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The materials uncovered in excavations in several provinces of the Roman empire provide important evidence to the frequent use of glass in most precious opera sectilia. Glass pieces imitating a dozen of ancient marbles and exotic stones have been identified in the collection belonged to Evangelista Gorga, samples of which were selected for analysis in order to obtain valuable information on the Roman glassmaking technology of the 2nd century AD, especially on coloration techniques. The monochrome and polychrome glass tesserae replicating precious marbles and rare stones were: porfido verde antico, litomarga verde, turquoise, pavonazzetto, gabbro eufotide, diaspro nero e giallo, semesanto, alabastro tartarugato, rosso antico, cipollino rosso, giallo antico, and onyx. XRF quantitative chemical composition was determined allowing the types of glass used and the colouring techniques to be identified. Results have been compared with similar data obtained by studies on coloured Roman glass and mosaic tesserae. Similarities and discrepancies are discussed and hypotheses are suggested on the technology of this extraordinary Roman production.

The study concerns the chromatic alteration of mosaic glass tesserae in the Sao Joao Chapel, Sao Roque Church in Lisbon. This precious chapel was commissioned by the King of Portugal, Joao V, in 1742. The polychrome marbles, stucco bas-reliefs, precious silver sculptures and three mosaic altarpieces were sent by ship to Lisbon. Alessio Mattioli, the glassmaker who produced the glass tesserae for the mosaics, was the famous inventor of new compositions for difficult colours in opaque glass such as red, orange, and the variety of flesh colours. In fact Mattioli succeeded in imitating the technique of painting to produce altarpieces that were “eternal”, or, at least, more durable in appearance than paintings. Our research is based on archive documents discovered in St. Peter's Fabrica in Rome concerning the decoration of the Sao Joao Chapel in the Sao Roque Church in Lisbon and the compositions of Mattioli's glass. The chromatic alterations producing conspicuous spots and shades of grey, brown, and white are similar to the alterations in many mosaic altarpieces in the Basilica di San Pietro in Rome, dating from the same period. The findings of the chemical analyses on glass and weathering products, are the first attempts to understand the weathering mechanism and to prepare a restoration and conservation plan.

Although Venice is regarded as being one of the most important centres of glass manufacture and trade in Western Europe, little is known about the origins of the Venetian glassmaking. Some archaeologists suggest there is a continuity in the history of glass making, in its transplantation from the Roman centres of the mainland (Altino and Aquileia), first to some islands of the lagoon, and later to the town. Some others, instead, think that there was no continuity at all and that the art of glassmaking was imported in Venice from the Islamic glass factories of the Middle East. Of great help in answering this and other technological questions is the information about the composition of ancient glass from well-dated excavations that allows to conclude on the early technology, sources of raw materials and long-distance trade. Some significant glass findings, including fragments of pots used for glass melting, were found on the island of Torcello and, more recently, in two emergency excavations in the historic centre of Venice. In this paper, the chemical composition of a large set of glass findings (and related materials) from the Venetian lagoon dating from the 7th to the 13th centuries is determined by X-ray microanalysis in order to throw new light on the glassmaking technology in the early centuries of the Venetian tradition. The results show that in this period there occurred a gradual transition from the Roman technique (a two-ingredient formulation with silica-lime sand and natron as a fluxer), towards the use of silica sand and soda plant ash. There is evidence, therefore, for a production continuity with the Roman period (melting pots and glass scraps with a natron-based composition), yet restricted to transparent blown items. No evidence was found for a local production of glass tesserae for the mosaics of the church of St. Maria Assunta in Torcello.