L. Maltby, Toronto, is a private conservation consultant, with an interest in numismatic preservation.

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