Vitreous tesserae from the calidarium mosaics of the Villa dei Quintili, Rome. Chemical composition and production technology

Highlights

• Vitreous tesserae from Villa dei Quintili have been analyzed.

• Chemical composition and production technology of vitreous tesserae have been determined.

• SEM-EDS and LA-ICP-MS have been applied on vitreous tesserae analysis.

Abstract

The archaeological excavation of the Villa dei Quintili in Rome (2nd century AD) brought to light one of the most impressive residential complexes of the Roman Empire. Among the astonishing number of findings, inside and outside the buildings, a large amount of glass mosaic tesserae were gathered from the thermal bath environments. This work reports the results of a microtextural and microchemical characterization of 29 colored opaque glass tesserae, by using an analytical approach that included the use of scanning electron microscopy with energy-dispersive X-ray spectroscopy, laser ablation-inductively coupled plasma-mass spectrometry. The glass is usually a soda-lime-silicate glass, with the exception of red and orange glass tesserae, which showed the typical composition of lead glasses mixed with plant ashes. Red tesserae were likely produced with a natron-based glass mixed with plant ashes, acting as flux and reducing agent. The opacifiers were all antimony-based with different natures according to the chemical composition: calcium antimonates (white in color) and lead antimonates (yellow in color).

In the 29 tesserae studied in this research, the colors of glass were strictly correlated to the concentrations of coloring agents, the two main coloring ions identified were copper, introduced in the majority of samples as bronze scrap, and cobalt. Finally, the levels of trace elements indicate the use of mature sand, rich in quartz and poor in heavy metals and clay minerals, for the production of all samples.

Introduction

The elemental characterization of archaeological objects, together with other kinds of structural and chemical analyses, are usually carried out to establish the nature and provenance of the raw materials, to understand the technique used to produce the artefacts and possibly to attribute it to an artist or to a region. Glass objects have been produced and used in Europe for more than 2000 years and have been found in excavations since pre-Roman times. The knowledge of Roman glass comes mainly from Plinius’ writings [1], from archaeological excavations and from chemical analyses of the finds. The most reliable hypothesis regarding the production and distribution of glass in the Roman Age is that most of the glass produced in the Western provinces of the Empire came from large primary furnace sites in Middle Eastern regions [1], [2], [3], [4], where sand and flux were melted together to produce large glass chunks. Following Plinius’ description, the melting process took place in several steps and in different ovens in order to obtain a workable melt, homogeneous and without bubbles. Then, the raw chunks were traded to be worked in widely distributed secondary production sites in the West, where the ingots were re-melted, colored (or decolorized) and shaped [5], [6], [7]. Within the panorama of Roman art, parietal mosaic decoration is well-attested, but generally limited to relatively small areas. One of the most interesting pieces of evidence regards the domestic nymphaea. In the Roman world, the presence of a fountain adorned with mosaic cladding is considered as luxury indicator, and its occurrence may be interpreted as an expression of the commissioner's willingness to confer higher status to the house, even in unpretentious habitations [8]. This value can become even stronger when stone tesserae are combined with rare and expensive materials such as colored glass. The use of vitreous elements, produced on purpose in the form of tesserae or obtained from other objects, represents a relatively well-attested solution [9].

This study presents an archaeometric investigation of the glass mosaic tesserae from “Villa dei Quintili”, located in Rome. In this archaeological site, several studies have been carried out on the characterization of glass tesserae, mortars, plasters, bronzes and pigments [10], [11], [12], [13], although those works have never dealt with trace elements in glass tesserae.

The mosaic elements are mainly made of colored opaque glass, which has been produced since ancient times for the manufacturing of vessels, enamels, tesserae, and specific artefacts, like game counters. These objects are usually characterized by contrasting intense colors. The most common coloring agents present in ancient glasses are metal ions, such as Cu(II), Fe(II), Fe(III), Mn(III) and Co(II), dissolved in the glass matrix, absorbing visible light [14]. However, in some instances, glass color can arise from light-scattering by minute dispersed particles, such as gold and copper. Opacity is normally accomplished by the presence of particles segregated from a transparent matrix. Frequently, these opacifying compounds also impart color: copper or copper(I) oxide in red opaque glasses, calcium antimonate and tin dioxide in white ones, lead antimonate and lead stannate in yellow ones [15].

As Roman vessels are almost invariably translucent, numerous studies on transparent glass excavated at various locations are available [10], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], while there is limited archaeometric data regarding Roman opaque glass to date, in particular tesserae [26], [27], [28], [29], [30], [31], [32], [33].

Twenty-nine tesserae were analyzed by scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS) and laser ablation coupled with inductively plasma-mass spectrometry (LA-ICP-MS).

The LA-ICP-MS combines micro-destructivity with the capacity for analyzing a great number of trace and rare earth elements with high sensitivity in a very short time. These characteristics make LA-ICP-MS a very powerful tool for geochemical characterization of archaeological glasses.

The aim was to investigate raw materials and production technologies of Roman opaque glass tesserae and, in particular, to gather information about chromophore elements.