Automated phase maps are an important tool for characterizing samples but the data quality must be evaluated. Common options include the overlay of phases on backscattered electron (BSE) images and phase composition averages and standard deviations. Both these methods have major limitations. We propose two methods of evaluation involving principal component analysis. First, a red–green–blue composite image of the first three principal components, which comprise the majority of the chemical variation, which provides a good reference against which phase maps can be compared. Advantages over a BSE image include discriminating between similar mean atomic number phases and sensitivity across the entire range of mean atomic numbers present in a sample. Second, principal component maps for identified phases, to examine for chemical variation within phases. This ensures the identification of unclassified phases and provides the analyst with information regarding the chemical heterogeneity of phases (e.g., chemical zoning within a mineral or mineral chemistry changing across an alteration zone). Spatial information permits a good understanding of heterogeneity within a phase and allows analytical artifacts to be easily identified. These methods of evaluation were tested on a complex geological sample. K-means clustering and K-nearest neighbor algorithms were used for phase classification, with the evaluation methods demonstrating their limitations.

Archaeometrists use a variety of analytical methods to determine trace elements in ancient Greek silver coins, for provenance studies, understanding social and technological change, and authentication. One analytical problem which is little documented is understanding the horizontal spatial heterogeneity of coin elemental composition in micro-sampled areas, which are usually assumed to be uniform. This study analysed ten ancient Greek coins representative of silver circulating in the Aegean region in the sixth to third centuries BC. Scanning electron microscopy with energy dispersive spectrometry was used to map the spatial distribution of elements on coins that were abraded to remove the patina. Time of flight-secondary ion mass spectrometry was then conducted on selected coins, mapping an area ~100 × 100 µm and depth profiling from 0 to 10 µm. These data revealed the three-dimensional elemental complexity of the coins, in particular, the heterogeneity both in the patina and beneath it. These data will guide future authentication and provenance studies of larger sample sets of ancient Greek coins including the use of line scanning for laser ablation inductively coupled plasma mass spectrometry data collection rather than spot analyses, and non-destructive analytical techniques such as X-ray fluorescence spectrometry.