Conservation of Coins


Conservation of Coins

Almost all coins are made of metals. The most common problem affecting conservation of metals is corrosion, an accelerated process of oxidation, caused by the presence of oxygen, chlorides or sulfur dioxide in the air. The process in intensified by dampness and dust (impurities that are deposited over the metals). If one leaves the dust deposited over the surface of coins to remain for a long time, it may form very small galvanic cells (1) that will stimulate corrosion. This is the reason why metals shall be kept clean and dry.

On the other hand, if two different metals keep in contact for a long time and an electrolyte (2) is formed by the action of dampness of mineral salts or impurities, an electrical current will circulate, and the less noble metal will be corroded, while the more noble will be preserved, although it may be covered by the residues of the other metal’s corrosion.

In the order of the electrochemical series, the nobler elements came at the end. The order is as follows:

Aluminum (Al), Zinc (Zn), Iron (Fe), Tin (Sn), Led (Pb), Copper (Cu), Silver (Ag), and Gold (Au). Therefore, if kept in direct contact for a certain time without cleaning or previous protection an aluminum and a zinc coin, the aluminum one will be corroded and the zinc coin will probably be covered and certainly marked by the product of this corrosion.

Therefore, one should, to the extent possible, avoid keeping side-by-side coins of different metals in the same medallist foil or drawer.

In case of lined – or veneered – coins, with different compositions for the core and the outer layer (copper lined with silver, as in Roman coins, for instance), there may be an electrolytic process between the inner and the outer parts of the same coin, leading to the destruction of the core.

Some items are clear from these data:

According to some European conservation manuals, medal drawers shall have openings for an adequate aeration of its interior. Other manuals, in general North American and Canadian, say precisely the opposite, and suggest the storage of very thin coins, such as the bracteates and other medieval coins, within compact transparent molded blocks of synthetic resin to seal the piece against air.

Brazil is a country featuring mostly tropical climates, with sharp oscillations of temperature and relative humidity of the air.

On the other hand, main coin collections concentrate in larger urban areas, with an air permanently loaded with gases and impurities.

Climatizing the areas designed for the storage of coin and medal collections might be an adequate resource for their conservation, but only on a permanent day-and-night basis. Interrupting the climatizing by some periods would cause the same effect of natural changes in temperature and relative humidity. A permanent climatizing would be hardly feasible due to the high maintenance costs attached to it. Therefore, other forms of coin conserving keep being sought, and some conclusions have been arrived at.

We have noticed that English medal drawers, with their structure of separated compartments for each drawer and small movable rods, forming a pigeon-hole for each coin, give the pieces of the collection so stored very good conserving conditions.

Regarding medal drawers that fail to offer the same conditions, a procedure started four years ago that seems to yield satisfactory results. The procedure has three phases: mechanical cleaning of the pieces; their protection with microcrystalline wax; and storage in envelopes. There are other methods for cleaning coins, as chemical cleaning, cleaning by electrolytic reduction, and ultrasonic cleaning, yet the mechanical process has been the only one seen as sufficiently safe and, mainly, economically feasible for the institution.

The process of mechanical cleaning consists of:

Additional Care.

Care on Cleaning.

1) Cells that, by chemical action or contact between two different metals of an interposed liquid, cause the establishment of an electrical current on a certain body.
2) Substance that, in either fusion or solution, may suffer electrolysis, i.e., chemical decomposition by the passage of an electrical current.
- Excerpt from "As Moedas do Brasil", by Eugenio Caffarelli