On the other hand, main coin collections concentrate in larger urban areas, with an air permanently loaded with gases and impurities.Ĭlimatizing 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. 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.īrazil is a country featuring mostly tropical climates, with sharp oscillations of temperature and relative humidity of the air. (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).Īccording to some European conservation manuals, medal drawers shall have openings for an adequate aeration of its interior. (2) that the storage of coins made of different metals in direct contact shall be avoided (1) that coins shall be cleaned before stored 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. Therefore, one should, to the extent possible, avoid keeping side-by-side coins of different metals in the same medallist foil or drawer. 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. The order is as follows:Īluminum (Al), Zinc (Zn), Iron (Fe), Tin (Sn), Led (Pb), Copper (Cu), Silver (Ag), and Gold (Au). In the order of the electrochemical series, the nobler elements came at the end.
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. This is the reason why metals shall be kept clean and dry. 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. The process in intensified by dampness and dust (impurities that are deposited over the 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.
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