Work Package B: Comparative analyses
Work Package B1: Laser Ablation Inductively Coupled Mass Spectrometry
Although Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) has been used for the analyses of coins for several years, there has been little detailed investigation of the techniques advantages and limitations. Most LA-ICP-MS protocols assume that the metal being analysed is essentially homogenous and although some work has been published looking at concentration gradients, little has been done to understand the problems caused by macro- and micro- heterogeneity. Using some of the coins purchased for metallography, the first post-doctoral researcher will rigorously evaluate LA-ICP-MS by undertaking a programme of duplicate analyses involving multiple bulk chemical analyses of drilled samples and multi-site LA-ICP-MS analyses together with SEM-EDS investigation of the microstructures. A major potential advantage offered by laser ablation analysis to be explored is the broad-brush, low resolution characterisation of lead isotopes in coins that are too thin or small for drilled samples.
Work Package B2: muonic X-ray Emission Spectography (μXES) analysis
Some of these coins analysed by μXES will have been sampled using drilled samples and laser ablation and examined by metallography; others that are too small or thin to be sampled will provide new data on composition for Work Package A. μXES offers the possibility of a completely non-destructive, multi-elemental analysis and is based on the measurement of characteristic muonic X-rays emitted after the muon has been captured by the nuclei inside the material (Hillier, Paul and Ishida 2016). The intensity of the muonic X-ray corresponds to the number of captured muons, which is related to the atomic density. The technique offers a revolutionary way of analysing coins and other complex archaeological materials, but still requires experimentation and evaluation: currently it is not sensitive enough to detect trace elements, but a trial using an early third-century Roman denarius clearly shows the surface enrichment through the divergent copper and silver contents converging in the heart metal of the coin at a depth of over 400 microns; furthermore, the detected heart metal fineness of the coin of 45% concurs with results derived from drilled samples and MP-AES. The coins will be analysed by Dr Adrian Hillier of the ISIS Neutron and Muon Facility, Rutherford Appleton Laboratory, Harwell, assisted by the first post doctoral researcher.
Work Package B3: Metallurgy Conference
At the end of the second year of the project, or the start of the third, the team will hold a two-day conference on the scientific investigation of money and minting, with a focus on techniques. The conference would be held in Liverpool, where most of the metallurgical work on the project will be undertaken. Papers will be presented by team members, and invited speakers.