Silver archaeology

Deteriora.tlon. Apart from physical damage that can result from carelessness, abuse, and vandaUsm, the main problem with metal objects Hes in thek vulnerabihty to corrosion (see Corrosion and corrosion control) (127,128). The degree of corrosion depends on the nature and age of the object. Corrosion can range from a light tarnish, which may be aesthetically disfiguring on a poHshed silver or brass artifact, to total mineralization, a condition not uncommon for archaeological material. 

Silver items, however, are also relatively rare in the archaeological record. The most common metal found is either copper, usually alloyed with either tin (bronze) or, in the later periods, zinc (brass), or iron. The latter contains very little lead and, because of severe corrosion problems, its survival rate is often low (but see Degryse et al., 2007). Fortunately, copper can also be characterized from its lead isotope signature, since the primary ore of copper is chalcopyrite (CuFeS2), which often co-occurs with galena (PbS) and sphalerite (ZnS). Even if the ore used is a secondary mineral formed by the oxidation of the primary deposit, the copper smelted from such a deposit would normally be expected to... 

Perhaps the simplest archaeological question that can be answered by chemical means is what is this object made from . The chemical identity of many archaeological artifacts may be uncertain for a number of reasons. Simply, it may be too small, corroded, or dirty to be identified by eye. Alternatively, it may be made of a material that cannot be identified visually, or by the use of simple tests. An example might be a metal object made of a silvery-colored metal, such as a coin. It may be pure silver (in practice, a silver alloy containing more than about 95% silver), or it could be a silver-rich alloy that still has a silver appearance (silver coins with up to 30% copper can still look silvery, in which case the precise composition may well... 

Continued analysis of Sasanian silver objects will be directed towards (1) a detailed statistical study of the data, (2) analysis of Sasanian and Umayyad coins, (3) determination of isotope ratios of lead extracted from the silver, (4) correlation between the analytical data and stylistic information obtained from an art historical and archaeological study and with information on methods of manufacture, toolmark, etc. 

ICP-MS has also been used to measure trace elements in archaeological native silver artifacts [345] in order to identify their geographical origins. The low detection limits provided by ICP-MS allowed analysis of trace elements on 3 to 15 mg of sample. The passivation of alloy steels using acid solutions has been studied by XPS measurements of the solid in combination of ICP-MS analysis of the passivation solutions [346,347]. When bullets are crushed on impact, striations cannot be used for identification. The percentage of antimony, trace element composition, and lead isotope ratios in bullets was measured for forensic evidence [348]. The lead isotope ratios were found to be the most useful evidence. 

Seven years ago we started the first systematic research program on the application of the lead isotope techniques to provenance studies in archaeology. Particular stress was placed on the sources of metals in the Mediterranean Bronze Age. For the first 2 years we worked mostly on the sources of lead and silver in Bronze Age Greece, Cyprus, and Egypt (32-36). In 1982, we pioneered the application of the lead isotope method for prove-nancing copper-based artifacts (15, 37-38). 

Law. R.W.. and J.H. Burton. 2008. Non-destructive Pb isotope sampling and analysis of archaeological silver using EDTA and ICP-MS. American Laboratory News 40(17) 14-15. 

---