CORROSION AND ARCHAEOLOGICAL MINERALOGY OF A SPANISH 2 REAL COIN

Coins provide a rich legacy of the period of Spanish influence in the Americas from the discovery until the 19th Century. Spanish coins were minted in several localities using precious and base metals, predominantly gold, silver, and copper, and circulated as the principal mediums of exchange. Large numbers of coins have been recovered and provide valuable insights into Spanish colonial culture. Many of these coins were lost, as ships foundered at sea. The survival of the coins in the marine sediments depended upon the degree to which solutions have dissolved metals or to which available anions have reacted with the metals to convert them into oxides, sulfides, carbonates, chlorides, or other mineral-analogous phases. Thus, coins may serve as long term geochemical indicators of the conditions to which they have been subject while buried. The present study is of a two Real coin recovered from the wreck site of the Infante which was part of a large fleet that sank in the Florida Keys in 1733. The coin, typical of two Real coins, was stamped as an approximately 2 cm long, by 1.5 cm wide, by 0.3 cm thick, mass of copper. When recovered in the 1960's, the coin appeared as a black mass with no evidence of being a coin or even a piece of metal. Detailed examination revealed that the surface of the coin was completely converted to a layer of crystals of cuprite, Cu₂O indicative of being held in an oxidizing sedimentary environment. Sections through the coin have revealed that, although dominantly copper, many small spherical inclusions are present which contain cores of cuprite surrounded by concentric envelopes rich in arsenic and bismuth. In addition, there are numerous inclusions (<5 microns) of nearly pure silver. The preservation of these phases within the copper has protected them from oxidation or sulfidation and may provide information that can be used to ascertain the provenance of the metals and the coin's mintage.