Glass Mosaic Research Articles

Study of mosaic glass tesserae from Delos, Greece using a combination of portable μ-Raman and X-ray fluorescence spectrometry

The present work discusses the results of an archaeometrical investigation on glass tesserae present in the mosaics from Delos (Greece) that was conducted in situ using two complementary non-invasive techniques, namely μ-Raman and portable X-ray Fluorescence spectrometry. The main objective of this work was to provide a compositional and technical characterisation of the glass tesserae. Since they are generally coloured and opaque or translucent, the study mainly focused on the identification of the employed colouring and opacifying agents.The three hundred and fifty four mosaics from the Cycladic island of Delos (130 to 88 BCE) form one of the most relevant mosaic corpus of the Hellenistic period. These numerous mosaics are often of high quality and stand out by the large use of artificial materials, in particular glass and faience, which is remarkable for the period.During three in situ analytical campaigns, around 270 glass tesserae were analysed by pXRF and nearly 40 tesserae using portable μ-Raman spectroscopy. This integrated approach allowed to gain an overview of the raw materials and the techniques employed in the production of these glasses. In particular, for all the tesserae the use of natron as a flux could be hypothesised. The majority of red glass tesserae were identified as a high-copper high-lead glass generally referred to as sealing-wax. The colourings agents used in the other tesserae (Fe, Co and Cu ions, Pb2Sb2O7) are the common ones used in Antiquity and the antimony-based opacifiers are typical for the considered period and geographic area. Blue and turquoise tesserae remarkably show a highly variable lead concentration which appears correlated with antimony. This correlation raised the hypothesis that an antimony ore rich in lead was used for the opacification of these glasses.

Multi-methodological characterisation of calcium phosphate in late-Antique glass mosaic tesserae

The present study focuses on calcium phosphate identified in some late-Antique glass tesserae, green or blue in colour, from Padova (Italy) and Tyana (Turkey). It is generally accepted that calcium phosphate is a primary opacifier, probably obtained by deliberate addition of bone powder to a transparent glass. However, no specific crystallographic–chemical–spectroscopic studies have been performed on this important type of opacifier until now. Multi-methodological investigations on calcium phosphate, on its reaction rim, and on interactions with surrounding soda–lime–silica glass were carried out in this work by means of scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), electron microprobe (EMPA), and micro-Raman spectroscopy, in order to clarify the nature of these inclusions and provide new insights into the production technologies of such kind of glass tesserae. Our results show that calcium phosphate grains are actually hydroxyapatite characterised by an evident reaction rim, which is enriched in sodium derived from the surrounding glass and replacing calcium of the opacifier. Such a cation replacement process between opacifier and glassy matrix is also confirmed by the presence of newly formed crystals composed of sodium calcium phosphate (β-rhenanite) and calcium silicate (wollastonite, polytype 1-T). β-Rhenanite and wollastonite are generally found in the proximity of calcium phosphate grains. Notwithstanding the geographical distance between Padova and Tyana (more than 2000km), comparable results were found in all the tesserae from both sites, suggesting similar and highly standardised production technologies of late-Antique tesserae with calcium phosphate as opacifier.

Hellenistic mosaic glass vessels in Bohemia and Moravia

Imported artefacts from the Late La Tène period also include mosaic glass vessels produced using millefiori, reticella and ribbon mosaic glass techniques. The artefacts are part of the assemblages from the oppida of Stradonice and Staré Hradisko and from the Jičina-Požaha hillfort of the Púchov culture. Their origin can be traced to a Hellenistic workshop(s) in the eastern Mediterranean that was probably in operation in the second and first centuries BC. According to their chemical composition determined by means of SEM-EDS, NAA and LA-ICP-MS, the chemical type of glass of the mosaic vessels is the same as the glass used to make La Tène ring ornaments – soda-lime natron glass.

Multi-methodological study of palaeo-Christian glass mosaic tesserae of St. Maria Mater Domini (Vicenza, Italy)

This study focuses on polychrome glass tesserae from the palaeo-Christian mosaic, still in situ , which decorates the vaults of the votive chapel of St. Maria Mater Domini in Vicenza (Veneto region, NE Italy). Aims are characterisation of the “base composition” of the glassy matrix and identification of opacifiers and colourants and these are performed by means of a multi-methodological approach (SEM-EDS, EMPA, XRPD, IS). Systematic comparisons between the Vicenza tesserae and those from the votive chapel of St. Prosdocimus in Padova, part of the remains of the only other palaeo-Christian mosaic located in the Veneto, coeval in age and with comparable colour palettes, are also carried out in order to improve our knowledge of production technologies of these materials during the 6th century AD in the Northern Adriatic area. Textural, chemical, diffractometric and spectroscopic data indicate multiple production technologies in the Vicenza tesserae , with evidence of several glassy matrices, opacifiers/pigments and/or ionic colourants/decolourants, all variously mixed in order to obtain the desired shades. In particular, the matrices of the Vicenza tesserae , mostly comparable with compositional reference groups typical of both Roman and Late Roman times, and opacifiers/pigments mainly composed of antimony-based compounds typical of Roman tradition, indicate the extensive availability of older glass for re-use and/or a preference for recycling, rather than the preparation of new batches. Peculiar is also the finding, in one Vicenza Aquamarine sample, of an opacifier composed of sodium antimonate, never found in ancient glass tesserae until now. Comparisons between the Vicenza and Padova mosaics indicate technological connections between the sites, as testified by the many chromatic groups with high degree of comparability. However, when differences are observed into chromatic groups compared, the Vicenza tesserae generally show older technological features than those of Padova. The hypothesis advanced here, related to the different geographical location of the two sites, explains such experimental evidence in terms of the different speeds at which innovative technologies spread in north-eastern Italy.

Baroque glass mosaics from the Capela de São João Baptista (Chapel of Saint John the Baptist, Lisbon): unveiling the glassmaking records

For the first time, 18th‐century glass mosaics from the Capela de São João Baptista (Chapel of St. John the Baptist, Lisbon) were analysed by Raman microscopy (RM). This masterpiece in baroque mosaic art had one of its major contributors the most famous glassmaker in Rome, Alessio Mattioli. Mattioli was celebrated because of the opacity of his mosaics and the astonishing number of hues he was able to produce for mosaic decorating Saint Peter's Basilica in Rome. This study had two goals in mind: (1) characterising the materials involved in the manufacture of these glass mosaics and (2) lengthening the understanding of what was left of Mattioli's glassmaking records. As expected the mosaics presented a high ratio of crystalline phases, making RM the ideal technique for non‐destructive analysis. The mosaics contained a white ‘background’ or opacifier added identified as Ca2Sb2O7. The yellow tesserae are opacified with lead antimonate (Pb2Sb2O7) and ternary oxides, structures related to lead antimonate but with other ions entering the position of Sb4+ (namely Sn4+). Those ternary oxides are pervasive in most colours, admixed with other colorants. The red, orange, pink and brown colours were accomplished with cuprous oxide (Cu2O) and admixed with a ternary oxide to create the latter three colours. The red copper‐based colours were made according to the procedure to make a ruby copper glass and with the exception of the red colour; all mosaics exhibited a dark layer on each side of the mosaic, named scorzetta. This layer is the outcome of an oxidation reaction because of a quick cooling process and is composed of CuO. Finally the blue and green colours are accomplished with cobalt oxide and copper oxide, respectively, and the purple/black colour with manganese oxide. Copyright © 2015 John Wiley & Sons, Ltd.

Preliminary Informative Results on Glass Tesserae from V<sup>th</sup>-VI<sup>th</sup> Century AD Mosaics in Albania

This paper discusses some preliminary results regarding raw materials used in ancient glass mosaic tesserae. The studied glass tesserae were recovered during the archeological excavation of the mosaics at the Christian basilicas in Byllis, Lin and Elbasan, which according to the archeologists were built between the end of Vth and beginning of VIth century AD. In the recent years, several studies are performed on opaque coloured glass but in Albania only few studies are performed in this field. These samples are analyzed using optical microscopy and micro X-ray fluorescence. Optical microscopy is used to analyse the microstructure of the glass tesserae. The main elements and trace elements are determined qualitatively from measurements performed with micro X-ray fluorescence. The glass matrix resulted to be very heterogeneous with too many inclusions dispersed on it. Several elements which are responsible for the color and opacity of glass tesserae are determined. Comparisons are made between the same colors of glass tesserae from different archaeological sites.

Production technology of Byzantine red mosaic glasses

The study of glass tesserae found in the excavations of the St Severo basilica at Classe, Ravenna, Italy, sixth century A.D., besides a comparison between these materials and those used for the decorations of the other churches of Ravenna built in the same epoch, has stimulated a discussion on the production technology of the red tesserae, which colour is due to the presence of metallic copper and/or cuprite (Cu2O) particles. As regards the opaque red glasses of the Iron Age, Roman and Byzantine periods, studies of English authors are particularly interesting, but the conclusions of the present study are partially in contrast with what reported in the literature that is based mainly on red enamels and much less on red mosaic glasses. For the studied Byzantine red mosaic glasses it is suggested the use of lead and lead-tin alloy scraps, besides additions of ash of the vegetal fuel, to attain the suitable reducing conditions during the melting process.

X-ray and optical spectroscopic study of the coloration of red glass used in 19th century decorative mosaics at the Temple of the Emerald Buddha

Abstract The Temple of the Emerald Buddha in Bangkok, Thailand is noted for its glass mosaic decorations on exterior walls and statuary. The original mosaic artwork dates to the early 19th century and is composed of variously-colored, mirrored glass pieces. In this work, we examine the chemical composition and optical properties of the red glass manufactured at that time. Through the use of X-ray and optical spectroscopies, we demonstrate evidence that the 19th century craftsmen produced “ruby-gold” glass, wherein the red coloration is caused by the dispersal of nanoscale metallic gold particles throughout the glass matrix.

Roman glasses coloured by dissolved transition metal ions: redox-reactions, optical spectroscopy and ligand field theory

A collection of Roman glass samples from Ancient Messene, Greece, was analysed non-destructively for colouring elements and non-colouring additives such as fining agents, opacifiers and decolourizers, by comparing the chemical composition with the observed optical spectra. The resulting information on ion speciation is important for the discussion of technological parameters such as the applied melting temperatures or the prevalent redox conditions. Speciation also helps to distinguish intentionally added dopants from impurities. This knowledge might be used in provenancing the raw materials. The Roman assemblage under study included purple coloured Mn3+-containing glasses, but also samples with high levels of colourless Mn2+. Manganese ions can either be considered a residue of recycling, or are indicative of an intentional addition, either as a fining or as a decolouring agent. Antimony oxide was either added as a fining agent, resulting in good quality transparent glasses, or as crystalline antimonates which act in glass mosaic tesserae as opacifiers. The much weaker molar extinction coefficient and the partial reduction of blue Cu2+ to colourless Cu+ explain why CuO is a weaker colouring agent than CoO. The colours of iron-bearing glasses range from light blue to green and from yellow to dark brown, reflecting a complex interplay between redox conditions and the presence of sulphur in the glass melt.Quantitative speciation of dopants will be reviewed on the basis of the ligand field theory. The different transition probabilities will be demonstrated on differently doped soda lime silicate glasses which were prepared for this purpose in the laboratory.

Characterization of colorants and opacifiers in roman glass mosaic tesserae through spectroscopic and spectrometric techniques

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 tesserae). Raman results and elemental analysis prove the use of Sn–Pb antimonates to create yellow glass and of Ca‐antimonates for the white tesserae. A mixture of Sn–Pb antimonates and copper ions was used to obtain the emerald green color, while Ca‐antimonates were employed in both copper‐colored and cobalt‐colored blue glass to obtain different shades (blue‐green, dark, and light blue). X‐ray powder diffraction analyses reveal the presence of metallic copper (Cu0) and Cu2O particles (cuprite) in red and orange tesserae, respectively. These results confirm the high technological level reached by the glassmakers of the Imperial Age. Copyright © 2014 John Wiley & Sons, Ltd.

The glass mosaics tesserae of “Villa del Casale” (Piazza Armerina, Italy): a multi-technique archaeometric study

A wide selection of glass tesserae from mosaics of the Roman “Villa del Casale” (Piazza Armerina, Enna, Italy) were characterized from a chemical and textural perspective, with the aim of providing archaeometric information about the raw materials and manufacturing techniques utilized at this archaeological site of utmost importance. The studied samples are vitreous colored tesserae, both opaque and transparent. The textures and chemical compositions of the coloring and opacifying agents were determined through a multi-technique approach based on scanning electron microscopy, X-ray diffraction, electron microprobe analyses for major elements, and inductively coupled plasma mass spectrometry for trace elements. On the basis of major element contents, the samples can be classified as natron-based silica–lime glasses. As regards the opacifiers/coloring materials, a substantial use of Pb and Ca antimonates, and metallic copper was observed. A slight enrichment of heavy rare earth elements (REE) relative to light REE, and a number of positive correlations between high field-strength elements suggest that the raw materials used for the production of the Piazza Armerina tesserae included a mature sand, enriched in quartz and heavy mineral phases.

The palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy): archaeometric characterisation of tesserae with copper- or tin-based opacifiers

This paper reports the results of archaeometric characterisation of the tesserae, intentionally coloured with or without copper- or tin-based opacifiers, in the palaeo-Christian glass mosaic of St. Prosdocimus in Padova. In particular, 54 tesserae belonging to colour types Orange, Red, Brown, Green, Blue and Yellow are examined here. The multi-methodological approach (SEM-EDS, EMPA, XRPD, imaging spectroscopy in some cases coupled with XAS) gave valuable insights into the complex technologies behind palaeo-Christian glass mosaic production, with identification of various glassy matrixes typical of both Roman and Late Roman periods, and of opacifiers, both crystallised in situ (e.g., metallic copper and cuprite) and ex situ (e.g., cassiterite and lead stannate) and colourants (mainly iron, manganese and copper), all variously mixed in order to obtain the desired shades. In addition, the combination of all textural, chemical, diffractometric and spectroscopic data allows us to hypothesise that all the tesserae, used to decorate the Paduan chapel, were produced in the 6th century AD. This is because each chromatic group examined here reveals at least one technical feature typical of the 6th century and, in this context, the type of glassy matrix and/or opacifier used turns out to be particularly discriminatory. In particular, tesserae with copper-based opacifiers (cuprite and metallic copper), although the latter had been used from the Bronze Age onwards, are all characterised by the matrixes typical of the Late Roman period; tesserae with tin-based opacifiers are characterised by matrixes typical of both Roman and Late Roman periods and by opacifiers, which was not systematically used before the 4th century. However, the close compositional, textural and technological similarities of the tesserae from each chromatic group, particularly evident into those with tin-based opacifiers, is consistent with a small number of specialised workshops and skilled workers. Peculiar relationships among the oxidation states of colouring elements, their contents in the matrix, the type of opacifiers used, and the final colour of the tesserae were all identified. The correlations between copper and other associated elements (e.g., tin, zinc, antimony, iron), together with micro-textural observations, allow inferences regarding possible sources. Technological connections between Padova and Ravenna, the capital of Byzantine mosaic production in Italy, were documented, although in the case of orange tesserae good chemical correspondence was also identified with the Near East. Lastly, soda ash identified in Opaque Red and Orange tesserae are indicative of Medieval restoration.

Calibration and Use of the Heating Microscope for Indirect Evaluation of the Viscosity and Meltability of Archeological Glasses

The heating microscope is a widely used instrument in many industrial applications, especially in the ceramic field, to obtain information on the sintering behavior and, indirectly, on the viscosity curve of glass. It is therefore important for the study of glass technology. Usually, heating microscopes work by taking images of the sample outline, during a preset thermal cycle, which define five characteristic temperatures corresponding to particular shapes of the specimen. The first two characteristic temperatures can also be extracted from the sintering curves provided by the instrument. In agreement with three published models, the five temperatures are associated with five viscosity values, producing the two best‐fitting plots (Log10(η) – T). In this work, the heating microscope was calibrated using a glass with known properties, to determine the most reliable model, so that it can be applied successfully in the field of archeological science. The selected methodologies were used to analyze two opacified Roman mosaic glass tesserae excavated in Pompeii, previously characterized chemically. A Pompeiian transparent glass, sampled from a large block of base (raw) glass, and its experimental reproduction were also studied. Finally, the relationship between the opacifiers and the thermal behavior of base glass was investigated.

Characterization on mosaic glass found at Phu Khao Thong, southern area of Thailand

Glasses in forms of ornament and decorative objects have been found in Thailand for several hundred years. The mosaic glass used in this work was only one piece that excavated at Phu Khao thong archaeological site in Ranong Province, southern area of Thailand. Micro-beam X-ray fluorescence spectroscopy (μ-XRF) based on synchrotron radiation was firstly carried out to analyze its elemental composition and distribution. Scanning electron microscope coupled with energy dispersive X-ray fluorescence spectroscopy (SEM-EDS) and (PIXE) were also used to characterize the composition. The main composition of this mosaic glass sample found in Thailand was a lead-based silicate glass. The colorations were affected from transition metals, especially iron, copper and manganese. It was shown that although it look-liked the same, but the main composition was differ to that of Persia and South Asia, especially the lead content. However, it demonstrated the long distance trade or exchange network of the ancient time.

Characterization on Mosaic Glass Excavated from Phu Khao Thong (Ranong), Thailand

Abstract There were many glasses in forms of ornament and decorative objects found in Thailand for several hundred years. The mosaic glass used in this work was fragments that excavated at Phu Khao Thong proto-historic site in Ranong Province, southern area of Thailand which was a shred of glass looked-like Roman mosaic glass. Scanning electron microscope coupled with energy dispersive X-ray fluorescence spectroscopy (SEM-EDS) and proton induced X-ray emission spectroscopy (PIXE) were used to characterize the composition. Micro-beam X-ray fluorescence spectroscopy based on synchrotron radiation (μ-SRXRF) was also carried out to analyze its elemental composition and distribution. It was shown that the composition was similar to that of Roman mosaic glass which produced during the early 1st century A.D. The main composition was lead-based glass which colored from transition metals such as iron and copper. It demonstrated the long distance trade or exchange network of the ancient time.

Jerusalem Obscured: The Crescent on the Temple: The Dome of the Rock as Image of the Ancient Jewish Sanctuary

To begin with, what is it? In order to answer this question one must, of course, qualify it by asking—to whom? Pamela Berger in The Crescent on the Temple: The Dome of the Rock as Image of the Ancient Jewish Sanctuary has done a great service by supplying us with a history of the iconographic representation of Jerusalem's Dome of the Rock (the Qubbat al-Sakhrah). While no publication could ever exhaustively summarize the countless visual and literary portrayals of this world heritage site, Berger not only makes a valiant attempt at such but necessarily changes the way that almost all scholars and untrained alike look at this edifice in the present. The images she examines are worth far more than a thousand words. Various depictions of the holy site perched on Mt. Moriah in the al-Haram al-Sharif (“the Noble Sanctuary”) found across the world in Islamic homes and public buildings, on advertisements produced by the Israeli Ministry of Tourism, and memorabilia purchased by pilgrims—make this little understood but extraordinarily beautiful structure one the most well-known on the planet. Rising above white marble and once multicolored gold and glass mosaic, its famous, now gold-covered Dome has captured the attention of those looking upon the old city of Jerusalem from several directions since the end of the seventh century. Inside marble and elaborate mosaics, the largest preserved set from before the twelfth century anywhere in the area surrounding the Mediterranean Sea, provide a glorious ring around sacred rock and cave. Frankly, upon consideration of its long history and Berger's work on the history of its representation, this review author doesn't even know where to begin.

Archaeological excavations at Gamzigrad - Romuliana in 2007-2008

Archaeological excavations at Gamzigrad - Romuliana in 2007-2008

The intriguing case of silicon crystals unveiled in ancient mosaic tesserae

Mosaic glasses are very heterogeneous materials, and their characterization highly benefits from investigation by micro‐analytical techniques. In this regard, ancient mosaic tesserae belonging to Daphni Monastery (Greece, XI century) were analysed by micro‐Raman spectroscopy. In all the samples, some silicon nanocrystals were disclosed within the silica amorphous matrix of glass mosaics. This result, together with statistical analysis, suggests that a singular glass production site was the origin of all the investigated glass tesserae supporting the hypothesis of local glass making in contrast with raw materials supplied only from Middle East or Egypt. Copyright © 2012 John Wiley & Sons, Ltd.

Computer assisted glass mosaic tiling automation

Abstract: Artistic mosaic tiling applications have been used in all periods of time since the oldest ages. Nowadays the mosaics, mostly called glass mosaics, are manufactured in square forms and applied by the factories. Tiling of the mosaics on a grid base is done manually in factories to reflect a predetermined design. In this study a fully-automated system has been developed for tiling the mosaics without human intervention. All the phases, starting from a digital photo, through to the tiling of mosaics, up to the packaging of panels in the production, are performed without any manual intervention in a continuous flow. The developed system has a modular structure, so the number of modules may be increased according to the desired speed and the number of colors. A software package has been developed in order to process the image to be tiled, to convert the colors of the image to the colors of mosaics, to perform simulation before the actual production and to control the various stepper motors, relays and sensors on the machine during manufacturing.

The palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy): archaeometric characterisation of tesserae with antimony- or phosphorus-based opacifiers

The present paper reports the results of archaeometric characterisation of the opaque tesserae, intentionally coloured with antimony- or phosphorus-based opacifiers, coming from one of the two only palaeo-Christian glass mosaics known in the Veneto region (Italy), i.e., the mosaic which decorated the votive chapel of St. Prosdocimus in Padova. In particular, 55 tesserae belonging to glass types “White”, “Blue”, “Yellow”, “Green” and “Brown” are examined here. The multi-methodological approach (SEM-EDS, EMPA, XRPD, imaging spectroscopy coupled, in some cases, with XAS) gave valuable insights into the complexities of palaeo-Christian glass mosaic production technologies. Two main groups are identified, one characterised by glassy matrixes and opacifiers (calcium or lead antimonate) typical of the Roman period and comprising all “White”, “Blue”, and “Yellow” tesserae and some “Green” ones, and the other characterised by glassy matrixes and an opacifier (calcium phosphate) typical of the 6th century AD, composed of “Green” and “Brown” tesserae. This suggests that, during that century there was a gradual change from older to “new” production technologies: although new opacifiers such as calcium phosphate started to be used, the frequent use of antimony-based ones (43/55 samples) supports the hypothesis that their systematic use was extended until the 6th century, although re-using old tesserae cannot be completely excluded. In addition, comparisons with compositional groups already identified in the “gold” tesserae of the same mosaic and tesserae from Ravenna demonstrate that both the same “base compositions” of the glass were used to produce transparent and opaque glass. This evidence, coupled with the results of the historical-artistic study, suggests technological connections between Padova and Ravenna, the capital of Byzantine mosaics in Italy. Micro-structural observations and chemical analyses of the Paduan antimony-based opacified glass demonstrate that different processes and raw materials were used in their production. Both in situ and ex situ crystallisation can be identified for calcium and lead antimonate in Paduan tesserae, whereas the production of tesserae opacified with calcium phosphate generally appears to be highly standardised. Although the opacifiers used in the Paduan tesserae support technological transitions, the colouring elements identified here, i.e., iron and manganese for white, yellow, brown and some green tesserae, cobalt for blue, and copper for blue and green, suggest continuity, because their use is widely testified in the production of both transparent and opaque glass artefacts dating from the Bronze Age until Medieval times, from whichever archaeological site the samples come. Peculiar relationships among the oxidation states of colouring elements, their contents in the glassy matrix, the types of opacifiers used, and the final colour of tesserae were identified. In addition, the correlations of cobalt and/or copper with other elements, together with identification of relics of colouring and “metallic” droplets, allow us to speculate on possible sources and production technologies. Lastly, identification of newly formed crystalline inclusions in tesserae also yields information on kiln temperatures, which ranged between 900 and 1150 °C, a range easily reached in the furnaces of the 6th century AD.