Marco Verità

Wet and dry atmospheric deposition on TiO2 coated glass.

To prevent the soiling of glass window used in the built environment, the use TiO(2) coated products appears an important application matter. To test the cleaning efficiency and the sustainability of self-cleaning glass, a field experiment was conducted under real life condition, on a site representative of the background urban pollution. Samples of float glass, used as reference, and commercialized TiO(2) coated glasses were exposed to dry and wet atmospheric deposition during two years. The crossed optical, chemical and microscopic evaluations performed, after withdrawal, allowed highlighting a sensible difference between the reference and the self-cleaning substrate in terms of accumulation, nature, abundance and geometry of the deposit. This experiment conducted in real site emphasized on the efficacy of self-cleaning glass to reduce the maintenance cost.

A study of smalt and its conservation problems in two sixteenth-century wall paintings in Rome

Abstract The durability of glass depends both on its chemical composition and on the environment in which it is conserved. The glassy particles of smalt in samples from two sixteenth-century wall paintings in Rome were investigated to study the technology of glass melting and colouration, and the origin of the pigment discolouration. A potash–lime silica and an unusual soda–potash–lime silica were identified through analysis. These compositions were compared with recipes recorded in Italian medieval and Renaissance treatises that help understand the technology used in the preparation of smalt. The mechanisms of weathering are discussed in terms of glass durability in relation to its chemical composition and to aggressive environmental conditions. In the case of the two wall paintings studied here, smalt discolouration was found to be due to several factors: leaching of alkali, leaching of cobalt, formation of micro-cracks and glass corrosion.

Technology and deterioration of vitreous mosaic tesserae

Abstract Glass tesserae have been produced for use in mosaic decoration for more than 2000 years and constitute an important part of cultural heritage. They were applied to the architecture of pagan and religious monuments initially in the Mediterranean area, and later spread throughout the world. The paper examines the properties of vitreous mosaic tesserae, with particular reference to the techniques of glass melting, the colouring and opacification of glass pastes and smalti and the preparation of metal-leaf tesserae. Brief general information on the nature of glass, including its optical and mechanical properties and chemical durability are discussed and the manufacturing techniques and their development are described on the basis of historical sources and scientific analysis of finds. Particular reference is made to the different mixtures of raw materials used to make, opacify and colour glass in different centres of production in past centuries. The weathering mechanism of the tesserae is discussed, both as a consequence of the chemical quality of the glass and its environment. A review of the published literature and the analytical techniques currently available summarizes the valuable research already undertaken, but also shows that much work is still to be done in this field in order to achieve a comprehensive understanding of the evolution of this technology and to determine the most appropriate restoration and conservation techniques.

SEM-EDS analysis of ancient gold leaf glass mosaic tesserae. A contribution to the dating of the materials

Metal leaf (gold, silver or their alloys) glass tesserae began to be used in wall mosaics in the first century AD (the first examples are in Rome) and their use has been uninterrupted up to day. The metal leaf could be obtained from circulating coins, jewellery or refining. According to various techniques that have changed over the centuries, the leaf was hot fixed between two glass layers. From an archaeological point of view, it is interesting to know when and where these tesserae were made, if they were new made or if they were reused tesserae recovered from earlier dismantled mosaics. The determination of the glass composition of the tesserae is not of great help in this connection, for the same kind of glass was used over long periods. Available information is still scanter for glasses produced between the 1st to 8th centuries when the batch of raw materials (a natural soda called natron and a silica-lime sand) was melted in large tank furnaces and chunks of raw glass were transported all over the Mediterranean to be remelted and shaped into manufacts in small pot furnaces. The SEM-EDS analysis is proposed in this study as a useful tool to investigate the composition of both the glass and the gold alloy in leaf tesserae from mosaics of the 1st - 9th centuries. The comparison of the composition of the gold leaf of the tesserae with that of circulating gold coins (for which an important analytical data base is available), adds further information to the glass analysis, allowing us to improve the dating of the tesserae and increase the knowledge that may result from scientific analyses. The results demonstrate that good quantitative analyses of the metal leaf can be performed and that metal leaves made of pure gold or gold-silver alloys were used.

New insights into Byzantine glass technology from loose mosaic tesserae from Hierapolis (Turkey): PIXE/PIGE and EPMA analyses

This study focuses on the Byzantine glass tesserae from Hierapolis (Phrygia, central Turkey). Fifty-seven samples of loose tesserae from two sites in the town (the theatre and the church of St. Philip) are analysed by particule-induced X-ray emission and particule-induced gamma ray emission and electron probe X-ray microanalysis to obtain the chemical composition and identify the colourants and opacifiers. The aims are to add new information to the scant knowledge of the Byzantine glassmaking technology, to constrain the chronology of the mosaics and to trace the supply routes of the tesserae. In the destruction layers of the theatre, tesserae produced following the Roman glassmaking technology (natron glass opacified by calcium and lead antimonate) were found. They were made using a Levantine 1 raw glass, generally attributed to the early Byzantine period (fifth to sixth c.). In the church, the samples attest a technological change from Roman tradition, and a complex pattern according to building history (two phases are attested, probably in the sixth and eighth to ninth c.), and a multiplicity of supply. Three glass types and some recipes not attested before in this chronological range for the production of tesserae are documented, such as the use of a local low-chlorine natron glass for the production of black and red tesserae, the blue colouring by a source of cobalt with zinc in a natron glass tessera and the opacification with tin oxide (both in a lead-free and in a high-lead natron glass), as well as with quartz.

Thermal Stress as a Possible Cause of Paintwork Loss in Medieval Stained Glass Windows

Abstract One of the main causes of the loss of paintwork from medieval stained glass windows is the propagation of cracks in the grisaille and in the underlying glass, which cannot be considered the result of a stabilization process, but the evidence of an ongoing stress. The thermal behaviour of different-coloured painted glass pieces exposed to direct solar radiation was investigated in order to study the effect of thermal stresses on the conservation of the paintwork. Experimental results have been compared with laboratory tests as well as with measurements carried out on original stained glass windows. The grisaille showed a thermal behaviour similar to that of the glass on which it is fired. A clear correlation was found between the glass thermal cycles, the increase in stress at the glass–grisaille interface and the growth of cracks in the grisaille. According to these results, protective glazing aids the preservation of the paintwork from the risk of damage and loss, reducing the thermal stresses on the glass of the stained glass windows in terms of amplitude and frequency.

Villa del Casale (Piazza Armerina, Sicily): stone and glass tesserae in the baths floor mosaics

The Villa del Casale at Piazza Armerina (Sicily) is world famous for its beautiful and extensive mosaic floors dating to the fourth-century A.D. Before a recent restoration of the Villa, it was possible to take a representative sampling of coloured glass and stone tesserae from the baths’ floors that were examined by chemical and minero-petrographic analysis, respectively, with the goal of determining the production-centre/s of the glass, and the provenance of the stones. The laboratory techniques used were SEM-EDS on polished sections, powder XRD and polarised OM on thin sections, extended to reference stones found in a Villa’s ancient storehouse. The obtained results had shown that the white and pinkish tesserae were exclusively made of local (from the Caltanissetta province) limestones, while the black ones were cut from an obsidian, likely of a Liparote origin; the turquoise and gold-leaf tesserae were exclusively made of glass manufactured according to the Roman and Byzantine tradition while both stone and glass were used for the dark and yellow hues (giallo antico was identified among stone yellow hues). As for the glass tesserae, the results of the chemical analysis of a few samplesxa0(mostly erratic) have allowed to date them to the fifth- to sixth-century and to the eighth- to ninth-century A.D. and thus to testify to a hitherto unknown ancient restoration of the Villa mosaics.

Correction to: Villa del Casale (Piazza Armerina, Sicily): stone and glass tesserae in the baths floor mosaics

The original version of this article, unfortunately, contained error. Modifications have been made to the Introduction, Results and discussion, Conclusions and figure caption. The original article has been corrected.

Scientific investigation and study of the sixteenth-century glass jewellery collection of Archduke Ferdinand II

Abstract The glass jewellery collection of Archduke Ferdinand II, today part of the Collection of Sculpture and Decorative Arts, Kunsthistorisches Museum, Vienna, Austria, is a rare example of early modern lampwork. The small glass items, made by shaping fine glass rods and tubes in front of the flame of an oil lamp, date from before 1596. Although lampworked glass objects must have been popular collectibles, only a few pieces have survived. Therefore, the examination of this collection is a unique opportunity for the study of sixteenth-century lampworking technology. The rarity of the glass objects limited the examination to non-destructive methods. Optical microscopy provided an overview of the glass colours and surfaces. Subsequently, non-invasive portable energy dispersive X-ray fluorescence analysis (XRF) was carried out to classify the glass types. Further invasive investigations were carried out with micro-invasive electron probe microanalysis (EPMA) and energy dispersive X-ray microanalysis in a scanning electron microscope (SEM-EDX) on samples obtained from fragments of the jewellery, to identify the composition of the glasses specifically produced for lampworking, and the glassmaking tradition to which they belong.

SEM-EDS, EPMA and MRS analysis of neo-crystallisations on weathered glasses

When a glass is exposed to the atmosphere the combined action of water and environmental elements leads to the weathering of the glass surface. Both the chemical composition of the glass and the environmental parameters influence the mechanism of glass weathering mainly consisting in the formation of a layer of hydrated leached glass and salt deposits. The study of these phenomena is very important for both industrial and cultural heritage applications. In order to study the weathering mechanism of glasses exposed outdoor the chemical composition of the glass and the chemical and mineralogical nature of the reaction products should be determined. X-ray micro-analytical techniques such as scanning electron microscopy and X-ray microanalysis (SEM-EDS) and electron probe microanalysis (WD-EPMA) are traditionally employed in this type of investigation. Nevertheless particular compounds are hardly detected with this technique (for ex. nitrates) and no direct information can be obtained on the mineralogical features of the reaction products. Moreover, X-ray diffractometry (XRD) cannot be successfully used because of the very low amounts of deposited material. This work reports the results of a combined complementary X-ray fluorescence spectrometry (WD-XRF), SEM-EDS, WD-EPMA and μ-Raman spectroscopy (MRS) investigation on the surfaces of an industrial float glass and a glass reproducing the low-durable glass of ancient stained windows exposed sheltered outdoor for 6 months. The investigation enabled the identification and characterisation of different reaction products (soluble salts) confirming the need of this analytical approach for the study of the glass weathering process.