ROMAN WINDOW GLASS: A COMPARISON OF FINDINGS FROM THREE DIFFERENT ITALIAN SITES

Thirty‐three samples of window glass and five glass lumps coming from three Italian archaeological sites—the Suasa excavations (Ancona, settled from the third century bc to the fifth to sixth centuries ad ), the Roman town of Mevaniola (Forlì‐Cesena, settled from the Imperial Age up to the fourth century ad ) and Theodoric's Villa of Galeata (Forlì‐Cesena, settled from the sixth century ad onwards)—were analysed to track the changes in the chemical composition and manufacturing technology of window glass through the centuries. The aims of this study were: (1) to establish the origin of the raw materials; (2) to verify the chemical homogeneity among samples coming from different sites and/or produced using different techniques; and (3) to sort the samples into the compositional groups of ancient glass. The analysis of all the chemical variables allowed two groups to be distinguished: (a) finds from Mevaniola and Suasa; and (b) finds from Galeata. All the samples had a silica–soda–lime composition, but the analysis of minor elements—in particular, of Fe, Mn, and Ti—made it possible to split the samples into two groups, with the higher levels of these elements always found in the Galeata samples (HIMT glass). In conclusion, it can be asserted that the main differences between the samples are related to their chronology.     

THE COMPOSITION AND MANUFACTURE OF EARLY MEDIEVAL COLOURED WINDOW GLASS FROM SION (VALAIS,SWITZERLAND)—A ROMAN GLASS‐MAKING TRADITION OR INNOVATIVE CRAFTSMANSHIP?*

Archaeological excavations between 1984 and 2001 at the early Christian cemetery church in Sion, Sous‐le‐Scex (Rhône Valley, Switzerland), 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.     

MEDIEVAL GLASS FROM ROCCA DI ASOLO (NORTHERN ITALY): AN ARCHAEOMETRIC STUDY

An archaeometric study was performed on 33 medieval glass samples from Rocca di Asolo (northern Italy), in order to study the raw materials employed in their production, identify analogies with medieval glass from the Mediterranean area and possible relationships between chemical composition and type and/or production technique, contextualize the various phases of the site and extend data on Italian medieval glass. The samples are soda–lime–silica in composition, with natron as flux for early medieval glasses and soda ash for the high and late medieval ones. Compositional groups were identified, consistent with the major compositional groups identified in the western Mediterranean during the first millennium AD . In particular, Asolo natron glass is consistent with the HIMT group and recycled Roman glass; soda ash glass was produced with the same type of flux (Levantine ash) but a different silica source (siliceous pebbles, and more or less pure sand). Cobalt was the colouring agent used to obtain blue glass; analytical data indicate that at least two different sources of Co were exploited during the late medieval period. Some data, analytical and historical, suggest a Venetian provenance for the high/late medieval glass and a relationship between type of object (beaker or bottle) and chemical composition.     

SCIENTIFIC ANALYSIS OF SEVENTH‐CENTURY GLASS FRAGMENTS FROM THE CRYPTA BALBI IN ROME*

Inductively coupled plasma emission spectroscopy, reflectance spectroscopy and X‐ray diffraction were used to study seventh‐century AD glass fragments from the Crypta Balbi in Rome. All the samples were found to be silica‐soda‐lime glasses. Iron determines the colour of blue‐green, green and yellow‐green transparent glasses; chemical composition suggests deliberate addition of iron and/or manganese in about half the samples. Copper was found as the main colourant in red, pale blue and blue‐green opaque fragments; elemental copper acts as an opacifier in red glass, and calcium antimonate in white, pale blue and blue‐green glasses. Detection of antimony in transparent fragments suggests recycling of opaque mosaic tesserae.     

WASTE GLASS,VESSELS AND WINDOW‐PANES FROM THAMUSIDA (MOROCCO): GROUPING NATRON‐BASED BLUE–GREEN AND COLOURLESS ROMAN GLASSES

A collection of window‐panes, vessels and alleged waste from Thamusida has been investigated by OM, SEM–EDS, ICP–MS, ICP–OES and XAS at the Fe–K and Mn–K edges. Glass samples have been characterized as natron‐based soda–lime–silica glasses, with low magnesium and low potassium. The results have been compared with 43 reference groups available for ‘naturally coloured’ and colourless glasses of both Roman and later ages. Two main types were distinguished: RBGY 1 (R oman B lue–G reen and Y ellow 1) and RBGY 2 (R oman B lue–G reen and Y ellow 2). Given their compositional similarity to the Levantine I or, to a lesser extent, HIMT glasses, the Syrian–Palestinian coast for RBGY 2 and Egypt for the RBGY 1 have been suggested for their provenance. Most Thamusida samples have been assigned to the RBGY 2 type. A small group of Thamusida colourless vessels was included into the RC (R oman‐C olourless) compositional field; the latter still being defined. The alleged waste pieces may define a local production that should have been of secondary type. The investigations performed on local raw materials seem to discount the possibility of a primary glass‐making site. XAS measurements determined that Fe²⁺ contents ranging between 30 and 52% are able to assure an aqua blue colour; below 30%, the glasses turn light green or light yellow.     

THE APPLICATION OF A PORTABLE X‐RAY FLUORESCENCE SPECTROMETER TO THE ON‐SITE ANALYSIS OF GLASS VESSEL FRAGMENTS FROM SOUTHERN THAILAND*

Ancient glass vessel fragments belonging to the seventh to ninth centuries ad , from the Ko Kho Khao, Laem Pho and Khuan Lukpad sites in southern Thailand, were studied. The glass vessel fragment samples are a collection belonging to the Department of Archaeology, the 15th Regional Office of Fine Arts of Thailand. The chemical compositions of the glasses were analysed using a modified portable energy‐dispersive X‐ray fluorescence spectrometer (OURSTEX 100FA‐II) by the introduction of a MOXTEK^® AP3.3 polymer window (5 mm²?) to the KETEK silicon drift detector for the measurement of light elements. The non‐destructive analysis was performed at the National Museum, Phuket, in Thailand. It is shown that the glass chemical compositions belong to mineral and plant‐ash based soda–lime–silicate glass. The origins of the glass artefacts are discussed in terms of raw materials and glass decoration, and compared with previously reported similar typological glasses from sites in the port city of Rāya and the Monastery of Wadi al‐Tur in Egypt.     

Glass Fragments from the Crypta Balbi in Rome: the Composition of Eighth‐century Fragments

Eighth‐century glass fragments from the Crypta Balbi in Rome were analysed by inductively coupled plasma emission spectroscopy. The samples included fragments of artefacts as well as ingots of raw glass and wasters. All the fragments proved to be soda–lime glasses. Manganese‐to‐iron atomic ratios are highly variable and determine the colour of a large number of samples. Fairly high copper contents, generally associated with relatively high amounts of antimony and lead, were detected in some green and blue–green samples: this suggests recycling of glass in the form of opaque mosaic tesserae. All three elements are higher in eighth‐century than in previously analysed seventh‐century fragments. This may indicate greater recourse to recycled glass, related to a reduction in trade exchanges in the Mediterranean.     

Radical changes in Islamic glass technology: evidence for conservatism and experimentation with new glass recipes from early and middle Islamic Raqqa,Syria*

The chemical analysis of excavated glass fragments from dated archaeological contexts in Raqqa, Syria, has provided a detailed picture of the chemical compositions of artefacts deriving from eighth to ninth and 11th century glassmaking and glassworking activities. Evidence for primary glass production has been found at three excavated sites, of eighth to ninth, 11th and 12th century dates; the first two are discussed here. The 2 km long industrial complex at al‐Raqqa was associated with an urban landscape consisting of two Islamic cities (al‐Raqqa and al‐Rafika) and a series of palace complexes. The glass fused and worked there was presumably for local as well as for regional consumption. Al‐Raqqa currently appears to have produced the earliest well‐dated production on record in the Middle East of an Islamic high‐magnesia glass based on an alkaline plant ash flux and quartz. An eighth to ninth century late ‘Roman’/Byzantine soda–lime recipe of natron and sand begins to be replaced in the eighth to ninth century by a plant ash – quartz Islamic soda–lime composition. By the 11th century, this process was nearly complete. The early Islamic natron glass compositional group from al‐Raqqa shows very little spread in values, indicating a repeatedly well‐controlled process with the use of chemically homogeneous raw materials. A compositionally more diffuse range of eighth to ninth century plant ash glass compositions have been identified. One is not only distinct from established groups of plant ash and natron glasses, but is believed to be the result of experimentation with new raw material combinations. Compositional analysis of primary production waste including furnace glass (raw glass adhering to furnace brick) shows that contemporary glasses of three distinct plant ash types based on various combinations of plant ash, quartz and sand were being made in al‐Raqqa during the late eighth to ninth centuries. This is a uniquely wide compositional range from an ancient glass production site, offering new insights into the complexity of Islamic glass technology at a time of change and innovation.     

COLOUR ATTRIBUTES OF MEDIEVAL WINDOW PANES: ELECTRON PARAMAGNETIC RESONANCE AND PROBE MICROANALYSES ON STAINED GLASS WINDOWS FROM PAVIA CARTHUSIAN MONASTERY*

Stained glass windows from the Carthusian Monastery of Pavia, dating back to the 15th century, were studied by combining two analytical techniques: electron probe micro‐analysis (EPMA) and electron paramagnetic resonance (EPR). Chemical compositions and oxidation states of the transition ions, present as minor elements, were investigated by EPMA and EPR, respectively, in order to ascertain the role played by chromophorous ions in the glass coloration. The investigated glass can be defined as K–Ca glass, and the panes with red, green and blue colours were produced using flashing techniques.     

The Production and Circulation of Carthaginian Glass Under the Rule of the Romans and the Vandals (Fourth to Sixth Century ad): A Chemical Investigation

Fifty‐seven glass samples from Carthage dating to the fourth to sixth century ad were analysed using the electron microprobe. The results show that these samples are all soda–lime–silica glass. Their MgO and K₂O values, which are below 1.5%, suggest that they were made from natron, a flux that was widely used during the Roman period. The major and minor elements show that these samples can be divided into four groups, three of which correspond to the late Roman period glass groups that were found throughout the Roman Empire: Levantine I, and ‘weak’ and ‘strong’ HIMT. Of particular interest is our Group 2, which is technologically and compositionally similar to HIMT glass and the CaO and Al₂O₃ values of which are similar to those of Levantine I. Glass of similar composition has been reported by several authors and is predominantly found dating from the late fifth to seventh century. This could represent a ‘new’ glass group; therefore further study is needed to determine its origin. Also, this study suggests that the Vandal invasion in North Africa did not disrupt the glass trade between Carthage and the Levantine coast.     

ICP–MS ANALYSIS OF GLASS FRAGMENTS OF PARTHIAN AND SASANIAN EPOCH FROM SELEUCIA AND VEH ARDAŠ?R (CENTRAL IRAQ)*

Forty‐one glass fragments were analysed by inductively coupled plasma – mass spectrometry, determining 40 major, minor and trace elements, including rare earth elements. The fragments came from excavations carried out at the archaeological sites of Seleucia and Veh Arda??r in modern Iraq, and were dated to the Parthian and Sasanian epochs. Analytical data indicate that all the samples are silica–soda–lime glasses. Magnesium and potassium oxide contents below 1% suggest that eight out of nine glasses from Seleucia, dating from between the first and the third century ad , could have been obtained by use of an evaporite as a flux; the same conclusion can be drawn for some of the Sasanian glasses dating from the fourth and fifth centuries ad . The other glasses from Veh Arda??r, as well as the remaining sample from Seleucia, are characterized by higher contents of magnesium and potassium, which suggests recourse to plant ash; different magnesium and phosphorus contents allow one to separate these samples into two main groups, pointing to the use of different kinds of plant ash. Aluminium and calcium contents, together with trace element data, may indicate that different sands were used for preparing glasses of different composition. Samples from Seleucia and Veh Arda??r are mainly blue–green and green to yellow–green, respectively; iron and manganese contents suggest that the furnace atmosphere was mainly responsible for the development of these hues.     

LATE ROMAN GLASS FROM THE ‘GREAT TEMPLE’ AT PETRA AND KHIRBET ET‐TANNUR,JORDAN—TECHNOLOGY AND PROVENANCE

Forty‐seven window and vessel glasses from the ‘Great Temple’ at Petra (Jordan), dating to the first/second and fourth centuries ce , were analysed by EPMA for major and minor elements. A subset of 29 samples from Petra and 15 samples from Khirbet et‐Tannur were additionally analysed for trace elements, using LA–ICP–MS. Six compositional groups were identified on the basis of the presence of decolouring agents. The majority of the glasses derive ultimately from the Levantine coast, but most of the groups show evidence for highly selective recycling. The observed differences between the two sites could be related to different chronological ranges, functions, relative wealth and trade connections.     

Potassium–Calcium Glass: New Data and Experiments*

The chemical composition of potassium–calcium ‘wood‐ash’ glass reflects the elemental pattern of the involved non‐volatile base materials in quartz sand, wood ash and possibly potash. The essential elemental ratio K₂O/CaO of wood ash varies between 0.2 and 0.8, and depends on the habitat and geological substratum of the wood rather than on the tree species; ratios between 1.0 and 3.0 in wood‐ash glass are only possible when potash is added as a third base material. Melting temperatures of wood‐ash glass sensu stricto, termed K–Ca‐2, produced with the two raw materials quartz sand and wood ash, are between 1250°C and 1400°C, while those of three‐component‐glasses, termed K–Ca‐3, are between 900°C and 1250°C, according to the amount of added potash. Experimentally produced glass displays different hues, from colourless to brown, olive‐green and pink, according to the chemical composition of the wood ash. Elevated MnO concentrations between 0.5 and 3 wt% may originate from wood ash and are hence not necessarily an indicator of colour‐inhibiting additives. Phosphate stemming from wood ash is an essential discriminator between wood‐ash glass and potash–lime glass. Because wood ash contains only minor amounts of sodium, wood‐ash glass with equal concentrations of potassium and sodium is a hybrid glass type, where besides quartz sand, wood ash, possibly potash and also soda‐rich cullet have been applied for glass production.     

ROMAN AND MEDIEVAL GLASS FROM THE ITALIAN AREA: BULK CHARACTERIZATION AND RELATIONSHIPS WITH PRODUCTION TECHNOLOGIES*

Compositional investigations were performed on 81 Roman and medieval glass fragments (first to 14th centuries ad ) from four Italian archaeological sites. The samples were soda–lime–silica in composition, with natron as flux for the Roman and early medieval glass samples, and with plant ash as flux for the late medieval ones. The varying colours are due to the differing FeO, Fe ₂ O ₃ , MnO and Sb ₂ O ₃ contents. Hierarchical cluster analysis identified six compositional groups related to age, which were compared with those found in the literature. In this way, technological continuity from the Roman to the early medieval period and the appearance of plant ash technology in the ninth century, 200 years in advance of the period previously believed, are demonstrated.     

AN ASSESSMENT OF COMPOSITIONAL AND MORPHOLOGICAL CHANGES IN MODEL ARCHAEOLOGICAL GLASSES IN AN ACID BURIAL MATRIX

The degradation mechanisms of glass in a buried context result in surfaces that have been depleted in various elements. The stability of the glass is primarily affected by the burial environment and the glass composition. However, in all archaeological glasses, the corroded layer that is formed on the surface tends to be low in alkalis, high in silica and lacking in cohesion. The extent to which the material has degraded, along with the physical nature of the corrosion, has a profound effect upon a wide range of factors affecting the stability of artefacts, as well as the choice of conservation techniques to be employed. This study has a number of objectives: determination of the morphology of the surface of the leached layer in glasses of two different compositions with different surface finishes; examination of the transition between the corroded material and the unaffected substrate; and investigation of concentration profile of different elements within the surface layers, as a function of depth. The study uses two glasses, fabricated under laboratory conditions, to replicate two common glass types found in the historical environment; a soda–lime–silica glass typical of those found in the Roman period throughout the Mediterranean and northwestern Europe, and high‐lime–potash glasses typical of those of Western Europe in the late medieval period. Three different surfaces have been prepared to mimic alternative manufacturing techniques such as blown, cast and ground surfaces for each composition. The glasses have been corroded under controlled laboratory conditions to replicate the buried environment. Imaging and chemical information is obtained using SEM–EDX and morphological information using IFM to produce 3‐D mapping from topographical surfaces.     

The Production of Lead–Tin Yellow at Merovingian Schleitheim (Switzerland)*

A Merovingian crucible fragment, with internally adhering yellow glass, and yellow glass beads of the same region and period were investigated by non‐destructive XRF, optical microscopy and SEM‐EDS. Although the microstructure and chemical composition of the yellow pigment (lead–tin yellow type II, ‘PbSnO₃’) are almost identical in both the beads and the crucible, in the latter the pigment occurs in a much higher concentration. However, the glass base in the beads and the crucible is very different, indicating that the beads were not manufactured directly from the crucible. Instead, the crucible most likely served to produce lead–tin yellow, which was subsequently mixed elsewhere with a colourless soda–lime glass to produce yellow glass beads.     

THE COLOURED TESSERAE DECORATING THE VAULTS OF THE FARAGOLA BALNEUM (ASCOLI SATRIANO,FOGGIA, SOUTHERN ITALY)

Twenty‐six tesserae (red, orange, yellow, light amber, green, blue and white) from the balneum of the villa at Faragola (Ascoli Satriano, Foggia) have been examined by colorimetry, ICP–MS, ICP–OES and SEM–EDS. Different types of calcareous sands have been used as the source of silica (network former), also providing the stabilizing agent. A natron‐type soda source served as the network modifier; however, the use of a sodium‐rich plant ash and the recycling process have been hypothesized for the production of two tesserae (FT 1 red and FT 3 orange). The colouring and opacifying agents were Cu oxide (cuprite, orange), metallic copper (red), Pb antimonates (yellow), Ca antimonates (white), a mixture of copper (Cu²⁺) and Pb antimonates (green), a mixture of cobalt (Co²⁺) or copper (Cu²⁺) and Ca antimonates (blue). The light amber tesserae should owe their colour to iron (Fe³⁺) alone or associated with sulphide (S^2?) and Ca antimonates. It is likely that the Faragola tesserae were locally produced in a secondary glass workshop.     

A STUDY OF GLASS BEADS FROM THE HALLSTATT C–D FROM SOUTHWESTERN POLAND: IMPLICATIONS FOR GLASS TECHNOLOGY AND PROVENANCE*

Eighty‐one samples taken from 68 glass beads found in southwestern Poland on sites of the Lusatian culture from the Hallstatt C and Hallstatt D subphases were analysed by EPMA. A subsample of 18 of these were additionally subjected to analysis by means of LA–ICP–MS in order to validate the results obtained by EPMA. Some glass was made using mineral soda and some using plant ash rich in sodium. Both high‐magnesium soda–lime glass (HMG) and low‐magnesium soda–lime glass (LMG) were identified. A large number of samples are characterized by low MgO content and medium K₂O content (LMMK glass), combined with low concentrations of CaO and high Fe₂O₃ and Al₂O₃. All the LMMK glass contains numerous silica crystals and inclusions composed of a number of elements (most frequently Cu, Co, Sb, As, Ag, Ni and Fe). The LMMK glass was presumably made in Europe during the Hallstatt C.     

CHARACTERIZATION OF ARCHAEOLOGICAL AND IN SITU SCOTTISH WINDOW GLASS

Scottish window glass from both archaeological sites and historic buildings was examined using portable X‐ray fluorescence (pXRF) and scanning electron microscopy – energy‐dispersive X‐ray micro‐analysis (SEM–EDX). The elemental composition of the glass provides information regarding the materials used and, subsequently, an approximate range of dates of manufacture. pXRF is shown to be more vulnerable than SEM–EDX to the effects of surface corrosion and matrix effects in archaeological samples. The study showed that the production of window glass in Scotland from the 17th century onwards appears to closely parallel that in England. It also demonstrated the potential of pXRF for in situ studies of window glass in historic buildings. pXRF was used to assess two Scottish buildings; one in state care and one in private ownership. The building in state care, the Abbot's House at Arbroath Abbey, showed a uniform glass type, suggesting that the building was re‐glazed completely at some point during the late 19th or early 20th century. The building in private ownership, Traquair House, had a range of glass types and ages, demonstrating a different maintenance and repair regime. This type of data can be useful in understanding historic buildings in the future, particularly if re‐glazing is being considered.     

A Quasi Non‐destructive Microsampling Technique for the Analysis of Intact Glass Objects By Sem/edxa

A novel sampling technique for the analysis of glass is described, which involves the removal of minute particles from an object with a diamond‐coated file, followed by energy‐dispersive X‐ray analysis in the scanning electron microscope (SEM/EDXA). The particles are fixed to adhesive carbon discs and carbon coated without grinding or polishing. Mean compositions are determined for 10 arbitrarily selected particles above a minimum grain size of 150 mm and normalized to totals of 100%. Tests were carried out on two standard soda–lime–silica glasses of well‐characterized composition, using two files of different grade. The analyses showed good agreement with the accepted values of all elements. Although the precision is somewhat reduced, this highly portable and quasi non‐destructive microsampling procedure provides almost the same information as that gained from samples embedded and polished in the normal way. Its application is thought to be especially useful for the investigation of intact glass objects and ceramic glazes.