Cleaning and Preservation of Coins - A Complete Guide
INDIVIDUAL DESCRIPTIONS OF COIN METALS, THEIR ALLOYS AND NATURAL CHANGES
It is important to know the meaning of the terms CORROSION and OXIDATION. Corrosion represents all naturally occurring chemical changes of the metal surface of coins or of the entire coin. Oxidation includes all processes in which substances absorb oxygen or give off hydrogen.
Precious Metals:
PLATINUM
Platinum (Pt) was used in Russia between 1828 and 1845 and in other countries such as Poland, Venezuela, France and Spain for coinage minting. Platinum has very high resistance to affects of air and soil: it does not oxidize, except for an extremely thin film invisible to the naked eye. Platinum is used as a bullion metal and also as a catalyst in chemical reactions.
GOLD
Gold (Au, Aurum) does not oxidize, except for an extremely thin film invisible to the naked eye, is not attacked by any acid except Aqua Regia (its components are described on page 21), and does not combine with sulfur. Some minor discoloration sometimes occur with gold.
Gold Alloys:
Pure gold is never present in coins for the following reasons: formerly gold could
not be prepared in pure state, unalloyed gold has excessive softness, and today
its pure state application is impractical due to financial reasons.
The basic gold alloys are: Red Gold - 90% gold and 10% copper; White Gold - 90% gold and 10% silver; Electrum - about 30% gold and 70% silver.
SILVER
Silver (Ag, Argentum) is the most important coinage metal because it can be found in many locations, can be easily obtained from ore, is not too soft for use or too hard to coin, and because it represents exactly the value of the coins involved, so that it can serve as the currency standard.
Silver Alloys:
Silver coins always consist of silver alloys. Chemical action occurs on the non-silver
alloy constituents; the higher the proportion of these, the more severe the effects.
Silver is chiefly alloyed with copper. Alloys with nickel are not possible.
Natural changes in silver and its alloys:
• Silver does not combine naturally with
oxygen, and thus it forms no oxides.
• Silver is tarnished by sulfur, generally in the form of hydrogen sulfide (gaseous) in the air, or through sulfur in perspiration on the hands, in leather of purses, etc. A salt, Silver Tarnish (AgS), is formed, and it is so microscopically thin that the metal is practically not attacked by it.
• Horn Silver, silver chloride (AgCl), is another salt formed by silver via combination of hydrogen chloride (HCl) in the ground with the metal. Horn silver forms a yellowish, gray to brown, and violet coating of varying thickness and is usually bumpy and fissured.
Heavy Metals of Very High Durability:
NICKEL
Nickel (Ni) is the hardest coinage metal, it does not oxidize or tarnish by sulfur compounds, except for an extremely thin film invisible to the naked eye. In the alloys with only 25% nickel, the alloy is changed from a red metal to a white one.
Heavy Metals of Low Durability:
COPPER
Copper (Cu, Cuprum) is the most important of the non-precious heavy metals for coinage. It is attacked by weak acids (verdigris) and sulfur compounds (tarnish), but after lengthy intervals the copper patina forms a second coating considered to be a mark of genuineness of age, which should not be removed in the normal state.
Copper was often used in the pure state in ancient times, but rarely so in the Middle Ages when it was more often employed as an admixture with silver that served only to achieve a silvery appearance (white copper).
Copper Alloys
Copper is quite important in the principal alloys for modern coins:
• Gun Metal, an alloy of 88% copper, 10% tin and 2% zinc, used for making cannons.
• Brass (Orichalchum in the Roman times), consisting of copper with 10-50% zinc, is used for jettons and metal marks.
• "Nickel" consists of 75% copper and 25% nickel. Franklinium is a cupro-nickel alloy developed by the Franklin Mint in the USA.
• Coinage Bronze consists of 90% copper and 10% zinc.
• Bell Metal contains bronze and 22% tin.
• Pewter, originally an alloy of tin, 15% lead and sometimes antimony and copper. Modern pewter is usually lead-free and contains copper and a small amount of tin.
• German Silver, Virenium, an alloy of 30-60% copper, 20-40% zinc, and 10-25% nickel.
• Potin, an alloy of copper, tin, lead and silver, occurring in ancient times.
• Aluminum Bronze, a gold-like, very hard alloy of 90-95% copper and 5-10% aluminum.
• Tombac, an alloy of 88% copper and 12% zinc.
• Goldine, an alloy of brass and 30% zinc.
• Billon, an alloy of copper (more than a half) and silver.
Natural changes in copper and its alloys:
1. Copper Tarnish (copper sulfide, CuS) forms
under the influence of hydrogen sulfide (H2S) analogously to silver, but copper
tarnishes considerably faster than silver, often requiring a few weeks.
2. Copper Carbonate (CuCO3) forms by first oxidizing of copper and then under influence of carbonic acid (H2CO3) that is present in the atmosphere at a concentration of about 0.03%. Copper carbonate surrounds the copper in the form of a second coating.
3. Verdigris. Before verdigris can form, the copper (or its alloy) must first be oxidized as above. The copper oxide is immediately transformed by acetic acid into copper acetate, Cu(CH3COO)2, or into basic copper acetate Cu(OH)CH3COO. Acetic acid is present in the atmosphere resulting, for example, from exhaust gases of wood fires. In general, verdigris consists of a mixture of copper acetate, basic copper acetate and copper carbonate, and forms very quickly, within a few years in the atmosphere, but much longer in water.
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