The mosaics of St. Mark’s Basilica have undergone many restoration works during the centuries, since the end of the 13th century.
The tremors, the structure deformations and the atmosphere aggressiveness are the main risk factors for the preservation of the mosaic heritage.
Nowadays the Procuratoria of St. Mark is in charge of the mosaics protection, maintenance and repairs and through its organs it is the keeper of this precious masterpiece.
The mosaics date to between the late 11th and mid 19th century. Today they are a combination of original parts and parts modified in various epochs. The Greco-Byzantine parts completed in the 13th century have been wholly replaced.
In restoration carried out before the end of the 13th century, the original character and concept was always respected whereas the retouching of the gold backgrounds and the redoing of individual parts in subsequent epochs show through. Some of the most evident restorations date to the renaissance period when restorers could not resist expressing their own stylistic consistency. The 14th century mosaics (baptistery and Chapel of St. Isadore) are Venetian school in which the teachings of the Greek masters were transformed through developing iconographic themes and techniques, linked to the production of mosaic materials which thenceforth were wholly supplied by the Murano glass-works.
In the 15th century Tuscan artists introduced the use of the cartoon for transferring the design to the wall (Mascoli Chapel). What remains of the oldest palimpsest, also in terms of quality and technique of execution, is guaranteed stability by the considerable structural restorations of the last hundred years.
From the 16th century to the end of the 17th century restoration took the form of renovation of ruined surfaces, taking advantage of even modest damage in order to totally renew large areas and pictures. The influence of the greatest painters of the day prevailed over mosaic artists, who were thus reduced to becoming executors of the great canvases of the most important cartoonists, chief among whom were Titian, Tintoretto, Veronese, and Jacopo il Palma the younger. Not to mention the Salviatis, Padovanino, Aliense, Maffeo Verona and Pietro Vecchia.
In the 18th century a cultural revaluation of the old mosaics began, to the detriment of the new which were often held to be inferior. Leopoldo Dal Pozzo innovated techniques and the use of enamels in the splendid lunette and vault of the portal on the left of the central one (cartoon by Sebastiano Ricci), operating carefully to recover the ancient mosaics. After him there was a rapid deterioration.
After the fall of Venice, what Austria inherited in 1814 from Napoleon was a church that had been abandoned for years. The Venetian state had died, and with it its chapel. It was to Napoleon’s credit that he separated the ducal basilica from a government that no longer existed and gave it to the Church. In the nineteenth century the lack of maintenance for about twenty years, since the turn of the century, and the increasingly precarious static conditions, called for the restoration of large surfaces. This was carried out until the 1860’s, with replacement of detached parts but with the intention of reproducing the pre-existing figures.
Subsequent restorations were increasingly refined and aimed at preservation, with the operation of removing the surface to be restored (“lievo“) and, in the first half of the twentieth century, even with “restoration from behind” by dismantling the masonry and rebuilding it after restoring the mosaic which remained intact in its original position.
In the first half of the 19th century a consolidation programme was implemented which involved the mosaics on a large scale.
The second half of the century was characterised by the commitment of director of works Pietro Saccardo who responded to John Ruskin and Alvise Zorzi’s pleas for rigorous respect for history. The damaged parts were restored, the criticised interventions modified and operational techniques were enriched and rendered more suitable. Restoration of enormous areas – more than 3000 square metres – was approached with the “new method” of preservation. The great period of mosaic restoration that runs from the 19th century to the first half of the 20th had the same value as that of the origins in the 12th and 13th centuries. In that period new life was given to a dying organism. Such wide-raging action was due to the urgent need to face a general state of maximum decay at the beginning of the 19th century when only the physical intactness of the monument was safeguarded, neglecting the mosaics.
This new cultural attitude led to the search for more modern techniques, and in the space of seventy years, from 1880 onwards, cupolas, vaulted ceilings and walls were restored without ever destroying or replacing them.
The main damage to the mosaics today results from detachment from the masonry, the falling off of tesserae, from structural alteration and from break-up of the basic layers that ensure adhesion. Effects due to atmospheric aggression and rising damp cause surface deterioration of the tesserae and break-up of the basic layer mortar. In the former case the tesserae crumble and in the latter they fall away, either individually or in small sections. Some situations are due to both causes.
In the case of detachment, evaluation of structural deformation in progress or foreseeable is of great help in identifying the zones where the covering may be damaged. To this end a glance at the events regarding the church’s structures may be useful.
The “magna gesia” of 1063, the present day church, replaced the one built in 828 to preserve St. Mark’s body.
Its creation certainly involved reuse of the pre-existing building situation, which the Greek architects designed and managed with the greatest rigour, but the subsequent addition of lead covered cupolas, the marble crowning of the roof, the atrium and the Baptistery altered the unitary weights on the sandy clay of the subsoil, which subsided in a different manner.
The first four centuries of its life were followed by earthquakes and fires which dramatically accentuated the church’s unbalanced state. The external frameworks of the north and south sides required continual attention with the addition of buttresses, tie-beams and shoring up of the walls.
No part was exempt from operations of replacement, sometimes only partial, but above all the vaulted ceilings and the perimeter walls suffered the repercussions of these unbalances, and with them the mosaics which more than once were detached from the walls, obliging the Procuratorate to take action.
Today there are no longer zones of greater or lesser quality in terms of mosaic preservation. Even the disfiguring error is now respected, bearing witness to the various periods of the church’s history.
In the case of detachment the traditional way of identifying the part of the mosaic that has separated from the wall is by rapping with the knuckles and noting the difference in sound. Pressing with the palm one notes any flexibility of the surface. The limits and depths of the unstuck areas can be defined by means of a “stethoscope” through the resonance of the cavities behind the mosaic on which a tuning-fork is placed to vibrate.
As well as the checking described above, modern and more sophisticated techniques are used to evaluate detachment of the mosaic: computerised measurement of the responses to vibrating emissions sent to the mosaic surface. This can give highly useful information for remote inquiry but it still needs to be fine-tuned.
Consolidation of detached parts is dealt with by means of a liquid mortar obtained by mixing milk of lime and finely powdered marble and gradually adding as required an acrylic resin – Primal – which is never more than 50% of the total but is normally used as a third part of the mix.
In general, and especially in cases of detachment, there are 4 phases to the operation:
1. extensive photographic documentation, graphic surveys and, where necessary, some tracings made on site;
2. identification of the detached surfaces with delimitation by means of marking the outline of the various types of detachment;
3. the various phases of consolidation;
4. washing, surface cleaning and documentation when work is completed.
When damage is due to crumbling of the tessera fixing mortar, the so-named “rovigno“, the operation is far more difficult. Consolidation of the mortar is sometimes nearly impossible because it may be soaked with salts and dampness and therefore untreatable with chemical agents. In these very few cases, if the surface has to be removed and the area is small, it is cleaned and the base layers of deteriorated mortar are replaced, known as “lievo“.
Surface deterioration of the tesserae, on the other hand, requires that attention be paid to the causes, rising damp or acid atmosphere. Any replacement of individual tesserae must be evaluated in accordance with its position in the work. The integration of tesserae into the fabric of the mosaic, unlike with paintings or frescoes, constitutes suitable protection of the base and, above all, ensures continuity of the concatenation of the mosaic structure. In these restorations the ancient tesserae are supplied from the Procuratorate’s stores. The replaced tesserae are marked with a dot of coloured enamel on each tessera, visible only from close up.
It is simpler to work on mosaics that have not undergone restoration over the last hundred years because the ancient techniques remained the same from the start, and are well known, with the variant of Roman mosaicists applying the tesserae on stucco. In these areas one still finds systems of fixing the mosaic to the masonry that bear witness to previous interventions (nails, washers etc.) and integration operations of particular crudeness such as those to repair structural cracks. Unfortunately in some cases saline moulds grow between tesserae treated with 19th century cement “mashes”. This is one of the greatest problems to solve because it is difficult to remove cemented zones, and without removal an efficacious safeguarding operation cannot be envisaged.
Historical knowledge of the works, over and above the damages, provides a useful “background” for a better definition of the interventions. With the establishment of the Mosaic Studio in 1881, Saccardo collected, documented and published the main activities in this sector.
The wholly St, Mark’s-oriented school of mosaics, in this century, has ensured the availability of craftspeople and permitted in-depth study of the problems involved in restoring the church mosaics.