1 6 A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 2 mechanically removes some corrosion products from the metal surface. The results were achieved in 2 hours, a fraction of the typical time for metal cleaning, and with a degree of cleanliness that revealed all the details of the copper coin surface. Different results can be obtained depending on the intensity of the applied current, and can affect the rate of reduction of corrosion products to the metallic state and mechanical cleaning by the action of bubbles of hydrogen on the surface. In general, it is advisable not to work with very high currents due to the complexity of the chemical reactions that could affect the cleaning process[2]. Figures 1a and b show the state in which the 1853 copper coin was received and Figs. 1c and d show the coin after electrolytic cleaning. Figure 2 shows the 1851 coin after cleaning. The clean and polished coins show their origin, year of issue, monetary value, legend, and two laurels. The material is primarily copper with alloying elements that do not play a major role in the chemical composition (Table 1). The calamine formed (green) patina on its surface behaved as a protective barrier over time preventing corrosion; that is why the coin has an almost perfect state of preservation. METALLOGRAPHIC OBSERVATIONS OF THE 1853 HALF CENT COIN Figure 3 shows an example of the half cent 1853 Chilean coin as listed in the Standard Catalog of World Coins, also known as the Krause catalogs. The obverse side says “REPUBLICA DE CHILE” with the five-pointed star in relief, and year of minting 1853 between two points. The reverse side says “ECONOMIA ES RIQUEZA” (in English: economy is wealth), and the denomination in words is surrounded by laurels with a four-pointed star on the bottom. Note than in the 1853 coin from the study, the letter Q of “RIQUEZA” has a short outer tilde. The coin’s microstructure was investigated after etching with an alco- holic solution of 2% ferric chloride Fig. 4 — Micrograph of the 1853 historical copper coin showing corrosion from pitting. Reagent: Alcoholic solution of 2% ferric chloride (FeCl3). Magnification: 100x. TABLE 1 — COIN CHEMISTRY COMPOSITION Coin % Zn % Ni % Fe % Mn % Cu % Pb % Si % As % Bi 1853, Half cent 0.076 0.099 0.090 0.015 96.2 0.10 0.26 0.091 0.11 1851, One cent 0.063 0.069 0.062 0.016 80.0 0.066 0.48 0.10 0.079 Fig. 5 — Example of the 1851 one-cent coin in good condition. into pennies, halfpennies, farthings, half farthings, and quarter farthings. In 1852, the Mint won a contract to produce a new series of coins for France. In this, the Mint was a pioneer in the minting of bronze. In 1853, the Royal Mint was overwhelmed with the production of gold and silver coins. They even re-minted copper coins for Chile. The Birmingham Mint won its first contract to mint finished coins for Great Britain: 500 tons of copper, minted between August 1853 and August 1855, with another contract in 1856. During the peak of operation, the four presses of hit around 110,000 coins a day. CLEANING BY ELECTROLYTIC REDUCTION The two copper coins in this study were in good condition, having a well-preserved metallic core and an original surface that was covered with non-deforming corrosion products that could be reduced back to the metallic state. The researchers decided to use electrochemistry to clean the coins by electrolytic reduction[1]. This treatment creates a galvanic battery in which the metallic object to be treated acts as a cathode and a galvanized steel sheet (zinc) or an aluminum sheet acts as an anode, with a 1% M sodium hydroxide as the electrolyte. When the galvanic reaction takes place, the less noble metal, in this case the aluminum or zinc, loses electrons in favor of the most noble (copper), producing a reduction of some corrosion products back to the metallic state. At the same time, the reaction produces hydrogen, which when released in the form of bubbles
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