Editorial Feature

Spectroscopic and Microscopic Analyses of Roman Mosaic Glass Tesserae

A new study combining spectroscopic and microscopic analysis has confirmed the constituents and age of a Roman Mosaic Glass Tesserae found in the Morbio Inferiore municipality in the Canton Ticino region in Switzerland. A preprint of this yet-to-be-published research paper can be found on The Social Science Research Network (SSRN).

Spectroscopic and Microscopic Analyses of a Roman Mosaic Glass Tesserae

Image Credit: Clara/Shutterstock.com

Techniques developed to study light-matter interactions have been widely employed in Archeology. Various spectroscopy and microscopy methods at different wavelengths can disclose in-depth information about archeological findings. On many occasions, these techniques either confirm the archeological theory placing a particular finding in a specific era or offer new insights into its historical context. The process of dating archaeological remains using scientific methods is referred to as archaeometry.

Mosaic Tesserae in the Roman Era

A particular direction of scientific interest comes from the study of colored and colorless glass mosaic tesserae in the Roman era. A tiny piece of glass cut into a cubic shape for mosaic artwork was called a tessera, which is Latin for "cube." Roman mosaics used glass tesserae to create vibrant, clean hues of blue, red, and green that were not possible to achieve with harder natural stones like marble and limestone.

The glass mosaic tesserae examined in this work are the only ones identified in the current Canton Ticino territory, and because they contain golden leaf components, they are essential to comprehending the Roman villa of Morbio Inferiore and placing it within the geohistorical regional context.

Scientific Study of Glass Mosaic Tesserae

The scientific study of colored and colorless glass mosaic tesserae focuses on a number of details, including the nature and provenance of the raw materials, production technology, aesthetic characteristics like color, opacity, usage of metal foils, and archaeological consequences.

The presence of transition metal ions, the dispersion of metal nanoparticles, and the existence of crystalline phases that also serve as opacifiers all contribute to the color of the tesserae. Following the fourth century CE, tin-based compounds replaced the commonly used calcium-antimoniates and lead-antimoniates as opacifiers. Natron was a common source of flux during the Roman era.

The goal of the archaeometry research is to better understand any relationships that might exist between the materials and the glass technology used in an effort to provide appropriate support for the temporal consistency of archaeological data from the 1st to 4th century CE. This case study also provided a rare opportunity to research the vitreous mosaic tesserae in Tessin Canton.

Historical Context

The Roman villa of Morbio Inferiore was discovered inadvertently in 1920 while construction was underway on a modern villa. The villa was to be built in an isolated location on a hill overlooking the early-medieval fortress of Pontegana dominating the Breggia river plain.

The location is in Tessin Canton, Switzerland's southernmost canton, just a few hundred meters from the Italian border. A crucial stop on commerce and military routes to and from the Alpine passes that connected Milan to Chur, the Swiss plateau, and the Germanic limes, this region was strongly tied to the surrounding political and economic center of Como in Roman times.

Archaeometric Details of the Current Study

The glass tesserae discussed in this paper are colored and colorless glass, both of which were used as wall decorations.

About 250 pieces made up the first batch of materials, which comprised polychrome tesserae with a variety of colors. The second group, which consisted of just around thirty pieces, was made up of glass tesserae that are translucent, non-iridescent, and faintly greenish.

The color and the outcomes of a preliminary non-invasive investigation utilizing hand-held X-ray spectrometry were used to select the glass tesserae for scientific analysis.

Samples for micro-invasive analysis were chosen from each preliminary group. Each typical tesserae had samples produced for analysis that were between one and two millimeters in size.

With a scanning electron microscope fitted with an energy-dispersive spectrometer, the micro-structural characteristics and chemical composition of elements were examined. Laser Ablation Inductively Coupled Plasma-Mass Spectrometry was used to determine the trace element concentrations of the archaeological samples.

GLITTER software was used to reduce the amount of data, and repeated analyses were conducted to evaluate precision and accuracy. X-ray powder diffraction was also applied to samples with random orientations.

Results and Discussion

Natron is the flux used to make glass. This proof makes it very evident that the tesserae date to the Roman era. Due to complete agreement between the measured values and those recorded in the literature for the Roman and Late Roman periods, the iron and titanium concentration enables for the time to be more precisely defined.

In Southern Italy, the same period's colored natron glass tesserae have been found to contain the same amount of Na,Ca-phosphate that was detected in the colorless tesserae.

Ca-antimonate is the opacifier used for the colored blue tesserae. According to other studies, Roman glass tesserae from the first to fourth centuries CE are compatible with the crystal structure, which shows that the precipitation came from a soda-lime glass melt.

A Roman settlement in Southern Tessin was confirmed by the research that combined non-invasive and micro-invasive investigation to comprehend the composition and technological method utilized to make colored and colorless glass tesserae.

The sort of flux, opacifiers, and colorants employed are evidence that glass technology is widely used at other Roman sites.

This scientific study, conducted on a small number of tesserae due to limitations in micro-sampling, confirms the existence of a villa and, subsequently, of an owner, of a high socioeconomic level in Morbio Inferiore despite the absence of pertinent stratigraphic and archaeological data. This can be inferred from the high quality of the mosaic's glass tesserae and, more importantly, from the discovery of a batch of gold-foil tesserae that are consistent with the late antique era.

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References and Further Reading

Cavallo, Giovanni and Verga, Ilaria and Riccardi, Mariapia, First Evidence of Roman Mosaic Glass Tesserae in Tessin Canton, Southern Switzerland. Available at SSRN: https://ssrn.com/abstract=4363160 or http://dx.doi.org/10.2139/ssrn.4363160

Angelini, I., Gratuze, B., Artioli, G., 2019. Glass and other vitreous materials through history. EMU Notes in Mineralogy, 20, 87-150.

Conventi, A., Neri, E., Verità, M., 2012. SEM-EDS analysis of ancient gold leaf glass mosaic tesserae. A contribution to the dating of the materials. IOP Conf. Series: Materials Science and Engineering 32, 1-8. doi:10.1088/1757-899X/32/1/012007.

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Written by

Ilamaran Sivarajah

Ilamaran Sivarajah is an experimental atomic/molecular/optical physicist by training who works at the interface of quantum technology and business development.

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