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:
(1) that coins shall be cleaned before stored;
(2) that the storage of coins made of different metals in direct contact shall be avoided;
(3) that it should be avoided, to the extent possible, prolonged exposure of coins to dampness and air (that, as mentioned above, contains oxygen, carbon gas and sulfur dioxide, which are accelerators of the corrosion process).
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:
The cleaning of the coin is made by friction of the piece in a cotton flannel with a small amount (a pinch) of calcium carbonate – a neutral product, with a very thin granulation not to scratch the piece (approximately the same granulation as toilet talcum). Calcium carbonate may be used in any metal.
The washing of the coin in running water, with a neutral detergent (Detertec 7, manufactured by Vetec) and a natural bristle brush, rinsing carefully the coin.
The drying in cotton flannel, followed by immersion of the piece in pure acetone (periodically changed). The coin shall be removed from the acetone bath with bamboo tweezers with rubber ends, of the type used to develop photograph negatives. Pure acetone, very volatile, promotes a quicker and more complete drying of the piece. Placing the coin over a piece of cotton fabric, a hair drier may be used for a safe drying process.
After the coin is dry, it is the time for waxing with microcrystalline wax. The microcrystalline wax, sold in slabs, is diluted in naphtha or deodorized kerosene until pasty, and applied with a flexible brush, as the drawing brushes made with marten hair. First, one applies the wax on the obverse, leaving it to dry. Afterwards, over the face, and then over the edge. When the wax is dry, its aspect is somewhat opaque. Friction with a piece of cotton fabric (the cotton flannel may leave loose threads in this phase) restores the coin a polished appearance. The polishing with microcrystalline wax seals the "pores" of the metal, protecting the coin by a inert and harmless film. Waxing has been used to replace varnishing, today inadvisable.
Envelopment of pieces has been made in ordinary coin envelopes, however one may use crystal paper or, preferably, neutral Ph papers (6-6½), disacidified (as the Salto paper, manufactured by Arjomari do Brasil, or similar papers produced by Piray).
Coins stored in envelopes shall be periodically examined because envelopes, even those made with disacidified paper, tend to acidify with time, and shall be then replaced. An acidified paper may be easily recognized by yellow spots in its surface.
Additional Care.
Always clean a coin that has been handled before storing it. Fat acids of the hand, which contain uric acid, in addition to being an ideal means for the proliferation of fungi, recompose quickly, even after washed with soap. Besides, pieces that have been handled and stored without previous cleaning may be stained by fingerprints, which may be difficult to remove.
Use a cotton flannel for quick cleaning. In museums and larger collections, the use of cotton gloves is advisable for handling coins and medals.
Contact of coins with any metal instrument or tool shall be avoided, since they may produce indelible scratches.
Pieces shall be protected when there is the need to transport them.
Care on Cleaning.
During the cleaning process, the pieces shall be protected from direct contact with hands by wearing plastic or rubber gloves.
The greenish patina over ancient copper and bronze coins is inert, imparts the piece a protective and attractive coating and shall not be removed.
No current surface cover is completely efficient to prevent attack by corrosive agents or dampness of the air. Its employment shall be coupled with the best possible conditions of storing and exhibition.
Notes:
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