The conservation of medieval mosaics
10 June 2008
Professor Geoff Allen of The Interface Analysis Centre has recently been investigating techniques that might help to restore the beautiful mosaics of the Basilica di San Marco in Venice.
The Interface Analysis Centre specialises in the analysis of surfaces and materials. Specific problems are dealt with by academics pursuing cutting-edge research who regularly apply their knowledge to solving commercial problems. The Centre’s Director, Professor Geoff Allen, with one of his PhD students, Diana Edwards, has recently been investigating techniques that might help to restore the beautiful mosaics of the Basilica di San Marco in Venice.
The Basilica di San Marco – the cathedral of Venice – is the city’s most famous church. It lies on St Mark’s Square and is one of the best-known examples of Byzantine architecture in the world. The building was begun in 829 AD as an ecclesiastical structure to house and honour the remains of St Mark that had been brought from Alexandria. It now contains examples of some of the world’s most beautiful mosaics dating from the 13th century. Unfortunately, the position of the Basilica adjacent to the Grand Canal creates damp conditions that have caused the mosaics to deteriorate so that today they require a long-term programme of restoration.
The location of the Basilica presents considerable challenges to the conservation of the mosaics. Piazza San Marco is the lowest-lying part of Venice and high tides flood the piazza with salt water twice a day. The salt water also floods the basement of the Basilica where it is absorbed by the stone in the Basilica walls. The water level in a normal stone wall is determined by the rate of absorption and evaporation, which in turn depends on the porosity of the stone. Usually this level does not exceed one metre; however, unlike a normal wall, the first five metres of the Basilica walls are coated in a layer of marble, which is impervious to water. Since the water is prevented from reaching the surface and evaporating, the water level in the wall increases and in some parts of the Basilica has reached a height of ten metres. The salt water penetrates the lime mortar of the mosaics causing detrimental effects such as salt explosions, which push the tesserae out of the mortar. As each tessera was individually positioned by the mosaicist so that any gold-leaf detail reflects light on to the observer, producing a golden sheen to the mosaics, it is important to be able to replace loosened tessera very precisely. Analysing the mortar that holds them in place is thus key to the restoration programme.
The city's most famous church contains examples of some of the world's most beautiful mosaics
Lime mortar is a complicated material to analyse. For example, polishing the surface to look at the structure will damage the sample and ruin the structure that requires analysis. So for these materials a more sophisticated technique is required involving the use of Focussed Ion Beam (FIB) technology, that can be used as a form of microsurgery to cut a section of the mortar without damaging the structure, allowing clear electron images to be obtained. Using this technique, unique evidence for organic matter in the mortar was discovered. While the inorganic regions have been identified as calcium carbonate, the precise identification of the organic matter is more difficult. However, work on mosaic mortars of a similar age in Hagia Sofia, Istanbul, has indicated the presence of egg white, which could have been incorporated to aid the positioning of the tesserae. In addition, X-ray Photoelectron Spectroscopy of the region between the glass tessera and the lime mortar shows a higher concentration of potassium and sodium salts, compared with the bulk mortar, indicating that the presence of these components may play a role in the deterioration of the mosaics.
As well as analysing the original mortar, innovative techniques and experimental design have tested the strength of modern-day mortars in ‘simulated mosaic’ experiments, with the aim of finding the best mortar with which to restore the mosaics to their original state. Using this technique, the lime mortar found to give the strongest bond with the glass tesserae was one prepared by mixing a Venetian dolomitic lime (a lime which contains magnesium) with Carrara Marble, in the ratio one of lime to three of marble. The next phase of the work will use all these results to create a lime mortar similar to the original, which will be tested to check whether it can withstand the harsh salt-water conditions of the Basilica. If it passes this final test, it will be used to sensitively restore the mosaics.
In the Young Persons’ Lecture Competition, sponsored by the Institute of Materials, Minerals and Mining, Diana Edwards won the south west regional heat for her lecture about this work, and came second in the national finals.