title: Method development for characterisation and classification of antique silver coins using LIBS and LA-ICP-MS
Creators: name: Willner, Jakob
Publisher: name: TU Wien
Year: 2019
Subjects: subject: LA-ICP-MS
subject: LIBS
subject: silver coins
subject: elemental mapping
subject: isotope ratios

description: Our knowledge of past cultures is, besides written tradition, for an unneglectable part based on remnants of those times as for example ruins of buildings, findings of excavations, grave goods, jewellery, weaponry, currency and furthermore. Historiography of older times often lacks in detail and chronology, making the mentioned remnants an important piece for nowadays historians to pick the puzzles of past happenings together. One subject of major interest in this context is the currency, because monetary changes were initiated by politically important events as changes of leadership, the start or end of wars or other events of historical importance. Conventional this is examined in the field of numismatics, which studies the appearance, coinage, shape, weight and other externalities in a historical context. Modern methods of analytical chemistry give access to other internal properties, as the chemical composition of the used materials, detection of trace components and contamination, isotopic composition, phase composition, crystallite size and shape and more. Thereby further questions can be answered regarding the tracing of raw material sources, the assessment of the quality of refining and manufacturing, and in terms of clarifying the identity, unity or difference of coins that appear the same or very similar. The enormous size and the long life time of the Roman Empire resulted in many changes in the composition of its currency, the Denarii, over time. One of the first documented major changes was under the rule of Emperor Traian (AD 98 - 117), who organized a widespread coin collection initiative to fuel the upcoming war efforts of the Dacian Wars, making the coins of the mentioned time span an interesting subject to chemical analysis. This work focuses on the characterisation and the comparison of coins from before, during and after this event. In this works focus was the method development for determining main-, minor-, and trace components, as well as the lead isotopic composition, using Laser Induced Breakdown Spectroscopy (LIBS), Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) as well as a Tandem approach (LIBS/LA-ICP-MS). The developed methods were applied on a set of 66 coins from the ruling time of Traian and his predecessor Nerva. The with LIBS determined composition of the main components, silver and copper represent the intended recipe of the alloy, whereas trace components (i.e. gold, lead, bismuth) are representative to the quality of the refining process as well as the used raw material. By obtaining spatially resolved information (LIBS imaging) a major difference from the bulk of the coins and the outer layer can be observed. These results give hints about the conditions of the storage over the last two millennia through corrosion products, with typical earth crust elements like silicon, aluminum, magnesium, calcium, sodium and potassium being present. Through presence or absence of certain elements in the bulk or the outer layer also a potential later treatment of certain coins might be assumed or excluded. The with LA-ICP-MS determined ratios of the lead isotopes 204Pb, 206Pb, 207Pb and 208Pb are locally differing from the mean natural abundance. Therefore, the determination of isotope ratios can be used to assign samples to the for manufacturing used raw materials and, if references are available, to certain mining areas. By analysing the isotope ratios of the 66 roman coins and comparing the results with regard to the traditioned consulate of Traians reign and time of original manufacturing, it could be observed that no clearly distinguishable clusters occur. While some coins of the same manufacturing time show similar isotope ratios, others are significantly different. These results suggest that, due to the size of the roman empire at that time and its numerous successful war efforts, plenty of silver sources were available in form of mines but also in form of loot from conquers. By mixing different sources of ores with other silver objects and melting them together in batches for coin production, the original lead isotopic signature is disguised and cannot be related back to the used raw materials. The developed methods were finally combined into a Tandem LIBS/LA-ICP-MS method, that acquires both the elemental composition and the lead isotopic composition quasi at the same time using the intrinsic identical sample volume, that is defined by the ablated material of the laser pulses. Thereby the gained information is maximised relative to the sample invasion, making the tandem setup to the minimal invasive method. This is of major importance in analysis of historical or art objects, as they are very valuable due to their limitation and uniqueness. While some drawbacks occur in precision of the isotope ratio determination, that can be compensated by repeated measurements, the obtained emission spectra are of the same quality as in the single LIBS method. The collected information is used to characterise and classify the set of analysed coins and to reach a better understanding of the history of individual coins. LIBS results show a significant difference of the coins bulk to the outer layer, that is often affected by contamination of the surrounding and corrosion and shows silver enrichment. The created elemental maps show inhomogeneous element distribution in most of the analysed coins. Further, particulate inclusion in the bulk of the coins are characterised. The determined lead isotope ratios show no particular classification of the coins relating to the traditioned manufacturing time. By further minor optimisation, the developed methods could also be applied on different kinds of samples, as characterisation in different sample matrices, the identification of forgeries or for technological applications.