Major to trace element imaging and analysis of iron age glasses using stage scanning in the analytical dual beam microscope (tandem)

Dark and clear silicate glasses formed during an iron age vitrification event approximate to 1500 years ago at the Broborg hillfort near Uppsala, Sweden have been analyzed using a scanning electron microscope equipped with a micro-X-ray fluorescence (mu XRF) spectrometer. Correlated mu XRF and electron beam-induced energy dispersive spectrometry (EDS) X-ray maps were collected via stage-scanning at constant velocity. This coupled procedure represents a new approach for the cultural heritage community to conduct analytical studies of archaeometric specimens composed of metal, ceramic, or mixed inorganic/organic materials, where major and trace element compositions are registered in space for areas up to the centimeter-length scale at micrometer-scale resolution. Overview images were used to select areas for EDS beam scan maps correlated with multispectral cathodoluminescence (CL) imaging and co-located quantitative EDS and mu XRF point analysis. Fe, Ca, Mg, Ti, P, Mn, Zr, Zn, and Y are enriched in the dark glass, while Si, Al, K, Na, Ba, Sr, Rb, and Ga are enriched in the clear glass. Unmelted material is comprised predominately of quartz (SiO2) along with trace apatite (Ca-5(PO4)(3)[Cl,OH]) and zircon (ZrSiO4). Multivariate statistical analysis was used to measure the area fractions of high variance components while lower variance components represented phase mixtures. Differences between calculated melt viscosities for the glass compositions are consistent with field and laboratory observations. Coupled large area EDS and pXRF imaging shows significant promise for informed selection of higher spatial resolution and higher sensitivity follow-up studies, e.g., those performed using synchrotron analysis.

Automated summary

This study analyzed dark and clear silicate glasses formed during an iron age vitrification event approximately 1500 years ago using a scanning electron microscope equipped with a micro-X-ray fluorescence (mu XRF) spectrometer. The results showed that the dark glass is enriched in iron, calcium, magnesium, titanium, etc., while the clear glass is enriched in silicon, aluminum, potassium, etc. Multivariate statistical analysis was also used to measure the area fractions of different components, providing important insights for future high-resolution and high-sensitivity studies.