Iron oxide and iron sulfide deposits identified alongside silver flakes on a silver coin encrustation examined by SEM (x 885). 

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... November 2003, four gold and four silver coins recovered from the wreck of the sidewheel steamer SS Republic were submitted to the Microscopy Core labs in the Department of Pathology at the University of South Flor- ida’s College of Medicine in Tampa, Florida, for optical and electron microscope evaluation. Follow-up work on selected samples from the initial group of samples was continued at the PITTCON Conference in Orlando, Florida, in March 2010. SEM and X-ray analysis was conducted on samples on both occasions. The Republic sank on 25 October 1865 en route from New York to New Orleans and was discovered by Odyssey Marine Exploration in 2003 at a depth of around 500m in the Atlantic Ocean, 150km off the coast of Georgia, USA. Extensive excavations were conducted between October 2003 and February 2005 using the ROV Zeus, which led to the recording and recovery of a diverse range of cargo, including 51,404 coins, 4,135 of gold and 47,263 of silver (Bowers, 2010; Cunningham Dobson et al. , 2010; Cunningham Dobson and Gerth, 2010: 39). Upon recovery, some batches of the coins were washed and stored in deionized water, others in buckets of AgrosoakTM (Agrosoak, Inter- national, Inc., Dana Point, CA.), to help safely transport the artifacts. Some coins were placed in a jeweler’s sonic cleaning bath. The majority were kept in saltwater. Prior to conservation and study, the coins were completely covered with an encrustation that colored the coins shades of dark brown and black. The objective of the current study was to analyze the source and nature of these encrustations. The coins used in this study were washed and stored in distilled water, which prevented sea salt deposition on the coins and were air-dried prior to being submitted for microscopy. The coins were larger in diameter than the ideal sample size for the scanning electron microscope intended to be used to examine the marine deposits coating the coins. All of the coins were covered with an encrustation, a biofilm produced by marine bacteria (McNeil and Little, 1999). The biofilm had dried onto the coins dur- ing transport to the laboratory. The film was the natural product of metabolism of marine bacteria feeding on the wood, coal and metal of the shipwreck. It served as a protective environment for the bacteria to live in while the artifacts were in the sea. In an effort not to damage the artifacts, yet to examine successfully the deposits contaminating the coins’ surfaces, carbon-impregnated double-sided tape was used to gently lift the deposits from the surface of the coins. The tape was applied to the deposits, rubbed in place and pulled off the coins. When the encrustation was pulled free from the surface of the gold coins, the beautifully polished surfaces below the deposits were revealed. The encrustation had formed such a tight bond on the surface of the coins that it produced a mirrored, bas-relief image of the entire coin surface. The underlying surface of the silver coins had oxidized and discolored from years of submersion in salt water. Some areas of the surface encrustation were mounted on the tape in such a way that the outer surface of the encrustation could be photographed, while other areas of the encrustation were mounted on the tape so the surface of the encrustation facing the coins could be examined. Various areas of the encrustation were first photographed with an optical microscope (Carl Zeiss Microscopy, LLC, Thornwood, New York) and a color camera. The same areas photographed with the optical camera were also examined and photographed with a Philips 515 Scanning Electron Microscope (SEM), (FEI Corporation, Hillsboro, Oregon) equipped with an EDAX 9011 Energy Dispersive X-ray Analyzer (EDS) (EDAX, Inc., Mahwah, New Jersey) at similar magnifications. Higher magnification photographs were also obtained with the SEM. The SEM was equipped with three different types of detectors. One detector gave general photographic information about the structure of the sample, the size and shape of objects in the sample at various areas (a secondary electron detector). Another detector provided information about the presence of lighter or denser materials, such as those composed of calcium or gold (a backscattered electron detector). A third detector yielded information about the presence of elements from the periodic table, in essence different types of atoms present in the sample, revealing what various parts of the sample were made from (Energy- Dispersive X-ray Analyzer, EDX). Scientific interpretation of the optical microscope photographs, in combination with the SEM information, would enable the materials producing the encrustation on the coins to be identified. Using the optical microscope, many areas of encrustation on both gold and silver coins were observed to reflect light from their surface, indicating that these areas were either rich in metal (Fig. 1) or were crystalline in nature (Fig. 2). Deposits of coral could be identified growing in areas on the outer surface of the encrustation (Figs. 3-4). Brown deposits of iron oxide were also identified, formed in pockets on the outer surface of the black encrustation (Fig. 5). Small pieces of bright-colored metal were observed embedded in the overall black or dark brown colored matrix of the encrustation as well (Fig. 5). On the inside of the encrustation, facing one of the gold coins, a perfect mirror image of the letters ‘TY’ from the word ‘LIBERTY’ showed how tightly the encrustation was bonded to one of the gold coins (Fig. 6). Following optical microscope examination of the samples, they were coated with a 20 nanometer-thick layer of carbon, applied by evaporation, to make the samples elec- trically conductive, preparing them for observation in the electron microscope. The same samples observed by optical microscopy were then observed and photographed in the SEM. Scanning electron microscopy began with the examination of the outer surface of the encrustation formed on the silver coins. At low magnification the skeletons of marine coral were observed on the surface of the encrustation, forming disc and spherical-shaped structures (Figs. 7-8). The coral colonies were easily observed at a higher magnification of x 300 (Fig. 9). Both iron oxide and iron sulfide deposits were identified on the silver and gold coins by X-ray analysis (Figs. 10-11). These deposits sometimes contained flakes of silver (Fig. 12). Higher magnification studies of the silver coin encrustations were conducted to examine the samples for evidence of sulfur-fixing bacteria (SFB), a reported source of the iron sulfide deposits on marine archaeological remains (McNeil and Little, 1999). High magnification analysis at x 1,000 revealed areas of deposits with sulfur- and iron- rich particles that may have been formed by bacteria (Fig. 13). Some bacteria have been reported to form such shells and to live inside them, forming colonies. A nodule of spherical-shaped particles, the same size as encrusted sulfur-fixing bacteria, was observed at x 4,000 magnification (Fig. 14). A small deposit identified on one of the coins at x 2,000 magnification revealed a cluster of particles from a region of the iron sulfide deposits (Fig. 15), found to be silver sulfide, using both backscattered (L) and secondary electron detectors (Fig. 25). Meanwhile areas of copper and iron sulfide were found on the surface of another coin, observed with the secondary (Fig. 16-17) and backscattered electron detectors (Fig. 18), respectively. In some areas (Figs. 17-18), the encrustation on a silver coin was rich in iron oxide, the product of iron-oxidizing bacteria, coloring it brown, resulting in a change in the appearance of the crystal structure of the deposits in the encrustation. Some tubular iron oxide particles deposited on a silver coin were visible at high magnification (Fig. 24). Particles of this nature have been described in the literature as casts of deep-water bacteria, the iron being deposited as a shell surrounding the bacteria, resulting from their metabolic processes (Konhauser, 1997). An additional area of oxide deposits and silver particles taken from the surface of one of the silver coins was exposed (Fig. 25). A region of iron and copper sulfide deposits removed from a silver coin was again analyzed (Fig. 26). Scanning Electron Microscope examination of the iron sulfide deposits on the gold coins revealed often-smooth surfaced deposits with small colonies of coral growing on their surface. These encrustations had adhered so tightly to the surface of the coins that the stamping marks of the coins were reproduced in mirror relief in the deposit undersurfaces (Figs. 19-20). Using a secondary electron detector and a backscattered electron detector, the same view was compared (Figs. 20-21). The secondary electron detector gives general topography of a sample, while the backscattered electron detector gives atomic number information of a sample. In this supplementary examination, using the backscattered detector, the higher the atomic number of an element present in an area of the sample, the brighter that area of the sample will appear. The bright areas observed were attributable to gold particles that lifted off the coin when the encrustation was removed from the coin, while the darker areas show the presence of only the iron sulfide encrustation itself (Fig. 21). Higher magnification photography at x 200 magnification of some of the gold removed in the encrustation from this coin was viewed in both secondary and backscattered electron detector modes (backscattered mode is on the right; Fig. 22). On 3 March 2010, the samples of encrustation removed from the gold coins recovered from the wreck of ...