Making the most of expert knowledge to analyse archaeological data: a case study on Parthian and Sasanian glazed pottery

Chemical compositional data sets of archaeological artefacts are often analysed using standard statistical procedures. Adopting a different approach, we examine the major element oxides found in Parthian and Sasanian glazed pottery by identifying statistically important ratios of oxides in conjunction with the expert knowledge of the archaeological scientist during, rather than after, the identification process. This results in meaningful ratios, both statistically and archaeologically, which help identify the recipes and production practices used by Mesopotamian glass and glaze producers. The application of logratio analysis to interrogate the chronological groups suggests that the silica sources used for glazes were significantly purer in later periods and exhibited less variation. Comparing the expert-assisted selection of ratios derived from a legacy data set to the ratios from compositional data of Parthian glazed pottery recovered at the early 3rd century CE Roman military outpost of Ain Sinu in northern Iraq supports the hypothesis that there was a shift to more standardised recipes in the production of glass used for glazes. If this translates to a centralised glass-making industry, it would suggest that there are, as yet, undiscovered glass production centres, potentially in areas associated with increased urbanisation in southern Mesopotamia around the time of the Parthian–Sasanian transition.

Relative variation of ionic composition in a karstic lake under the theory of unweighted vs weighted logratio analysis

The use of logratio analysis in limnological studies has proved to be effective for solving the problems of the constrained nature of compositional data. The method offers a graphical tool, the relative variation biplot, to explore relative changes of the ions. However, recent methodological developments have shown that the results can be perturbed by low-valued ions with high variances and proposed downweighting their influences as in correspondence analysis. Additional to all properties of the unweighted version, this weighted logratio analysis extends the previous work and has the advantage of the principle of distributional equivalence. As a motivating application, we chose a karstic lake with dominating ions calcium and sulphate causing other ions to be present relatively in low absolute levels. Besides, one of the collected compositional samples was suspected to be unusual which, in part, contributes to high relative variances. This paper is therefore concerned with the choice of the best method for the analysis of such extremely saline water systems by comparing performances of both unweighted and weighted logratio analyses. We concluded that introducing weights captured more features of ionic relationships with almost all compositional variability explained. We observed that the ratio of calcium to sulphate, ammonium or phosphorus (to a lesser extent) was particularly valuable in understanding the natural chemical process of the lake. A constant log-contrast model based on calcium, ammonium, nitrate and total soluble phosphorus appeared as an equilibrium equation.