After the conquest of Constantinople in 1204, Venice had access to a great quantity of precious marbles from the sacred and civic buildings of the capital of the Eastern Roman Empire.
A great many marble articles were sent to St. Mark’s and used to decorate the facades and interior.
The most varied marbles were used with a symbolic function depending on their characteristics and colour. The most precious stone is red porphyry, symbol of imperial and divine power. Among other things this marble was used for the Tetrarchs group (south faï¿½ade) and the doge’s tribune (interior).
The marble elements decorating the basilica are of extreme interest from the viewpoint of both the coverings and the liturgical furnishings. Most of these are reused materials taken chiefly from buildings in Constantinople or associated regions. Importation to Venice of these items was documented as early as the 9th century, but it was after the venture of the Crusade of 1204 that the flow of marbles became more intense.
The late-antique criterion was followed in decorating St. Mark’s. As regards marbles this also took into account their features of colour and composition which were used with a symbolic function. Marbles were employed to underline determined functions or the importance of certain spaces in accordance with a practice which from late antiquity was to survive in the symbolic-decorative tradition of the Byzantine empire and also, in part, in the Middle Ages in the west.
The most precious stone was red porphyry, linked to the imperial symbology of the late-antique period and associated with purple, a substance and colour that symbolised royalty and divinity. In the period when the Venetians built St. Mark’s, purple, and consequently porphyry, were linked to a powerful imperial anddivine symbology proper to the Byzantine empire: an object made of porphyry was something connected with an imperial purchaser. In St. Mark’s the use of porphyry is associated with solutions whose purpose was to underline Venice’s political greatness and glory, without any religious implications: such as the group of the Tetrarchs in the corner of the Treasury, highlighting the entrance towards the ducal palace; the columns decorating the central door of the basilica’s west faï¿½ade, almost like a triumphal arch, or at the corners of the faï¿½ade as if delimiting a royal space. The only porphyry elements in the interior are in the so-named southern “ambo”, originally the doge’s tribune, another symbol of power. Sometimes when porphyry was not available, Iassense marble was used. Dark red with white veins it was especially employed in wall coverings but with a solely decorative intention.
Another precious marble with violet or reddish markings – Docimeum or pavonazzetto marble – is always used in a privileged position, such as the apse columns.
After porphyry in the hierarchy of imperial marbles come the green marbles (e.g. serpentine, used in St. Mark’s for small objects, or Thessaly green) followed by Aquitaine white and black. Thessaly green and Aquitaine white-black are used in an imperial context for sarcophagi and covering slabs. Aquitaine breccia is present in St. Mark’s in the form of column shafts decorating the narthex doors and the main portals of the west and south faï¿½ade. Green Thessaly breccia, much more in evidence, is used not only for column shafts but also for covering slabs, elements of liturgical furnishings such as the northern ambo for liturgical readings and the altar ciborium. There is also an altar mensa in Thessaly green, used as wall covering on the north faï¿½ade, and a slab of the same marble, perhaps from a sarcophagus, inserted into the Treasury wall.
Lastly, veined marbles were used with a decorative function, exploiting the pattern of the veining itself. For example, the columns in Proconnesio, a white marble with greyish veins, are set out in such a way as to respect correspondence and symmetry on the basis of the horizontal pattern of the veining.
As for wall coverings, the slabs were cut in such a way that the veining formed geometric decorations. Clear examples may be seen in the interior coverings where the veining of the slabs form broad zigzag or lozenge fascias laid out vertically and horizontally.