Use and decay of coloured stones in the former bizantine churches of Istanbul

Many coloured stones were used for wall cladding into the Byzantine churches of Istanbul (Ayasofia, Kalenderhane, Kariye, Molla Zeirek etc.). The stones came from the whole Mediterranean basin and, in many cases, were fit in the churches from ancient roman buildings. The most employed ones were: Proconnesian, Cipollino rosso, Cipollino verde, Pavonazzetto (veined marbles); Porfido rosso antico (volcanic); Grand Antique, Occhio di pavone (limestones); Verde antico (ophicalcite). The walls were clad, from the ground to the springing line of the arches, with large panels encircled by narrow mouldings; the veined marbles were almost always cut in the "open book" style using two or four panels. The stones show different decay phenomena often linked to the mineralogical composition and to the structure: cracking along white swirling bands (Cipollino rosso) or green chlorite veins (Cipollino verde), polish and colour losses (ophicalcite), bowing of huge and thin panels (mainly Proconnesian).