Mosaic covers approximately 8,500 square metres of the walls, vaults and domes of St Mark's Basilica, laid in millions of coloured and gold glass tesserae (Adami et al., 2018, p. 2). The surfaces on show today were made between the end of the eleventh century and the second half of the nineteenth, so the interior records about eight hundred years of work rather than one commission (Adami et al., 2018, p. 2).
The official chronology of the Procuratoria di San Marco puts the start of the mosaic decoration in 1071 to 1084, under Doge Domenico Selvo, on a church whose rebuilding had begun in 1063 and which was consecrated in 1094 (Procuratoria di San Marco, 2026). Later campaigns added the Baptistery and the Chapel of Sant'Isidoro between 1343 and 1354, and the vault of the Mascoli chapel in the first half of the fifteenth century (Procuratoria di San Marco, 2026).
Two cycles repay a slow look. The atrium carries 110 panels of Genesis from about 1275, which follow a fifth or sixth century illuminated manuscript now in the British Library, and the west wall of the south transept carries scenes from the childhood of Mary from about 1250 (Ene Draghici-Vasilescu, 2024, pp. 213, 216). Neither is easy to photograph and both sit high, so a pair of binoculars does more for a visit than any ticket upgrade.
How many square metres of mosaic are in St Mark's Basilica?
Surfaces made of millions of coloured tesserae cover approximately 8,500 square metres of walls, vaults and domes (Adami et al., 2018, p. 2). The subjects run across both Testaments and take in allegorical figures and episodes from the lives of Christ, the Virgin, St Mark and other saints, alongside the story of the city itself (Adami et al., 2018, p. 2).
The figure is large partly because the building is complicated. The basilica has three naves, a transept and a series of chapels and apses arranged over two levels, with arches, vaults and domes of varied and irregular shape (Adami et al., 2018, p. 6). The highest domes stand about 28 metres above the floor, which is why so little of the decoration can be read properly from ground level (Fassi et al., 2017, p. 229).
Scale also explains why the surveyors of the basilica work to such fine tolerances. The joints between one tessera and the next measure roughly 1 to 2 millimetres, so any photographic record meant to show individual tesserae has to reach a pixel size of about 0.5 millimetres, even in the domes (Fassi et al., 2017, p. 232).
When were the mosaics of San Marco made?
The Procuratoria di San Marco publishes a chronology that fixes the main dates. The body of St Mark was brought from Alexandria to Venice in 829, the first church of San Marco was consecrated in 832, and that church burned in the revolt against Doge Candiano IV in 976 before being rebuilt under Doge Orseolo I. Rebuilding of the present church began in 1063 under Doge Domenico Contarini, the mosaic decoration was begun between 1071 and 1084 under Doge Domenico Selvo, and the church was consecrated in 1094 under Doge Vitale Falier (Procuratoria di San Marco, 2026).
After that the additions are datable one by one. The Fourth Crusade of 1204 brought marbles, the four horses, the Nicopeia icon and the enamels of the Pala d'Oro into the building. The Sant'Alipio mosaic of 1265 is valuable to historians because it shows the exterior of the basilica as it then stood. The Baptistery and the Chapel of Sant'Isidoro were built between 1343 and 1354 under Doge Andrea Dandolo, and Florentine artists worked on the mosaics in the first half of the fifteenth century (Procuratoria di San Marco, 2026).
Art historians push the earliest surviving figures slightly further back than the official start date, to the last three decades of the eleventh century and perhaps to about 1070, with the older group of mosaic inscriptions dated between 1060 and 1099 (Ene Draghici-Vasilescu, 2024, p. 212).
The Genesis cycle in the atrium and the manuscript behind it
The atrium, the vaulted porch you pass through before reaching the nave, holds 110 mosaic panels telling Genesis, made around 1275 (Ene Draghici-Vasilescu, 2024, p. 216). Within that space there is a cupola given to Abraham, a set of lunettes and three cupolas given to Joseph (Ene Draghici-Vasilescu, 2024, p. 217).
The cycle is tied to a specific book. The Cotton Genesis, held in the British Library as Cotton MS Otho B VI, is a fifth or sixth century Greek illuminated Genesis thought to have been produced in Alexandria, and one of the oldest illustrated biblical codices to survive. Most of it was destroyed in the Ashburnham House fire of 1731, leaving eighteen charred scraps of vellum in reasonable condition, though some of its miniatures had been copied in the seventeenth century and those copies are now in Paris (Ene Draghici-Vasilescu, 2024, p. 216).
A 1986 monograph proposed the link between the manuscript and the atrium, and a German symposium volume of 2014 reopened the argument and concluded that the mosaicists copied the manuscript's scenes closely rather than simply taking them as a guide (Ene Draghici-Vasilescu, 2024, pp. 207, 218). The practical consequence for a visitor is unusual: the panels above your head preserve a fuller visual record of a burnt Late Antique book than the book itself now does.
Mary's childhood in the south transept
The west wall of the south transept carries a second cycle, dated to about 1250, on the childhood of the Virgin, sometimes called the cycle of Anne and Joachim. As in the atrium, the background is gold tesserae and the figures are built from variously coloured pieces (Ene Draghici-Vasilescu, 2024, p. 213).
The scenes come from the Gospel of James, an apocryphal text written around AD 150, and two survive in a legible state: Joachim making his offering in the Temple, and Anne holding the newborn Mary in a nursing position, known in Venice as the birth of the Virgin (Ene Draghici-Vasilescu, 2024, p. 213). The captions worked in tesserae do not match the scenes they sit above, and a further inscription on the wall describes an episode that is no longer there, which suggests the wall once held a longer sequence (Ene Draghici-Vasilescu, 2024, pp. 213-214).
The likely cause is a restoration campaign completed in 1690 under Dominicus Gigola, who reworked tableaux that had stood since 1084 and lost some scenes in the process (Ene Draghici-Vasilescu, 2024, p. 214). Comparison with mariological cycles in Cappadocia, Constantinople and Cyprus indicates what has gone: an image of Mary's parents before her birth on the left, and her blessing by the priests and her presentation in the Temple on the right (Ene Draghici-Vasilescu, 2024, p. 215).
How are the gold mosaics made?
A gold tessera is a sandwich. A thin sheet of beaten gold is fixed between two layers of glass, a thick support below and a thin cover above called the cartellina. The technique is described for the first time in a late eighth century manuscript, the Manoscritto di Lucca, which specifies that the gold leaf used in mosaics is held between two layers of glass (Conventi et al., 2012, p. 5).
Measurements on gold leaf tesserae from Italian mosaics of the first to ninth centuries, made by scanning electron microscopy, give a sense of the quantities. The cartellina is mostly 0.3 to 0.7 millimetres thick, while the gold leaf itself varies from nothing, where it has torn, to 0.9 micrometres, with most readings at 0.4 micrometres give or take 0.2. Beating 19.3 grams of gold, one cubic centimetre, yielded between 2.5 and 3.0 square metres of leaf for tesserae (Conventi et al., 2012, pp. 4-5).
For San Marco itself the question has been where the glass came from. Analyses by a British Museum team found tesserae made from a high potash, lime and silica glass typical of western Europe, rather than the soda, lime and silica glass then being made in the eastern Mediterranean, which points to local production from the twelfth century onwards (Ene Draghici-Vasilescu, 2024, p. 222).
Who laid them, Byzantine masters or Venetians?
Most researchers hold that the first mosaics were laid by masters from Constantinople, commissioned by Doge Domenico Selvo, and that some of the same workshop had already been employed on the cathedral at Torcello (Ene Draghici-Vasilescu, 2024, pp. 210, 212). The style of the eleventh century work has its closest parallel in the mosaics of the narthex at Hosios Loukas in Greece, which predate 1050 (Ene Draghici-Vasilescu, 2024, p. 212).
The documentary trail is thinner than the style suggests. Written evidence for Greek mosaicists at San Marco is almost non-existent. A Marcus Grecus is recorded as a mosaicist in 1153, with nothing to show whether he worked on this building (Ene Draghici-Vasilescu, 2024, p. 212).
Work on the main inner porch adds a caution about reading nationality from surfaces. In that portal the Greek alphabet is used only for the central figures of the Virgin, Peter and Paul, which may mark their rank rather than the language of the workers, and the costly bright orange known as arancione becco di merlo, which needed large amounts of lead, appears only in a few places such as the faces of the evangelists (Cantone, 2016, p. 145). Changes of style within one row are weak evidence of a change of date or of hand, because a mosaic was a collective job run by a master with a team under him (Cantone, 2016, p. 145).
Restoration, decay and the survey of every tessera
Until the end of the eighteenth century damaged parts were replaced, keeping the iconography but renewing the work to the taste of the day, which is how mosaics from cartoons by Titian and Tintoretto entered the building (Adami et al., 2018, p. 2). In the second half of the nineteenth century the approach turned to conservation, with the lievo technique lifting a mosaic on glued canvases so the mortar behind it could be replaced. The first half of the twentieth century brought restoration from behind, rebuilding vaults section by section with the mosaic left in place, and the 1980s brought consolidation in situ, injecting lime based mortars into the voids, first used at scale at Torcello (Adami et al., 2018, pp. 2-3).
The threat is water: as it works out through the masonry, salts crystallise at the contact between materials and the mortar loses its grip until tesserae fall (Adami et al., 2018, p. 3). More than 3,000 square metres were restored by that newer method in the later twentieth century (Fassi et al., 2017, p. 230).
Five mosaicists work on the surfaces daily, cleaning tesserae and replacing the worst from a store of pieces that have fallen over the centuries (Adami et al., 2018, p. 2). The floor was photographed between 2003 and 2008 in just over 2,000 frames at half a millimetre per pixel. Laser scanning was tested and rejected: gold and glass scatter the beam, leaving noise of about a centimetre (Adami et al., 2018, p. 6).
