Technology and deterioration of vitreous mosaic tesserae
Marco Merita
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.
Introduction
A mosaic consists of small fragments of different materials and colours, known as tesserae, which are held in place by mortar and generally are separated by visible interstices. The earliest examples of monumental decoration in a mosaic-like technique, using natural stones worn and polished by water action, have been found in Mesopotamia and Egypt. The introduction of cube-shaped fragments of stone tesserae, which gradually superseded the use of natural pebbles in floor-covering mosaics, probably dates back to the third century BC in the Greek-Hellenistic area [1].
It may be assumed that the glass tesserae decoration of the walls and vaults of Christian churches derived directly from earlier pagan examples, even though the earliest surviving large wall mosaics date back only to the era of Cons tan tine. The opus musivum (work in mosaic) probably evolved from wall decoration used in nymphea from the middle of the first century BC. Initially the walls and vaults were studded with shells, marble chips, and fragments of glass vessels. At a later date the need to provide a richer polychrome decoration led to the introduction of cut glass tesserae, which gradually replaced the earlier materials. Glass tesserae were water-resistant and could be produced in a range of vibrant colours. By the second century, glass mosaic was extensively employed to decorate both public and private buildings throughout the Mediterranean, later spreading across the
Roman Empire.
With the advent of Christianity the expressive potential of mosaic was fully exploited, for example at Rome and Ravenna; however, it was in the Byzantine world, where glass tesserae continued to be produced, that mosaics reached their zenith. In most cases the style of the surviving eleventh- to fourteenth-century mosaics in Europe reveals a Byzantine influence, for example at Rome, Venice and Monreale. In the following centuries mosaic was appreciated mainly for its permanence of colour and resistance to weathering, and its use was confined to circumstances where it was desired to render a painting immortal, a quality noted by Vasari: '...it is certain that mosaic is the most durable picture that exists' [2, p. 252). However, in Rome, Florence and Venice, the medium was still widely appreciated and in the sixteenth century great artists, such as Titian, Tintoretto,
Veronese and Raphael, furnished cartoons for mosaic decoration.
From the fifteenth to sixteenth centuries Venice was the main centre of mosaic material manufacture. New glass pastes called smalti were created with much wider ranging shades of colour

and there was an increase in production from a few hundred, to several thousand different hues. In the eighteenth century the Vatican glass workshop carried out a research programme, which led to the production of tesserae in an almost unlimited colour range. In 1775 a new type of manufacturing process was devised, by which the molten glass was first threaded onto thin glass rods, which were then cut into tesserae measuring less than a tenth of a millimetre in diameter. These were produced in every possible shade of colour and used to create images known as minute mosaics. In the nineteenth century, both in Italy and elsewhere, mosaic production was characterized by the use of shrill colours.
All mosaics underwent restoration during the ensuing centuries. Lost tesserae were replaced with new ones, and sometimes the restoration was more deleterious than beneficial: for example, entire medieval mosaic cycles were destroyed, to be replaced with new decoration in the style of the day
[1].
Glass structure and properties
Glass is an amorphous, solid material made of oxides, which forms by progressive solidification
(viscosity increase) of a liquid obtained by melting crystalline minerals [3]. The main difference between a crystalline and a vitreous solid is in the transition from the solid to the liquid state and vice versa. In a crystalline solid it occurs instantaneously at a well-defined temperature (melting temperature), whereas in glass it occurs progressively in a range of temperatures called the glassy transition range, where the material changes from complete rigidity to a state of decreasing viscosity.
The ability to form a vitreous material with normal cooling rates is a feature of very viscous liquids, like silica (SiO
2
), in the vicinity of their solidification range. When an alkali oxide (sodium, potassium, etc.) is added to a silica glass, the network continuity partially breaks down, so that each oxygen atom cannot be linked strongly with two silica atoms as before. The oxygen will modify a
Si-O-Si bond into two Si-O
-
units:
The cations are located in the network holes in the vicinity of non-bridging oxygens, so as to form ionic bonds weaker than the silicon-oxygen bond. The breakdown of the network at several points weakens it and affects the properties of the glass. In particular, viscosity at high temperatures decreases, thus allowing the glass to be melted at lower temperatures than pure silica glass. This effect is more marked with an increasing proportion of alkali oxides. Network modifiers include alkaline earth oxides (CaO, MgO, BaO) and lead(II) oxide (PbO). The incorporation of divalent oxides into the glass structure can be described as follows:
The Ca-O bond is stronger than the Na-O bond, because of the valence 2 of the calcium ion, so that the two non-bridging oxygens are both linked to the cation. A model for the structure of a soda-lime silica glass can be set up (Fig. 1). Ternary glasses facilitate glass formation considerably.
Some physical and chemical properties are characteristic of glass. Viscosity is one of the most important properties, because it is of greatest practical relevance during the forming processes.
Glass in the molten state can be regarded as a series of fragments or macromolecules formed by the rupture of network bonds and with a limited mobility. As the temperature decreases, the number of bonds increases, thus giving rise to larger fragments with lower mobility. Viscosity is strongly influenced by the chemical composition of the glass: for instance, the introduction of modifier oxides and the consequent depolymerisation of the network following the breaking of the Si-O-Si bonds will lead to a decrease of viscosity.

The degree of thermal expansion depends on the glass composition. The introduction of network modifiers that break stronger primary bonds will yield glasses with higher expansion properties. For the same amount of added modifiers, the cation that gives weaker bonds, for instance, potassium more than sodium, lead and calcium, will cause the highest increase in thermal expansion. Due to the low thermal conductivity, glass objects must be cooled slowly after forming, to avoid the creation of stresses between the surface and the mass. In complex artefacts such as mosaic tesserae, where crystals or metal foils with different thermal expansion are hot fixed in the glass matrix, permanent and irreversible stresses will arise and any variation in the temperature of the tesserae leads to an increase in stress.
It is well known that glass is a brittle material. To explain this particular behaviour, Griffith [3] suggested the hypothesis that fragility should be ascribed to innumerable microscopic defects associated with micro-cracks resulting from mechanical or chemical damage to the surface through abrasion, corrosion, devitrification, etc. Surface defects weaken glass because stress concentrates at the tip of the micro-cracks, thus generating an extremely high load at a localized point when even a modest stress is applied [4], Static fatigue occurs in glass subjected to a continuous load, where the material's mechanical strength lowers progressively
Fig. 1 Two-dimensional representation of the network structure of quartz, vitreous silica and sodalime glass. as the period of loading increases. This phenomenon accounts for the ease with which water is capable of breaking the network bonds located at the tip of micro-cracks in glass under load through stress corrosion. Mechanical loads similar to those described are generated in mosaic tesserae by the presence of stresses resulting from incomplete annealing or arising at the interface between glass and crystals and glass and metals. Hence, fragility may become a critical issue in mosaic tesserae, all the more so in the presence of extensive surface deterioration causing weathering, abrasions, micro-cracks, etc.

Glass technology
Glass melting
In the manufacture of glass intended for making mosaics, the homogeneity of the material is not as important as it is for blown glass objects. Of special importance, is the brilliance of the surface, the largest possible variety of colours and control of transparency through partial or complete opacification.
Until the nineteenth century the melting of the glass batch (the mixture of raw materials that yields glass upon melting) was carried out in two phases. A mixture of finely ground silica, alkaline and earth-alkaline carbonates was first calcined for several hours in a reverberatory furnace at temperatures around 800 °C to obtain a white, homogeneous mass called the frit. This intermediate crystalline product no longer contained the original raw materials, but their reaction products in a solid state, comprising alkaline and calcium silicates that melt at temperatures just over 1000 °C.
The second phase, i.e. the melting, took place in another furnace where the frit was mixed with scrap glass and possibly colouring or decolourising components and opacifiers. This was heated for several hours, or even for days for more refined glasses, in refractory containers (pots) at temperatures slightly above 1000 °C, until a workable glass was obtained [4]. In the nineteenth century this two-phase process was replaced by one stage in a single pot furnace capable of reaching temperatures above 1300 °C.
The raw materials of glass have also changed over the centuries. The use of calcium carbonate as a stabilizer was only fully understood in recent centuries, with the development of modern chemistry.
This component, essential for the chemical stability of glass, was previously added unintentionally with other raw materials [4]. In Roman times, and probably until the eleventh to twelfth centuries, the batch consisted of natron and sand. Natron, a natural mineral consisting mainly of sodium carbonate, was mined from the salt lakes of Egypt and was used as a flux. Sand, the vitrifying agent, was brought from a number of precise locations, including the mouth of the rivers Volturno in Italy and Belus in Palestine. This sand contains silica and calcium carbonate in optimal ratios for glassmaking, as was demonstrated by recent analyses [5, 6]. Another system in use, probably from the eighth to ninth centuries until the nineteenth century, prepared batches with vegetable ash and silica sand or ground quartz pebbles. Sand, which contained silica only, was chosen for its purity.
The composition of the vegetable ash varied in the different glassmaking centres. Usually, soda-rich ash obtained by calcination of coastal plants was used in the Mediterranean area, while, less frequently, mixed alkali ash coming from Spain and southern France with a comparable sodium and potassium content was used [7]. Soda ash contained sodium and calcium carbonate in addition to smaller quantities of potassium, magnesium, sulphates, phosphates, chlorides and other elements.
Potash ash from continental plants was used in Northern Europe and also at some Italian sites, probably from the thirteenth century. Potash ash consisted mainly of potassium and calcium carbonates and smaller amounts of magnesium, barium, phosphates and sulphates.
In the fifteenth century production of new mosaic materials called smalti began [8, p. 11]. Beside conventional components, considerable amounts of lead oxide (between 10 and 50%) were introduced in the form of minium or litharge, and the resulting glass was also known as lead glass.
From the second half of the seventeenth century, the natural raw materials for glass were gradually replaced by synthetic products, such as nitrates, borax, arsenic, etc., and sand of increasing purity was used. This evolution enabled glassmakers to develop new glasses, but several vitreous materials of low durability were produced before reliable formulae were devised. The advent of modern chemistry in the late eighteenth century led to a scientific understanding of glassmakers' secrets and finally freed this technology from its traditional empiricism.

Colouring and opacification
The traditional colouring agents for transparent glass are a limited number of metal ions, which, by absorbing part of incident light, give rise to colouring. The colour depends on the nature of the ion, its oxidation state and coordination and on the composition of the glass in which it is melted.
Cobalt, for example, gives only a blue colour, while copper gives blue-green (Cu 2+ ), dark green
(Cu
+
) and red, if reduced completely to the state of metal copper. In the latter case the colouring is no longer of an ionic type, but colloidal. Similarly, colloid gold particles give a red colour (gold ruby). Among the commonest ionic colorants is manganese (used for violet and brown) and iron
(from light green to blue and yellow-amber). Iron is a natural contaminant of the raw materials of glass and in order to obtain a clear uncoloured glass a decolourizer, such as manganese, antimony or arsenic must be added.
Combinations of various oxides are used to prepare other colours, such as black (manganese-ironcobalt) and dark green (copper-cobalt). Only in the eighteenth century were new ionic colourants introduced, such as chrome (yellow and green) and later selenium, alone or in combination with cadmium (pink and red) and rare earths such as neodymium, cerium, etc. [9].
The technique of colouring with pigments is also used, where coloured materials that are stable at high temperatures are dispersed in the glass. The pigments can be added to the molten glass as a fine powder, which is then rapidly incorporated and cooled to avoid dissolution, producing what are known as primary crystals. Alternately, in what is known as the devitrification process, the pigment particles can be made to separate during cooling from a homogenous melt containing suitable components to produce what are known as secondary crystals. The glass produced will be opaque or translucent according to the nature, size and quantity of the crystals formed and the colour will be the outcome of the combination of the colour of the transparent glass and the crystalline pigment.
This technique was mainly used to produce yellow, orange and red glass and to modify green and brown shades.
White opaque or semi-transparent glass was prepared by adding crystalline material, bubbles or saline droplets, which create discontinuities in the refractive index, thus causing the light to scatter.
Various kinds of opacifiers were used at different historical periods. The information available on this subject is still fragmentary and a precise chronology cannot be constructed. Calcium antimonate was employed in Roman times and it was later replaced by tin oxide. The use of antimony and tinbased opacifiers, common in Byzantine and Islamic glasses, does not appear to have been adopted by European glassmakers [10]. The use of tin oxide in Western glassmaking dates back to the early fourteenth century and sporadically is found even earlier in certain localities [11, 56], whereas by the eighth century it was already in use for ceramics in the Islamic world [12]. Calcium phosphate bone ash, a less intense opacifier, was probably introduced from the fourteenth century and lead arseniate, introduced by the Venetian glassworks, appears to have been used from the seventeenth century. Calcium fluoride was introduced in the nineteenth century and it is still the most widely used opacifier in the modern glass industry.
The technique of colouring glass tesserae with metal foil is widely used in mosaic work. In the
Roman period it was discovered that thin metal leaves of gold, silver and their alloys could be made to adhere to glass. The leaves are prepared by beating metal fragments to produce very thin sheets, less than 1um in thickness. At present 20 grams of gold (a cubic centimetre) yield about 6 square metres of mosaic slab. Silver leaf is prepared in greater thicknesses, since it is a less ductile metal.
The metal leaf is either made to adhere to cast glass slabs 5-10 mm thick (the support) or to a thin sheet of blown glass less than 1 mm thick (the cartellina), which will protect it against oxidation and add to its brilliance. The whole piece is heated in a furnace until the glass softens and it is then pressed to ensure good adhesion of the three layers. Several difficulties have to be overcome in producing these tesserae, and only with much experience can durable adhesion be ensured. In times of technological decline, for instance, in the eighteenth century, these difficulties led to the use of tesserae prepared by cold gilding on glass.

The historical sources
The preparation of mosaic glass requires a sophisticated technical expertise that has been preserved jealously for centuries in glassmakers' books of secrets (recipe books). Even today, the quality of mosaic tesserae is still strictly dependent on the experience and skill of the technician who selects the raw materials and mixes them in optimal ratios, prepares the intermediate products to be added at later stages and carefully controls the operative reproducibility of the melting process [13]. An important source of information about the techniques used to prepare mosaic glass slabs in the past are the 'collectors of secrets', whose texts include significant information obtained from glassmakers. In one of the most celebrated manuscripts in the history of artistic techniques, De
Diversis Artibus, compiled in the twelfth century by the German monk Theophilus, there are thirty- one chapters on glassmaking [14]. Although his work does not relate to the production of mosaic glass specifically, Theophilus was well-acquainted with stained glass technology. In Book II,
Chapter 12, he mentions the re-use of mosaic tesserae being applied to metal substrates to produce enamels:
'Different kind of glass, namely, white, black, green, yellow, blue, red and purple are found in mosaic work in ancient pagan buildings. These are not transparent but are opaque like marble, like little square stones, and enamel work is made from them on gold, silver and copper...' [14, p. 59]
This may suggest that the production technology of mosaic glass was generally unknown in
Western Europe at that time.
Another interesting text is the so-called Ricettario Darduin, the earliest recipe book compiled by the
Venetian glassmaker Giovanni Darduin (1585-1654), who produced enamels, beads and mosaic slabs. He collected and arranged logically the recipes of his father, Nicolo, as well as recipes that he had tried out from two other manuscripts in his possession [15]. A large number of recipes for mosaics, which Darduin transcribed from a text dating from 1523 are very similar to those included in the Montpellier codex described below, and some are identical, thus demonstrating a common earlier origin.
Archive documents are an additional source of information, for example, the Orvieto Archives provide a detailed description of the way the mosaics were made for the facade of the Cathedral between 1359 and 1390 [8, pp. 351-5; 16], Manuscript 797 in the Florence State Archives consists of three treatises of different provenance dating back to the early fifteenth century, first published by Milanesi in 1864 [17]. Part of this manuscript is concerned with the preparation of glass pastes for mosaics. As Zecchin points out in a series of articles on these treatises, the second and third books are probably of Venetian origin [8, pp. 211-26; 18, pp. 103-21]. The sixteenth-century manuscript H. 486 in the library of the Ecole de Medecine in Montpellier consists of ancient
Muranese glassmaking recipes, which have been transcribed and translated by Zecchin [18, pp. 247-
76]. The first thirty pages, dated 1536, contain recipes for making coloured glass, 25 of which relate to smalti for mosaics. The first printed book on glass technology, L'Arte Vetraria by Antonio Neri, published in 1612, does not include any recipe for the preparation of mosaic glass [19].
These historical sources give much information about glassmaking in general and mosaic production in particular. Manuscript 797 in the Florence State Archives, gives a recipe for the preparation of lattimo (milk-white opaque glass), which prescribes the calcination of tin and lead in a ratio 2:1 until a white powder of metal oxides is formed, which is then ground and melted with silica and soda ash. A soda-lime-lead glass is thus obtained, in which white microcrystals of tin oxide (cassiterite) are dispersed. Other recipes suggest the same ingredients in varying proportions.
The Florentine treatise also includes a recipe to make turquoise mosaic glass opacified with bone ash.
Darduin's recipe book indicates a way to make lattimo by adding calcium antimonate to the batch, as in Roman opaque white glasses. It is not known how this lost technique reappeared [9,
10]. Darduin mentions it as a novelty, less expensive to produce than the conventional calcined lead

and tin. This manuscript includes another recipe for opaque green glass for mosaics in six to seven different hues by adding increasing amounts of giallolino (a vitreous opaque light yellow intermediate product) to a dark green soda-lime-lead-silica glass opacified with calcined lead and tin or bone ash. This compound is prescribed in several recipes by Darduin and in the Montpellier treatise, although it is not mentioned in the three Florentine books. It is one of the secrets of the new smalti, which extended the previously limited colour range of traditional glass paste mosaics. It was usually prepared by adding lead oxide and calcined lead and tin to a molten lead glass, which was then cast and ground into powder in the form of lead stannate. Other yellow pigments mentioned in the recipe books are lead antimonate and a mixture of lead, tin, antimony and zinc. Shades from orange to brown were obtained by adding small quantities of iron oxide to the batch. Natural yellow stannate and antimonate minerals are rarely found [20] and, at least in the medieval period, it is likely that their use in making mosaics was restricted to the artificially-produced minerals.
Darduin also reports on an interesting prescription to prepare orange glass, another colour used in
Roman mosaics and then lost during the Middle Ages. The rare tesserae of this kind found in medieval mosaics were despoilt from Roman works. This colour was obtained by adding cupreous oxide and uncalcined tartar to a soda-hme-lead glass. The microcrystals of cuprite (Cu
2
O), which result from the reducing effect of tartar, are orange at first and become intense red as their size increases.
All the recipe books examined report formulae for the preparation of opaque red glass, one of the most difficult colours to produce. At different historical periods the production technique was lost and red glass was replaced with other materials, such as pink marble, red stone or terracotta fragments. The colour and opacity of red glass are due to the separation of red cuprite microcrystals, which form in the melt in the presence of iron oxide and reducing components. The crystal size and the amount of iron and lead added determine the colour, which varies in a wide range of shades from bright red to brown. Other recipes detail the method of preparing different colours, such as carnation, by adding manganese to opaque white glass, and blue, by adding zaffera (an intermediate product produced by calcination of cobaltite ore diluted in a large amount of finely powdered silica), green, black and turquoise.
The earliest description of the technique used to manufacture gold-leaf tesserae is given by
Theophilus in chapter 15 of Book Two, explaining that 'Byzantine...cover tiny square pieces on one side with gold leaf, coating them with the very clear ground glass.... Then they lay them side by side on an iron plate,...and fire them in the window-glass kiln' [ 14, p. 60 ]. This is a fairly fanciful description; indeed, Theophilus had never seen these mosaic tesserae being made. The first plausible description is in the manuscript in the State Archives in Florence, which explains how to blow a thin glass sheet (the cartellina), cut the gold leaf, make it adhere to the cartellina with egg white and then place a thicker slab on top. After heating in the furnace, the artefact is pressed to ensure good adhesion and then slowly cooled (annealing). The Montpellier codex reports a similar recipe using a soda-lime-lead-silica glass. The introduction of lead, comprising half the total quantity for the support glass and one third for the cartellina, was intended to improve the adhesion and durability of the cartellina, and add to its brightness. The same recipe is found in Darduin's book and in the nineteenth century it was used by Lorenzo Radi to make gold-leaf mosaics in
Murano, thus reviving a technique which had been lost for a long period [13].
The recipe books also contain prescriptions to produce particular kinds of tesserae. A special technique involved painting a layer of coloured glass powder dispersed in egg-white on a slab of ordinary glass. Once dry, the slab was placed in a kiln until the glass softened. On melting, the powder formed a uniform layer, which adhered to the support slab. The resulting tesserae were thus coloured only on the surface. The Montpellier recipe book, for example, describes this technique to prepare carnation tesserae, in which the coloured layer is produced with an opaque white enamel and fired clay fragments, and red mosaic glass by applying a layer of opaque white enamel in which a red iron oxide (probably haematite) is dispersed.

The mosaic tesserae
The tesserae made during more than 2000 years of glass mosaic history may be classified into three main groups: glass pastes, smalti and metal-leaf tesserae. Other kinds of tesserae, such as cold painted glass or fragments of glazed ceramics, have been used occasionally, especially at times of crisis in glassmaking technology [9].
The term 'glass pastes' indicates the coloured tesserae produced in a few shades of colour. The brightest coloured pastes are made of dark, transparent, homogeneous glass. The lighter shades are prepared by adding increasing quantities of white opacifiers and/or by reducing the quantity of colorant in the glass. Special hues are produced by adding coloured crystalline materials to the molten glass, such as fragments of brownish-orange fired clay, or yellow fragments of giallolino.
In ancient times intense, brightly coloured lead glass was produced, but the colours were limited to red and yellow and the intermediate hues. The production of new smalti in the Murano glassworks from the fifteenth century allowed a hitherto unknown intensity of colour to be manufactured, thus greatly extending the range of colours at the mosaicists's disposal. Smalti differ from glass pastes, not only in their colour properties, but also in their greater surface brilliance and the ease with which the tesserae can be cut due to the high percentages of lead oxide present.
The ultimate appearance and colour of metal-leaf tesserae depend on various parameters, such as the metal used (pure gold, silver or their alloys), the nature of the support and the chosen colour of the cartellina. Because the metal leaves are so thin, they often fail to form a coherent layer and discontinuities occur, through which the glass support can be seen. Thus, the colour of the support can influence the chromatic effect of the tesserae. Beside the transparent yellow, green or violet glasses of the more ancient tesserae, opaque red glasses were used from the eleventh to twelfth centuries and glasses of various colours at other times, especially between the seventeenth and nineteenth centuries.
The colour, thickness and homogeneity of the cartellina also affect the appearance of these tesserae.
The cartellina was made with a glass that was as homogeneous and colourless as possible, unless it was deliberately coloured yellow or violet to extend the colour range of the tesserae. The condition of the surface of the cartellina is also important, as a deteriorated glass layer can result in a bloomed effect and cause the tesserae to lose their brightness as a consequence of decay over time.
Sometimes other types of vitreous tesserae have been used in mosaic works. In some, glass is the support material for cold-painted decorations, such as imitation gold-leaf tesserae made with coldpainted gold resinates, in others, thin layers of expensive or very special glass, for example red or carnation-coloured glass, were made to adhere to a common glass support while hot. Fragments of glazed ceramics have also been used frequently as an alternative to glass during times of crisis.
Scientific analysis
A mosaic is an extremely complex work of art and the tesserae from which it is made range widely in composition owing to several factors. The mosaicist could obtain the tesserae from various production centres, each using different techniques and raw materials and specializing in the production of particular colours. In addition, the practice of reusing tesserae from earlier mosaics has long existed and is still in operation. Moreover, during successive restoration work, some of the lacunae were filled with tesserae that differed in composition from the originals.
In the last decades of the twentieth century, chemical analysis of ancient glassy materials made rapid progress. New analytical techniques allowed non-specialized laboratories to analyse materials that previously required very complex and time-consuming tests. Consequently much analytical data has been published and new information is available on the history of glassmaking as a result of the development of rapid, non-destructive, highly sensitive techniques for a better classification of glasses [21, 22].
Mosaic tesserae consist of more than one phase, so that global elemental analysis (vitreous matrix and dispersed phases) must necessarily be combined with the analysis of each single phase.
Scanning electron microscopy (SEM) examination of polished cross-sections, combined with X-ray microanalysis, allows a complete characterisation of individual phases to be performed. In addition,

X-ray diffraction is indispensable for the exact determination of the mineralogical nature of the crystalline phases.
Glass composition
The investigations carried out by several authors into the history and technology of glassmaking and the chemical composition of ancient glasses have shed considerable light on these topics [23-25].
The chemical composition may-supply clues as to the raw materials used and the production techniques and the conditions of manufacture for a particular time and place, thus helping to define the history of glass technology. Moreover, comparison of compositional data can help with the identification or authentication of glass objects. Conservators are able to use chemical analyses of the glasses and their weathered layers in choosing the most appropriate restoration procedures and conservation conditions [24-26].
The raw materials used for historic glass production were naturally occurring and of relatively low purity; therefore, glasses obtained by melting only two or three components will actually contain more than twenty different elements, including major, minor and trace constituents. The procedure to determine the quantitative composition of glass is rather complicated because of the large number of elements to be analysed [25, 27]. Besides insufficient knowledge of ancient glassmaking, such complexity is one of the reasons why many scientific publications report only incomplete and qualitative compositions that are restricted to major and minor components, neglecting those ones which are often decisive to the accurate authentication of the glasses.
Glass is usually a very homogeneous material and even small flakes are representative of the chemical composition of the object. This is not true for mosaic tesserae, where several phases can be dispersed in the glass matrix. However, care should always be taken, for all ancient glass artefacts are covered by modified deterioration layers with a chemical composition different from the bulk. Before analysis, these layers should be carefully removed from the fragment to be sampled, especially when surface techniques are used [21].
A wide range of techniques, each with its own advantages and disadvantages, has been used to analyse glass composition, including wet chemical analysis, atomic absorption (AA), inductively coupled plasma atomic emission spectrometry (ICP), neutron activation (NAA), X-ray fluorescence
(XRF), using both laboratory-based and portable equipment, and X-ray microanalysis, using either an electron microprobe with wavelength dispersive spectrometry (EPMA) or energy dispersive spectrometry associated with a scanning electron microscope (EDS+SEM). Other factors, such as whether the analysis is destructive or non-destructive, the danger of the method (NAA, portable
XRF), the cost of the device (EPMA) or of each analysis, etc., are also important and have influenced the diffusion of the different techniques.
Wet chemical analysis was the most common analytical technique in use up to the 1960s; it is a very accurate and sensitive method but is time-consuming, requires well-trained staff and largesized samples. It has rarely been used to analyse all the elements present, usually determining only the ma]or and minor components [25]. Atomic absorption and ICP are widely used today; both methods require small amounts of material for very sensitive and accurate analyses [28]. A promising technique seems to be the inductively coupled plasma mass spectrometry (ICP-MS) associated with laser ablation (LA-ICP-MS). In this technique the low destructive laser ablation is combined with the high sensitivity and accuracy of the analytical method [29]. Nuclear activation analysis (NAA) is a useful method because of its high sensitivity and good accuracy (only a few elements need a process of matrix correction), but can be dangerous due to the high energy of the radiation (X-rays) and the radioactivity induced in the sample; therefore, this technique is available in only a few institutions [30]. X-ray microanalysis is being widely adopted, especially in the form of EDS+SEM, owing to the low cost of the instrument. The advantages of this technique include small sample size (less than 0.lg) and the ability to distinguish and analyse different phases, inclusions, weathered areas, opacifiers, etc. Furthermore, the technique is faster than other methods and

Table 1 Examples of chemical composition (weight % oxides) of glasses melted with different fluxes. Tesserae from Venice [42], Florence [5] and Orvieto [37]. the samples are not destroyed, so they are available for further investigation. The high degree of accuracy that can be obtained in quantitative analysis of glass has also been demonstrated [27, 31].
Traditional X-ray fluorescence requires the sampling of several grams of glass. It has been used for the analysis of glass tesserae dating from the first centuries AD [32], and from the Byzantine [33] and Medieval eras [34]. Nondestructive portable XRF using an energy dispersive detector is a qualitative tool for chemical analysis. It has proved useful, especially in preliminary and rapid in situ analyses of mosaic tesserae. The detection of certain components, such as opacifiers or colourants, enables the amount of tesserae to be sampled for laboratory analysis to be reduced and, in some cases, it can rapidly distinguish between original and replaced glass [35]. A new nondestructive portable technique is being assessed to perform in situ examinations involving setting up a portable colorimeter, which will allow mosaic tesserae to be classified according to chromatic coordinates, and will also supply useful information about colorants used, deterioration, etc. [36].
The results of the chemical analyses performed up to now confirm that there are only a few main compositional categories of historic glass: soda-lime-silica, potash-lime-silica, soda-potash-limesilica (mixed alkali) and a glass obtained by adding lead oxide to one of the previous compositional groups (Table 1). Widely varying compositions are found within each group, resulting from the introduction of different types of flux and from the preparation of the batch with different silica to ash ratios [9].
EPMA chemical analyses of Roman glass tesserae of the first to fourth centuries are reported by
Henderson and compared with the compositions of contemporary blown glass and enamels [37].
Mosaic tesserae of the eighth century found in Ahus (Sweden) were studied by Callmer and
Henderson [38].
Similar research aimed at defining the compositional categories of Byzantine glass tesserae was carried out by Freestone et al. [39] based on a limited number of EPMA analyses. Analyses of tesserae of the nymphea in Pompei are reported by Marchese et al. [40] and some data for a few mosaic tesserae of the cathedral of Orvieto by Mambelli et al. [41]. More than 250 AA analyses of tesserae from the Roman, Byzantine and medieval (Venice and Rome) periods and of the 1715 restoration work in San CIcmcntc in Rome have been published in 1999 [42]. The description of the finds and the analytical data reported will be an important source of information for future archaeometric studies. EPMA analysis of mosaic tesserae in several Roman churches of the fifth
[43], seventh [44] and eleventh to twelfth centuries [45] are also available, reporting on the

chemical composition of the glass, the characterization of the crystalline phases and the state of deterioration. A similar study was carried out on the mosaics in St Mark's Basilica in Venice, examining a group of tesserae dating from the thirteenth to fourteenth centuries and other tesserae introduced during restoration work in the nineteenth century [46].
Analyses of opacifiers, pigments and inclusions
A review of opacifying agents in ancient opal glass was published by Turner and Rooksby [47] and
Byzantine red opaque tesserae were studied using XRF by Shugar [57]. Among the opacifiers added to the melt in the form of a thin powder, calcium pyroantimonate (Ca
2
Sb
2
O
7
) was identified, together with a similar compound (CaSb
2
O
6
). The former has been identified as the naturally occurring mineral romeite, which can readily be synthesized by reacting the appropriate mixture of calcium carbonate and antimony oxide at a temperature of about 1200°C. The authors state that no mineral having the latter formula is known. In the medieval period tin oxide, in the form of cassiterite (SnO
2
), dispersed in a lead glass was found to have been used.
The opal state can occur during cooling of the melt. Turner and Rooksby identified lead oxyarsenate
(3Pb
3
(AsO
4
)
2
.PbO) and calcium fluophosphate (Ca
3
(PO
4
)
2
.CaF
2
), corresponding to mineral apatite added in the form of bone ash, as opacifiers used in this case. During the last quarter of the nineteenth century, fluoride opals (CaF
2
and NaF) were introduced and have had widespread use in the twentieth century. Interesting information on the use of opacifiers and pigments in Roman times is reported by Mass et al. [20] who discuss hypotheses on the use of natural, rather than synthesized, antimonates on the basis of EPMA and SEM analyses.
Yellow pigments have been identified in yellow, green and brown tesserae, where they were used to enlarge the colour range [47, 48]. Yellow lead pyroantimonate (Pb
2
Sb
2
O
7
), the natural mineral bindheimite, can be synthesized by firing the appropriate mixture of lead and antimony oxide to about 900 °C. Other yellow pigments identified were lead stannate (Pb
2
Sn
2
O
6
) and a solid solution
(mixed crystals) of Pb
2
Sn
2
O
6 in Pb
2
Sb
2
O
7
. Another primary pigment is fired clay, found in small quantities in carnation and green tesserae, where it was used to modify the hues [44].
Secondary pigments (crystals separating from the melt) were used to prepare red, brown-red and orange-yellow tesserae. These colours were produced in glasses melted with soda ash, even in
Roman times when glass generally was made with natron. The colouring-opacifying pigment is always cuprite (Cu
2
O), which forms red crystals and yields progressively darker colours on the addition of iron. In certain conditions where small crystals have separated in glass also containing lead(II) oxide (PbO), cuprite forms small crystals, typically orange-yellow in colour [44].
Beside glass, opacifiers and pigments, crystalline or metal particles are sometimes found in mosaic tesserae, known as 'inclusions'. They have various sources, such as high-melting raw materials
(quartz), refractory fragments from the pot or the furnace and metal residues from the tools used or from colouring minerals such as copper, etc. These particles have no aesthetic effect, but their identification provides significant information about the technology and raw materials used [25].
Ancient glassmaking technology did not achieve a perfect fining (removal of bubbles) because of the low temperatures attainable in the furnaces, thus a certain amount of bubbles is always present.
In order to obtain a particular translucent or semi-transparent effect, bubbles could be produced deliberately by adding appropriate minerals to the melt. For example, carbonates decomposed on heating causing a release of gas, while undissolved sulphates and chlorides in the glass formed galls
(small immiscible drops that crystallized on cooling).
Weathering processes
During many centuries of exposure to the environment glass slowly undergoes surface transformations and alterations. Because these processes occur extremely slowly and affect thin surface layers, the research aimed at understanding the alteration mechanisms only advanced in the late twentieth century, with the development of surface analysis techniques [49].

Glass chemical durability and alteration processes
With the exception of hydrofluoric acid, which destroys glass rapidly, attack by acids and aqueous solutions with a pH below nine can be regarded as a reaction of ion exchange (leaching) between the alkalis in the glass and H (or H
3
O ) ions in water:
The velocity of the leaching reaction gradually slows down in time, due to the formation of a hydrated silica-rich gel layer acting as a protective coating. A second kind of reaction (corrosion) occurs when the glass surface is in contact with solutions of pH higher than nine. This reaction, more rapid as the pH increases, destroys the Si-O-Si bonds in the network and brings all the components into solution (congruent dissolution):
In this case, no protective layer is formed and the alkaline solution can penetrate and react undisturbed. In glass, both alteration reactions are influenced by temperature. With an increase of
10 °C the alkali extraction rate is doubled and the corrosion rate increases still more quickly.
The composition of the glass determines its durability and ranges between durable (scarcely reactive) and low-durable glasses. This distinction applies only for leaching at a pH of less than 9, while corrosion occurs almost independently from glass type. The relationship between chemical composition and durability is not as strict as it is for other properties of glass. Pure silica is the most durable glass and durability decreases with increasing amounts of network modifiers. Flowever, the leaching rate is not the same for all alkalis. With equal concentrations of total alkalis (Na
2
O + K
2
O), the amount of alkalis leached out from potash glass is twice as much as from soda glass. Moreover, in a glass containing both sodium and potassium in comparable proportions, the leached alkalis are lower from the purely soda glass (mixed alkali effect). The presence of stabilizers, such as calcium oxide (CaO) markedly increases the chemical resistance of alkali silicate glass [4].
In durable glasses the thickness of the protective layer is usually below 1 µm. In low-durable glasses the hydrated silica-rich gel layer has a very open, porous and fragile structure. It acts as a partially protective layer, slowing down the leaching process but not arresting it completely. Its thickness can reach more than 100 µm.
The alteration of a glass surface due to interaction with the atmosphere is referred to as weathering.
The main atmospheric agents responsible for attacking glass are water, in the form of vapour or rain, and acidic gases (chiefly CO
2 and SO
3
). However, the water adsorbed generally involves just a few molecular layers and the weathering is negligible. The increase in relative humidity with the formation of condensate or the presence of meteoric water accelerates interdiffusion reactions [50].
Alteration mechanisms in the atmosphere are similar to the above-mentioned reactions of glass in contact with water, but are complicated by the presence of deposits and cyclical variations in microclimate [49, 51]. The effect of weathering on glass preservation occurs in two main states: dynamic and static. The first condition is conducive to leaching reactions, while the second promotes corrosion reactions [49]. Leaching affects the whole surface, creating a uniform layer of altered glass in leached layer, while corrosion begins in single points where water droplets have dried out or at surface microdefects such as dust deposits, micro-cracks, etc. (pit corrosion) [4].
The hydrated silica-gel layers are very brittle, mainly where their thickness is greater than a few
µm, and are often traversed by micro-cracks. These fractures have catastrophic effects on durability.
Water can infiltrate in the micro-cracks, crossing the protective barrier and causing leaching to recommence. Crystallisation also occurs in the micro-cracks with a disaggregating effect similar to

that of ice in rock fractures. The cyclic nature of this process leads to a deep propagation of microcracks, with a consequent detachment or exfoliation of hydrated flakes.
The surfaces of ancient glasses, crizzled by prolonged environmental exposure and scratched by restoration treatments, accelerate deterioration phenomena. A similar effect is caused by deposits of hygroscopic salts or partially-soluble materials.
The deterioration action of micro-organisms is a modest, slow, yet continuous, process that can cause severe damage to glass. In appropriate conditions, such as in the presence of humidity and the absence of direct solar radiation, etc., the presence of fungi, bacteria and algae can cause localized retention of moisture, with an increasing concentration of carbonic acid as a result of the microorganism respiration. In addition, elimination products, which usually are basic, accelerate chemical decomposition [52]. These weathering processes are extremely complex and are not yet fully understood [4].
Alteration of the tesserae
Mosaic works are located on both interior and exterior walls. In the lat.ter case they are directly exposed to the rain, which markedly accelerates glass deterioration. This phenomenon has been clearly observed, for example in medieval stained glass windows [4]. Similar data are not available for mosaics, as published work concentrates almost exclusively on sheltered mosaics. The results, summarized below, refer to mosaic tesserae dating from the first centuries AD to the twentieth century.
The tesserae of a mosaic work often exhibit different states of preservation: some are well preserved, some exhibit evident crizzling of the surface, bloom and loss of colour, and others may be so deteriorated as to break up into small fragments under light finger-pressure. This variation in the degree of preservation is due to the different chemical composition of the tesserae and to microclimate conditions, which may change markedly from one point to another in the mosaic work, for example as the result of a localized area of condensation or water infiltration.
Soda-lime-silica tesserae produced with natron can be classified according to composition as durable or low-durable glass; tesserae produced with soda ashes generally are durable. The thickness of the leached layers is of a few µm and surfaces are smooth, sometimes iridescent and free of adherent deposits and corrosion initiation points (micropits).
Potash-lime-silica glasses are generally low-durable: the primary reasons are the low concentration of silica (less than 50%) and the fact that the potassium ion can be extracted more easily by water than the sodium ion. The chemical resistance of these glasses is so poor that their silica-gel hydrated layers may slowly disaggregate as an effect of acids (corrosion at a pH of less than 9) [4]. In lowdurable glasses, deterioration may cause a complete loss of colour in the tessera (blooming) and an increase in brittleness. The surfaces are generally rough and may be covered with adherent deposits
(crusts) in adverse climatic conditions.
In order to increase the brightness of the glass, conventional ashes were sometimes replaced completely or in part with purified, calcium-free ash. Several such prescriptions appear in glassmakers' recipe books. The analysis performed on the vitreous tesserae of the fourteenth century in St John's Baptistery in Florence testify to the use of this kind of glass, which has deteriorated up to complete disaggregation [9, 53].
In the nineteenth century the search for glass with improved brightness prevailed over the traditional prudence of glassmakers, who took insufficient account of its durability. This is one of the reasons why nineteenth-century tesserae often exhibit greater deterioration in comparison with those manufactured in earlier centuries, as has been was observed, for example, in St Mark's
Basilica in Venice [46].
The frequency of cleaning treatments may be critical for glass preservation. Maximum corrosion is expected where the accumulation of dust and other deposits has caused moisture retention and unfavourable pH conditions, but cases also should be noted where glass has been damaged by cleaning with coarse abrasives or basic solutions during conservation treatment. Critical

microclimatic conditions arise along the scratches and localized corrosion phenomena develop [13],
The deterioration of smalti may lead to the formation of coloured deposits, especially in polluted environments. A yellowish patina or brown and black layers will form, depending on the type of salts present, as a reaction of the leached lead with the pollutants [46, 54].
One of the main problems in the conservation of mosaics is the detachment of the cartellina in metal-leaf tesserae [55]. In these tesserae the leaching phenomena occurs not only on the surface but also at the interfaces between the support, metal leaf and cartellina. After an initial loss of brilliance, the cartellina may detach, with the consequent loss of the metal leaf, which is no longer protected from the environment. In such cases, only the support glass remains, thus causing noticeable discontinuities in luminosity. This phenomenon is still more marked in silver-leaf tesserae. Silver is readily oxidized with sulphidric acid, which is often present in polluted atmospheres. It is not surprising that most of the silver-leaf tesserae in ancient mosaics have been lost. Metal-leaf tesserae have also been damaged in the past when mosaics were removed from the wall, a technique frequently used when the underlying masonry required reinforcement [13].
Soda-lime-silica glass tesserae from Italian wall mosaics dating between the fifth and twelfth centuries have been investigated [51]. The analysis demonstrated that the composition of the lowdurable glasses can be classified into two groups. In comparison with durable glass, one group has
Table 2 Examples of chemical composition (weight % oxides) of durable (B-l; T-l), low-durable low Ca (B-2) and low-durable high Na (T-2) glasses and weathered layers of tesserae.
B: SS. Cosma e Damiano, Rome, blue; T: S. Clemente, Rome, turquoise [52]. a lower calcium oxide (CaO) content ranging between 1.5 and 5.0%, and one has a higher sodium oxide (Na
2
O) content ranging between 18 and 23%. Examples are reported above, together with the composition of the weathered layers (Table 2). The deterioration mechanism seems to be similar for both low-durable compositional groups. The weathered layers show a sharp depletion of sodium, while the calcium content remained practically unchanged (selective leaching). A partial extraction of calcium occurred only in completely degraded tesserae. While the composition of the other oxides remained more or less constant, the total percentage of the components of the leached layers

ranged between 80 and 90%, the remaining 10-20% being made up of water. When observed under the SEM, the surfaces of the brittle tesserae appear crizzled and partially covered with salt deposits.
The leached layers extend beneath the surface to depths varying between a few µm and 0.3 mm
(Fig. 2).
In scanning electron microscopy back-scattered electron (SEM-BSF.) micrographs of polished sections of disintegrated tesserae, an irregular propagation of micro-cracks can be observed (Fig. 3).
The micro-cracks are surrounded by a leached layer. This demonstrates that fractures are an excellent vehicle for water penetration through the protective layer. The evaporation of water during dry periods causes the crystallization of salts inside the micro-cracks, mainly sodium carbonate formed from a reaction between sodium extracted from the glass and carbon dioxide dissolved in atmospheric water. Only in heavily polluted environments, such as at San Vitale in Ravenna, are the deposits mainly composed of sodium sulphate [54].
The deterioration mechanism can be summarized as follows:
Depletion of Na
+
and replacement by H
+
and water occurs. This layer has either a complete or partial protective effect, depending on the glass composition.
In low-durable glasses a slow growth of the leached layer up to several µm occurs, and microcracks appear.
Water penetrates the micro-cracks, leading to accelerated sodium depletion and formation of a leached layer around the micro-cracks.
Fig. 2 SEM-BSE micrograph of the polished cross-section of a uniformly leached layer (W, dark grey area) crossed by micro-cracks (black lines) in a low-durable Na
2
O-CaO-SiO
2
glass (G).
(photo: Marco Verita).

Fig. 3 SEM-BSE micrograph of the polished cross-section of a disintegrated tessera; the low-durable glass (g) is crossed by micro-cracks surrounded by a leached layer (dark grey area), (photo: Marco Verita).
• Crystallisation of sodium (or calcium) carbonate in the micro-cracks follows water evaporation and promotes propagation of the micro-cracks in the glass.
• In polluted environments sodium (or calcium) carbonate is transformed into sulphate. This phenomenon is likely to accelerate the further propagation of the micro-cracks and the consequent pulverisation of the tesserae.
The results of analysis are beginning to reveal information useful for the understanding of those phenomena referred to by conservators in the simple, but expressive, phrase 'glass sickness'. A thorough knowledge of deterioration mechanisms will help to establish cleaning techniques, to plan appropriate consolidation work and to define appropriate environmental conditions for the tesserae's conservation. However, much work remains to be done, especially regarding unsheltered mosaics on exterior walls, where the effects of accelerated weathering necessitate urgent treatment with new techniques.
References
1 Farneti, M., Technical-Historical Glossary of Mosaic Art, Longo Editore, Ravenna,
1993.
2 Maclehose, I.., Vasari on technique, Baldwin, G., ed., Dover Publications, New York, 1960, p.252.
3 Scholze, H., Glass, nature, structure and properties, Springer Verlag, New York, 1991.
4 Newton, R. and Davison, S., Conservation of glass, Butterworth, London, 1989.
5 Brill, R.H., 'The scientific investigations of the Jalame glass and related finds' in Weinberg,
CD., ed., Excavation at Jalame, site of glass factory in late Roman Palestine, University of
Missouri, Columbia, 1988, pp.257 93.
6 Verita, M. and Vallotto, M., 'Analisi chimica di reperti vitrei del IV secolo rinvenuti a
Sevegliano (Udine)', Quaderm Friulani di Archeologia 8, 1998, pp. 7-19.
7 Barrera, J. and Velde, B., 'A study of French medieval glass composition', Archeologie
Medievale XIX, 1989, pp. 81-130.
8 Zecchin, L., Vetro e vetrai di Murano, Vol. 3, Arsenale Editrice, Venezia, 1990.
9 Verita, M., 'Tecniche di fabbricazione dei materiali musivi vitrei attraverso indagini chimiche e mineralogiche' in Borsook, E., Gioffredi Superbi, F. and Pagliarulo, G., eds., Medieval
Mosaics: Light, Colour, Materials, A. Pizzi Editore, Milano, 2000 (in press).

10 Freestone, J., 'Theophilus and the composition of medieval glass' in Vandiver, P.B., ed., Material Issues in Art and Archaeology III, Materials Research Society 267, Pittsburgh,
Pennsylvania, 1993, pp.739-46.
11 Hoffmann, P., Bichlmeier, S., Heck, M., Theune, C. and Callmer, J., 'Chemical composition of glass beads of the Merovingian period from Graveyards in the Black
Forest, Germany', X-Ray Spectrometry 29, 2000, pp. 92-100.
12 Mason, R.B. and Tite, M.S., 'The beginnings of tin-opacification of pottery glazes',
Archaeometry 39, 1997, pp. 41-58.
13 Verita, M., 'Glass mosaic and smalti: technique, materials, decay and conservation' in
Moldi Ravenna, C, ed., / colon delta luce. Angela Orsoni e I'arte del mosaico, Marsilio Editore,
Venezia, 1996, pp. 41-97.
14 Theophilus, De Diversis Artibus, edited and translated by Hawthorne, J.G. and Smith,
C.S., Dover Publications, New York, 1979.
15 Zecchin, I,., // ricettario Darduin, Arsenale Editrice, Venezia, 1986.
16 Harding, C, 'The production of medieval mosaics: the Orvieto evidence', Dumbarton Oaks
Papers 43, 1989, pp. 73-102.
17 Milanesi, G., Dell'arte del vetro per musaico: tre trattatelli dei secoli XIV e XV ora per la prima volta pubblicati, Romagnoli, Bologna, 1864.
18 Zecchin, L., Vetro e vetrai di Murano, Vol. 1, Arsenale Editrice, Venezia, 1987.
19 Nen, A., L'arte vetrana, 1612, II Polifilo, Milano, 1980.
20 Mass, J., Stone, R.E. and Wypyski, M., 'The minerological and metallurgical origins of
Roman opaque colored glasses' in McKray, P., ed., Prehistory and History of Glassmaking
Technology, The American Ceramic Society, Westerville, Ohio,
1998, pp. 121-44.
21 Verita, M., 'The chemical analyses of ancient glassy materials: problems and perspectives' in
Da Olio, S., ed., Proceedings of the Conference: Syntheses and methodologies in inorganic chemistry, Bressanone, 1997, pp. 94-99.
22 Mass J.L., 'Instrumental methods for the study of the deterioration of vessel glass' in Tennent, N.H., ed., The conservation of glass and ceramics, James & James,
London,
1999, pp. 15-42.
23 Turner, W.E.S., 'V - Studies in Ancient Glasses and Glassmaking Processes,
Part III: The Chronology of the Glassmaking Constituents', Journal of the Society of Glass
Technology 40, 1956, pp. 39T-52T.
24 Sayre, E. and Smith, R.W., 'Some materials of glass manufacturing in antiquity' in Levey, M., ed., 3rd Symposium on Archaeological Chemistry, University of Pennsylvania Press,
Philadelphia, 1967, pp. 279-311.
25 Brill, R.H., 'The scientific investigation of ancient glasses' in Proceedings of the 8th
International Congress on Glass, ICG Press, Bruxelles, 1968, pp. 47-68.
26 Verita, M. and Toninato, T, "A comparative analytical investigation on the origins of the Venetian glassmaking' Rivista della Stazione Spenmentale del Vetro 20, 1990, pp. 160-76.
27 Hreglich, S. and Verita, M., 'Determinazione mediante microsonda elettronica della composizione chimica di alcune tessere di mosaico vetroso, campionate nella cappella sud della
Basilica di S. Maria Assunta a Torcello' in Farioli Campanati, R., ed., Atti del III Colloquio
Internazionale sul Mosaico Antico, Edizioni del Girasole, Ravenna, 1984, pp. 553-4.
28 Hatcher, H., Tite, M.S. and Walsh, J.N., 'A comparison of ICP-Plasma emission spectrometry and atomic absorption spectrometry analyses on standard reference silicate materials and ceramics', Archaeometry 37, 1995, pp. 83-94.
29 Gratuze, B., Giovanoli, A., Barrandon, J.N., Telouk, P. and Imbert, J.L., 'Apport de la methode ICP-MS couplee a l'ablation laser pour la caracterisation des archeomateriaux', Revue d'Archeometrie 17, 1993, pp. 89-104.

30 Gratuze, B., Giovanoli, A. and Barrandon, J.N., 'Analyse non destructive de poids et estampilles islamiques en verre au moyen de techniques nucleaires' in Proceedings, of the 2nd International Conference on non-destructive techniques for study and conservation of works of art, Vol. 2, Section V, Istituto Centrale Restauro Press, Roma, 1988, pp. 1-11.
31 Verita, M., Basso, R., Wypyski, M. and Koestler, R., 'X-ray Microanalysis of Ancient
Glassy Materials: a Comparative Study of the WDS and EDS Techniques', Archaeometry 36,
1994, pp. 241-51.
32 Fiori, C. and Macchiarola, M., 'Studio e confronti fra la composizione chimica di vetri del
II-I secolo A.C. provenienti da Delos (Grecia) e vetri provenienti dalla Capitanata (Foggia):
Herdonia, I-II secolo D.C., Arpi e Ascoli Satriano, II secolo D.C in Meconcelli Notarianni, G., ed.,
7/ vetro dall'antichita alVeta contemporanea: aspetti tecnologici, fitnzionalt, commercial!,
Edizioni ET, Milano, 1998, pp. 139-46.
33 Fiori, C. and Fiorentini-Roncuzzi, I., 'Studio di tessere di smalto di mosaici bizantini',
Ceramurgia 24, 1994, pp. 271-82.
34 Fiori, C, Vandini, M., Ercolani, G. and Mingazzini, C, 'I vetri del mosaico absidale di S.
Ambrogio a Milano', Rwista delta Stazione Sperimentale del Vetro 29, 1999, pp. 21-29.
35 Maioli, P., Scafe, R., Seccaroni, C. and Tognacci, A., 'Studio delle paste vitree utilizzate nei mosaici della cappella del Sancta Sanctorum' in Sancta Sanctorum, Electa F.ditore, Milano, 1995, pp. 280-90.
36 Polato, P. and Verita, M., 'Catalogazione di tessere vitree mediante misure colorimetriche', Rtvista della Stazione Sperimentale del Vetro 29, 1999, pp. 285-95.
37 Henderson, J., 'Chemical characterization of Roman glass vessels, enamels and tesserae' in Vandiver, P.B., ed., Material Issues in Art and Archaeology II, Materials Research Society 185,
Pittsburgh, Pennsylvania, 1991, pp. 601-7.
38 Callmer, J. and Henderson, J., 'Glassworking at Ahus, S. Sweden (eighth century
AD)', Laboratw Arkeologi 5, Arkeologiska Forskningslaboratoriet Stockholms, Stockholm,
1991, pp. 143-54.
39 Freestone, I.C., Bimson, M. and Buckton, D., 'Compositional categories of Byzantine glass tesserae
7
, Annales du lie Congres de l'Association Internationale pour I'Histoire du Verre,
A.I.H.V., Amsterdam, 1990, pp. 271-9.
40 Marchese, B., Marino, O., Vallario, P. and Garzillo, C, 'Analisi SEM/EDS di tessere policrome in pasta vitrea di ninfei di Pompei' in Mascolo, G., ed., Atti giornate di studio
Materiali e Tecniche per il Restauro, Idea Stampa Editore, Cassino, Italy, 1997, pp. 119-33.
41 Mambelli, R., Fabbri, B., Fiori, C. and Donati, F., 'Studio di materiali vitrei provenienti dal
Cathedral di Orvieto' in Fiori, C, ed., Mosaico e restauro Muswo, Vol. I, CNR, Faenza, 1988, pp.
54-65.
42 Brill, R.H., Chemical analyses of early glasses, 2 vols., The Corning Museum of Glass,
Corning, New York, 1999, Vol. 1, pp. 105-11, Vol. 2 pp. 216-37.
43 Verita, M., 'Analisi di tessere musive vitree', in Tiberia, V, ed., // restauro del mosaico della Basilica dei Santi Cosma e Damiano a Roma, Ediart, Perugia, 1991, pp. 75-9.
44 Profilo, B., Santopadre, P. and Verita, M., 'Analisi di tessere vitree del mosaico absidale della cappella del santi Pnmo e Feliciano in S. Stefano Rotondo a Roma' in Iannucci, A.M., ed., Mosaici a San Vitale ed altn restaun, Longo Editore, Ravenna,
1992, pp. 99-105.
45 Verita, M., 'Analisi delle tessere in pasta vitrea' in Tiberia, V, ed., / mosaici del XII secolo e di
Pietro Cavallini in Santa Maria m Trastevere, Ediart, Perugia, 1996, pp. 209-12.
46 Verita, M., 'Analisi di tessere musive vitree del battistero e dell'atrio della Basilica di San
Marco in Venezia' in Proceedings of the Conference: Scienza e Tecnica del Restauro della Basilica,
Venice, May 1995, Vol. 2, pp. 567-85.
47 Turner, W.E.S. and Rooksby, H.P., 'A Study of the Opalising Agents in Ancient Opal Glasses throughout Three Thousand Four Hundred Years', Glastechnische Berichte 8, 1959, pp. 17-
28.

48 Rooksby, H.P., 'A yellow cubic lead tin opacifier in ancient glasses', Physics and Chemistry of Glasses 5, 1964, pp. 20-5.
49 Clark, D.E., Pantano, C.G. and Hench, L.L., Corrosion of glass, Magazines for Industry
Inc., New York, 1979, pp. 1-52.
50 Fitz, S.j 'Glass objects: causes, mechanisms and measurements of damage' in Proceedings of the European Symposium on Science, Technology and European Cultural Heritage,
Butterworth-Heinemann, Oxford, 1991, pp. 180-9.
51 Verita, M., 'Deterioration mechanisms of ancient soda-lime-silica mosaic tesserae' in
Proceedings of the 18th International Congress on Glass [CD-ROM], The American Ceramic
Society, Westerville, Ohio, 1998, ICG543, pp. 1-7.
52 Krumbein, W.E., Urzi, C.E. and Gehrmann, C, 'Biocorrosion and biodeterioration of antique and medieval glass', Geomicrobiology Journal 9, 1995, pp. 136-60.
53 Antonelli, E. and Raddi, G., 'I mosaici di San Giovanni: restauro di un coretto',
Opificio delle Pietra Dure Restauro/ Oiiaderm 6, 1994, pp. 134-43.
54 Fiori,C. and Donati, F., 'Studio dei materiali della decorazione musiva dell'arco di ingresso al presbiterio in S. Vitale a Ravenna' in Fiori, C. and Muscolino, C, eds.,
Restaun ai mosaici nella Basilica di S. Vitale a Ravenna, IRTEC, Faenza, 1990, pp.60-80.
55 Fiorentini Roncuzzi, I., 'Les smalts a fond d'or et d'argent dans les mosaiques anciennes' in
Farioli Campanati, R., ed., Atti del III Colloquto Internazionale sul Mosaico Antico, Edizioni del
Girasole, Ravenna, 1984, pp. 563-70.
56 Heck, M. and Hoffmann, P., 'Coloured opaque glass beads of the Merovingians',
Archaeometry 42, 2000, pp. 341-57.
57 Shugar, A.N., 'Byzantine opaque red glass tesserae from Beit Shea Israel, Archaeometry 42,
2000, pp. 375-84.
Author
Marco Verita graduated in chemistry and joined the Stazione Sperimentale del Vetro of Murano-
Venice as a researcher in 1977. His activities concentrate on analysis by X-ray instrumentation
(SEM, EDS-WDS-microanalysis, XRF, XRD) of glass, including ancient glasses, glazes, refractory and related properties, glass surfaces, mechanisms of glass corrosion and the study of ancient glass technology. He teaches the course on the History of Glass Technology and on the Chemistry of
Materials Conservation and Restoration at the University of Venice. He is also the co-ordinator of projects on the study of ancient stained glass and of the chemical composition and production technology of ancient glasses.