Hunting colours: origin and reuse of glass tesserae from the Wierum terp (original) (raw)
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Minerals
In the current study, a set of 60 glass tesserae from two disrupted Roman mosaics located in Pordenone and Trento (northeastern Italy) are analyzed, with the aim of investigating the coloring and opacification techniques, with a focus on the causes of specific textural features. All the available colors and textures were selected for archaeometric analyses, in order to guarantee the full characterization of both assemblages and comparisons between the two sites. The applied analytical protocol comprises micro-textural and preliminary chemical characterizations of the tesserae by means of OM and SEM-EDS, mineralogical analysis of the opacifiers by XRD and chemical analysis of the glassy matrices by EPMA; in addition, on specific tesserae, micro-Raman spectroscopy, FORS, and EPR were also performed to clarify the type of opacifer, coloring ion and oxidation state, respectively. Results show that both the base-glass and the coloring/opacification techniques identified are consistent with the presumed Roman dating of the mosaics. All the tesserae are natron-based and chemically comparable with major Roman compositional groups, except for red samples. Antimony-based opacifiers are identified in most of the blue, turquoise, white, yellow and green tesserae, and copper-based opacifiers in the red ones; cobalt and copper are the most frequent ionic colorants used to obtain various shades of blue, turquoise and green colors. Despite the general comparability of both assemblages with the published data on glass tesserae coeval in age, the present study shows differences in the technological solutions used for obtaining the same color, and less common coloring and opacification techniques in three samples from Pordenone. The banded textures of some tesserae were also carefully investigated, and multiple factors influencing the changes in color (different distribution or relative abundance of opacifiers, crystal size, micro-texture, chemical composition of glassy matrix) are identified.
Roman highly coloured glass is well represented amongst mosaic tesserae, occasionally found in Italy from the middle of the 2nd century BCE and commonly used from the early 1st century CE. SEM-EDS microstructural and chemical analysis has revealed colouring elements and opacifiers. Chemical analysis has identified both natron and plant ash glasses, the former fitting five compositional types of ancient natron glass (Levantine I and II, HIMT, Wadi Natrun and Egypt II) and of Roman colourless glass. The apparent Levantine and Egyptian provenance for the ʻraw glassesʼ (once the colourants and opacifiers were removed) is discussed critically in the light of Nd and Sr isotopic results in part II.
Journal of Archaeological …, 2012
Little is known about the origin, supply pattern and production technology of Byzantine glass mosaic tesserae. In this study, we have analysed forty-eight glass tesserae from Sagalassos (Asia Minor) of different colours and from two archaeological contexts that were stratigraphically dated to the sixth century CE. The main aim was to identify the raw materials, colourants and opacifiers as well as secondary working practices that are reflected in the composition (EPMA, LA-ICP-MS analyses) and the microstructure (XRD, SEM-BSE) of the tesserae. The set of samples retrieved from the Roman Baths complex at Sagalassos is compositionally very homogeneous, representing possibly a single commission, and can be tentatively dated to the late Roman period. In contrast, the assemblage associated with the construction of a Byzantine church around the turn of the sixth century CE is more diverse, suggesting that these tesserae were produced from more than one silica source. This highlights a diversification in the supply and manufacture of glass tesserae during the Byzantine period.
Colour and Chemistry of the glass finds in the Roman villa of Treignes, Belgium
Youth in the Conservation of Cultural Heritage, Yococu 2012, 2013
Optical spectroscopy allows the identification of ionic species and, under certain conditions, the quantification of Fe 2+ and Fe 3+ . The ratio of the oxidation states of iron gives an insight into the technological aspects of production. Moreover from the transmission spectra it is possible to calculate the CIE Lab colour coordinates and the UV absorption edge. The latter parameter is strongly related to the presence of heavy elements because they disrupt the silica polymer network. The optical parameter highlights differences in the sample population allowing the definition of subgroups. A comparison between colour coordinates, iron redox ratios, UV absorption edge and the chemical composition is presented. The results provide important information about the proportion between different compositional groups available from the archaeological site and underline the potentiality of UV-Vis-NIR absorption spectroscopy as a first-step screening method for large sets of archaeological or historical glass fragments. The present case-study demonstrates the results of optical spectroscopy on a colour (Belgium).
Compositional variation in Roman colourless glass objects from the Bocholtz burial (The Netherlands)
…, 2009
We investigated the major and trace element composition and Pb and Sr isotope characteristics of a series of about 20 colourless glass objects from a single high-status Roman burial from the Netherlands (Bocholtz). The major elements show a relatively homogeneous group, with one outlier. This is corroborated by the Sr isotopes. Based on the Sb and Pb content, three major groups can be discerned, with two other outliers. This grouping is corroborated by the contents of the trace elements Bi, Sn, Ag, As and Mo, and by variations in lead isotopic ratios. On the basis of these results, we conclude that the glass of all objects was probably made with sand and lime from the same source. The variation in trace elements and lead isotope composition is most likely the result of variations in the composition of the sulphidic antimony ore(s) that were used to decolourize the glass. The composition of the Bocholtz glass is compared with that of other Roman glass, and implications for production models, trade and use of colourless glass objects are discussed. On the basis of isotopic and major element variation, we conclude that the antimony ore presumably originated from different mines.
Pp. 361-380 in: Archaeometry 47(2), 2005
, brought to light more than 400 pieces of coloured window glass dating from the fifth or sixth centuries AD . The aims of this paper are threefold: first, to characterize the shape, colour and chemical composition of the glass; secondly, to understand whether the production of the coloured window panes followed traditional Roman glazing techniques or was of a more innovative nature; and, thirdly, to provide some indications as to the overall design of these early ornamental glass windows. Forty samples of coloured glass have been analysed by wavelength-dispersive X-ray fluorescence. The results of the chemical and the technological studies showed that most of the glass was produced using recycled glass, particularly as a colouring agent. Some of the glass was made of essentially unmodified glass of the Levantine I type. The results taken together seem to confirm that raw glass from this region was widely traded and used between the fourth and seventh centuries AD . The artisans at Sion were apparently still making use of the highly developed techniques of Roman glass production. The colour spectrum, manufacture and design of the windows, however, suggest that they represent early examples of ornamental coloured glass windows.
Archaeometry, 2009
In this work we attempt to elucidate the chronological and geographical origin of deeply coloured and black glass dating between 100 bc and ad 300 on the basis of their major and trace element compositions. Samples from the western and eastern parts of the Roman Empire were analysed. Analytical data were obtained by means of a scanning electron microscope – energy-dispersive system (SEM–EDS, 63 samples analysed) and laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS, 41 samples analysed). Among the glass fragments analysed, dark brown, dark purple and dark green hues could be distinguished. Only among the dark green fragments could a clear compositional distinction be observed between fragments dated to the periods before and after ad 150. In the early samples (first century bc to first century ad), iron, responsible for the green hue, was introduced by using impure sand containing relatively high amounts of Ti. In contrast, a Ti-poor source of iron was employed, containing Sb, Co and Pb in trace quantities, in order to obtain the dark green colour in the later glass samples. The analytical results obtained by combining SEM–EDS and LA–ICP–MS are therefore consistent with a differentiation of glassmaking recipes, detectable in glass composition, occurring in the period around ad 150.
We have investigated the provenance of highly coloured Roman glass from Italy by determining strontium and neodymium isotope signatures. The results suggest that the main production area was the Levantine coast and that other potential areas are the central-western Mediterranean and possibly Egypt. The Levantine isotopic values are variable, possibly attesting to the existence of sub-zones; they overlap with values from Apulia and Ba-silicata in Italy. Mosaic tesserae and raw glasses have been compared with other isotope values and this suggests that colorant-rich raw materials were added at or near primary production sites. The isotopic signature of one glass cake from a 4th–early 3rd century BCE Sardinian wreck suggests that western Roman production might be rooted in the Phoenicio-Punic tradition. We have observed a mis-match between the five chemical types of later natron glass and the isotopic provenance signatures.
Naturally coloured, blue or green are the most common glass colours found in assemblages from the Roman world from the end of the 1st century BC onwards. In the 4th century two different compositions have come to dominate this group, 'HIMT' and 'Levantine 1' glasses, both of which are now thought to have been produced in the eastern Mediterranean. Using Romano-British glass assemblages from the 4th and 5th centuries, it is shown here that although the two naturally coloured glass types predominate, by far the most common composition in British assemblages is HIMT, although older, earlier blue-green compositions are still present. The earliest date HIMT could be identified in these assemblages is around AD 330, although two distinct compositions can be identified within this group which relate to changes in composition over time. A similar change over time is seen in the Levantine 1 glasses. The reasons for these patterns within the assemblages are explored within the archaeological evidence currently available for glass production and consumption in the Roman world.
Journal of Raman Spectroscopy, 2014
Several glass mosaic tesserae were recovered during the archeological excavation of the thermal baths at the ‘Villa dei Quintili’ in Rome and dated to the second century ad. This work reports the results of an archeometrical investigation performed, through a multi‐technique approach, on 19 colored opaque tesserae. The aims of the study were (1) the characterization of coloring and opacifying agents used for the production of the glass tesserae and (2) the definition of the technological processes involved. Colorimetric measurements allowed us to classify the tesserae in color groups, while the glassy matrix and the dispersed crystallites were characterized in detail through micro‐Raman spectroscopy, field emission scanning electron microscopy with energy dispersive X‐ray spectroscopy, laser ablation‐inductively coupled plasma‐mass spectrometry, and X‐ray powder diffraction analyses. Most of the glass shows the typical soda‐lime‐silicate composition (except for the orange and red te...