A Complete Guide To Cleaning and Preservation of Coins, page 4
Descriptions of Heavy and Light Coin Metals, Their Alloys and Natural Changes
HEAVY METALS of LOW DURABILITY:
IRON
Iron (Fe, Ferrum) is very susceptible to oxidation occurring as rust. Iron is therefore used for coins only in times of emergency. To give some protection against rust, the iron coins minted after 1915 were provided with an artificial protective film of zinc that also resulted in tarnish.
Natural changes in iron:
1) Iron Rust - oxide forms of various compositions such as FeO, Fe2O3, Fe3O4, always depend upon the presence of moisture (water). Absolutely dry iron and completely dry oxygen do not react with each other. The iron oxide layer formed on the surface is not airtight thus allowing the atmospheric oxygen attack further the iron lying under the oxide layer.
2) Iron Carbonate (FeCO3) forms the same way as copper carbonate: after oxygen has acted and iron oxides have formed, carbonic acid exerts an influence.
ZINC
Zinc (Zn, Zincum) is very susceptible to surface oxidation. The original pale-silver appearance becomes even more pale as coins assume "field-gray" looks. Zinc is therefore used for coins only in times of emergency. The oxidation coating formed on zinc behaves oppositely to iron rust: it has the property of protecting the underlying layer against further harm of corrosion. For this reason zinc coins are never given a protective treatment in minting. Zinc is easily distinguishable from iron coins because it is nonmagnetic.
Natural changes in zinc:
1) Zinc Tarnish, basic zinc carbonate (4ZnOCO24H2O), forms under the influences of both oxygen and carbon dioxide in the presence of moisture. This white coating occurs in the form of a cohesive film, non-permeable to air and tightly adherent. The purer the zinc, or the smoother the surface, the more resistant are coins of this metal.
2) Zinc Sulfide (ZnS), white, is a further corrosion of zinc resulting from the action of sulfur.
3) Zinc Carbonate (ZnCO3), also white, is formed by the action of carbon dioxide similar to the case of copper: after oxygen has acted and zinc oxide has formed, carbonic acid exerts an influence.
TIN
Tin (Sn, Stannum) is readily melted at 449.6 °F (232 °C) and is not susceptible to oxidation. However, these advantageous properties are counteracted by its low hardness. It is thus used to a small extent for medals, and otherwise in copper alloys.
One tin alloy called Speculum (alloy of tin and bronze) was used for coins by Belgic people in 200 BC.
Natural changes in tin:
1) Tin Oxide (SnO2) protects the metal against continued corrosion, and its film is so thin as not to be visible, evidencing itself only in the form of a dulling effect.
2) Tin Plague constitutes an infectious alteration of the crystalline, but not the chemical, structure of tin, associated with a reduction in density. At temperatures below 13° C (56° F), tin's crystalline structure changes from tetragonal (beta-tin) to cubic (alpha-tin), thus pure metallic tin gradually turns into a non-metallic white powder. The rate of change reaches a maximum around -30° C (-22° F).
Tin Plague appears to spread like a disease. In this phenomenon gray spots are formed on tin medal, medallions or tin objects, which expand, proliferate, and then crumble, forming holes. Eventually the entire coin or tin object disintegrates. This phenomenon is also called a "tin disease" or "tin pest."
LEAD
Lead (Pb, Plumbum) has been used for coins on various occasions, but is the softest of all coinage metals. Lead loses its metallic luster rapidly in the air. Hard water scarcely attacks lead, but soft water, and especially distilled water, does.
Natural changes in lead:
1) Lead Oxide (PbO) forms immediately when lead comes into contact with atmospheric oxygen similar to the case of tin: the bright lead is thereby dulled.
2) Lead Carbonate (PbCO3) forms under the influence of carbon dioxide similar to the case of copper.
LIGHT METALS:
ALUMINIUM
Aluminium (Al), Aluminum in the US, does not resist oxidation (formation of alumina) when the surface is bright. Because of its light weight aluminum is used for cheap coinage only during the inflation or in emergencies. Aluminum alloy with 1% copper (for hardening) was used for coins by the German government from 1919. Another alloy called Nordic Gold (89% Cu, 5% Al, 5% Zn and 1% Sn) has been used for some Scandinavian coins.
Natural changes in aluminum:
Aluminum Tarnish (Al2O3) is an oxide form due to the effect of oxygen. The oxide film is very thin and has little mechanical resistance, therefore, if it is destroyed, the underlying metal is again attacked, with the second attack becoming visible.