A Complete Guide To Cleaning and Preservation of Coins, page 2
Descriptions of Precious and Heavy 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.
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 (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.
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 (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 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; however, the silver sulphide does not form a protective layer against surface corrosion.
• 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.
Also chlorine gas (from bleach) and sodium chloride (salt from human secretion) react with silver to produce silver chloride. Pure silver chloride is a white or transparent compound. Encrustation of dust and grime combined in the AgCl corrosion layer on a silver coin's surface can turn the coin into a darker colour. The corrosion products, made up with the sulphides and chlorides, on a silver coin's surface might also fuse with the silver substratum. That is why, to avoid accidental and irreversible damage to the coin, each silver coin should be closely examined before determining the best cleaning method for it.
HEAVY METALS of VERY HIGH DURABILITY:
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 (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 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:
NOTE: Corrosion processes that affect cupreous (copper-base alloyed) coins are very complex and involve many chemical compounds and formulas. The below-given information only "touches" the tip of the iceberg as chemistry of copper corrosion is beyond the scope of this article.
1. Cuprous Oxide (Cu2O) or Copper(I) Oxide or Cuprite is the first coating surrounding a pure copper. The cuprite's color ranges from dark red to red-brown - normal to bronze and copper based metals. On surface of a copper or copper-alloyed coin, a thin and protective layer of Cuprite is formed immediately after the coin has been struck. Cuprite degrades to Copper(II) Oxide - a black solid, in moist air. and even black
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%. The color of copper carbonate is aesthetically pleasing deep green and blue-green. Copper carbonate surrounds the copper in the form of a secondary-layer coating and is a usual form of Verdigris.
3. Verdigris is the common name for the natural patina formed when copper, brass (copper, zinc, and often lead) or bronze (copper and tin) is weathered and exposed to air or seawater over a period of time. Before verdigris can form, the copper (or its alloy) must first be oxidized as described above. In general, verdigris may consist of a mixture of the following four chemical compounds:
• It is usually a basic Copper Carbonate (CuCO3 - described above).
• Near the ocean or sea, it will be a basic Copper Chloride (CuCl2) - a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate (incorporates water molecules in its crystalline framework). Cuprous chlorides are very unstable mineral compounds that cause a complete disintegration of copper and copper-alloyed objects or coins through chemical corrosion process known as "Bronze Disease" (described below).
• If acetic acid (vinegar - CH3CO2H) is present at the time of weathering, the copper oxide is immediately transformed by acetic acid into Copper(II) Acetate [Cu(CH3COO)2] - a dark green crystalline solid, or into basic Copper Acetate [Cu(OH)CH3COO]. Acetic acid is present in the atmosphere resulting, for example, from exhaust gases of wood fires. Verdigris forms very quickly, within a few years in the atmosphere, but much longer in water.
4. 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.
5. "Bronze Disease" is irreversible chemical corrosion process that takes place when the copper and copper-alloyed objects containing cuprous chlorides are recovered and exposed to air. Cuprous chlorides in the presence of moisture and oxygen are hydrolyzed to form hydrochloric acid (HCl) and basic cupric chloride (Oddy and Hughes 1970:188). The hydrochloric acid in turn attacks the uncorroded metal to form more cuprous chloride. The reactions continue until no metal remains. Details on how to "cure" the Bronze Disease on the copper and bronze coins are given on next page.