Tüskés Anna (szerk.): Omnis creatura significans - Tanulmányok Prokopp Mária 70. születésnapjára (2009)
Antik és középkori művészet
Omnis creatura significans There are, however, rare exceptions: such as San Vitale in Ravenna and a corner of the atrium of San Marco in Venice. I refer, of course, to the work of Ce- sare Fiori and Marco Veritä,3 two physical chemists. These cases concern the identification of glass as well as pigments. Other studies, but again there are far too few of these, take a more archaeological approach concerning the ways tesserae were set as well as distinguishing between original pieces and those added during subsequent restorations. I am thinking especially of Irina Andreescu-Treadgold’s work at San Vitale and Torcello.4 But even here we have no report regarding the type or types of glass used at these sites. Hopefully in the case of Prague’s golden portal the story will be different thanks to the Getty Conservation Center and the Prague restorers. We need all the information we can get if we are to understand what we are looking at and how the work can best be taken care of. Before coming to questions concerning the technique and condition of the Prague mosaic, we should briefly review what is known of their historical context and what it shares with other examples of the period. As you know, contemporary chronicles inform us that the mosaic was made in 1370-1371 at the behest of the Emperor of Bohemia, Charles IV, who had just returned from an Italian visit .5 The royal nature of this commission was entirely in keeping with the use of the medium elsewhere. Mosaics were so costly and prestigious that it was still a medium reserved for mon- archs, popes and such rich republics as Venice and Florence. What would Charles have seen in Italy to inspire the Prague commission? Concentrating on facade mosaics in Rome, he would still have seen those at Old St. Peter’s, St. Paul’s outside the Walls, the Lateran, Giotto’s Navicella and Santa Maria Maggiore as well as the facade mosaic of San Frediano in Lucca (see note 5). Then, he might have seen the facade of Orvi- eto Cathedral and St. Mark’s in Venice where work was still in progress. To create such mosaics a large supply of tesserae was needed. As for the glass tesserae, this presumed the existence of a local glass industry because the quantity was such that it would have proved impractical to import them. The only stable glass industries in Italy then were at Murano and Gambassi in Tuscany.6 Although Murano at times exported small quantities of tesserae to Orvieto, for instance, production was limited and there were occasions when there were not enough to supply the mosaicists at St. Mark’s.7 Pisa cathedral, on the other hand, had a supply of surplus tesserae which occasionally were sold to Florence.8 But it is unknown who produced them—some were made in situ and others were purchased from vendors.9 Probably, glass works for stained-glass windows and tesserae were created for specific projects which when completed were dismantled. This was the case at Orvieto about which a great deal is known thanks to the studies of Catherine Harding and Lucio Riccetti.10 As for Bohemia, in Charles fVs time, Hettes informs us that there were at least 20 glass factories in the area so that glass for the tesserae for St. Vitus could have come from nearby.11 In many respects the Prague mosaic and those originally at Orvieto Cathedral have much in common. Both are facade mosaics—Orvieto’s measures about 121 square metres and Prague’s about 85 square metres.12 Both are roughly contemporary and both used tesserae of stone as well as glass. Carlo Bertelli has even suggested that some of the Orvieto mosaicists might have contributed to the Prague mosaic either via designs or actual setting in situ.'3 Whether Prague also used terracotta or recycled tesserae (i.e. spoils) remains to be seen. Also the type of glass used in both monuments seems to have been the same: potassic glass but there might have been other types of mixed alkalis as well.14 Potassic glass is less durable than soda glass—the kind preferred in Venice.^ Obviously, facade mosaics are subject to weathering which is particularly hard on potassic glass because water leaches out the alkali more easily from the glass mix.16 Why, then, was potassic rather than soda glass used in Orvieto and Prague? It is well known that the Venetians had a virtual monopoly on soda glass which used either natron (a mineral rich in soda imported from Egypt) or plant ash high in soda content—the lumen catino shipped to Venice from Syria. >7 The Venetians passed laws from 1275 onwards through the 14th and 15th centuries, prohibiting its sale or export.18 Interestingly enough, there may have been an attempt at Orvieto to circumvent this difficulty by purchasing some Venetian tesserae. A well known document of 1360 describes how an emissary was sent to Venice for the purchase Fig. 5. London, Victoria and Albert Museum. Birth of the Virgin, detached mosaic from fagadé of Orvieto Cathedral. 118
