Hunting colours: origin and reuse of glass tesserae from the Wierum terp

Abstract

Mosaic glass tesserae were imported to Dutch sites during the Early Medieval period, probably to address the demand for coloured glass needed in ornamental bead manufacture. Although challenging, because of the uncertainty of the material’s context, the Wierum Early Medieval collection represents an extraordinary opportunity, being the most significant find of glass tesserae in the Netherlands to date. The combined use of electron microprobe analysis (EMPA) and hand-held X-ray fluorescence spectrometry (HH XRF) allowed us to examine glass tesserae and other vitreous samples from the site. Low levels for magnesium and potassium oxides (<1.5 wt%) and the chemical components linked with the silica source, fluxes, opacifiers and chromophores are compatible with an older Roman soda-lime-silica glass production. Alumina and calcium oxide contents together with the adoption of antimony-based opacifiers are compatible with mosaic tesserae of a first to third century AD Roman tradition. We assume that these tesserae were collected during the spoliation of a lavish building and reused for glass objects produced locally. The hypothesis seems to be confirmed by the presence in the collection of tesserae still embedded in mortar, stone tesserae fragments and a rounded fragment of Egyptian blue.

Glass in Rome during the transition from late antiquity to the early Middle Ages: materials from the Forum of Caesar

“…However, there is reason to believe that cullet travelled far beyond the borders of the peninsula. The occasional finds of loose Roman tesserae along the roads crossing the Apennine passes suggest that cullet and perhaps recycled glass in the form of ingots were transported overland from central Italy to the north [72] to reach the large recycling workshops in northern Europe and Scandinavia, where glass was not easily available [73]. Interestingly, the data available for eight-to eleventh-century glass from the emporia of the Adriatic coast offer a slightly different scenario, with recycled Roman glass and pre-seventh-century base glasses occasionally flanked by fresh natron and plant ash glasses imported from the Middle east and Asia Minor [74,75].…”

Evaluation of spectral collection strategies for identification of Dalbergia spp. using handheld laser‐induced breakdown spectroscopy

“…The handheld LIBS employed here ablates the sample and collects atomic emission spectra in a sequence of rapid laser pulses. A typical measurement would entail a small number of "cleaning shots" to remove a contaminated surface (e.g., 2) followed by a series of "data shots" that return optical emission spectra (e.g., [4][5][6][7][8][9][10][11][12]. Such a scheme is often repeated over one to 12 locations, separated by 24 μm, in a serpentine pattern.…”

“…Many spectroscopic instruments are commercially available in handheld or portable form, including Raman, infrared (IR), X‐ray fluorescence (XRF), and laser‐induced breakdown spectroscopy (LIBS). The ability to readily collect large quantities of data in the field, with minimal sample transport, preparation, and storage, makes handheld spectrometers an intriguing tool in metallurgy, 2,3 botany, 4 archeology, 5,6 mining, 7 geology, 8,9 recycling, 10 and forensics, 11 among other fields. While many traditional laboratory analyses can now be performed in the field with handheld analogs to benchtop instruments, handheld spectrometers generally have poorer resolution and signal‐to‐noise (S/N) performance.…”