Spolia (Latin, 'spoils') is the re-use of earlier building material or decorative sculpture on new monuments. The practice was common in late antiquity: Roman examples include the Arch of Janus, the earlier imperial reliefs reused on the Arch of Constantine, the colonnade of Old Saint Peter's Basilica; examples in Byzantine territories include the exterior sculpture on the Church of Panagia Gorgoepikoos in Athens); in the medieval West Roman tiles were reused in St Albans Cathedral, porphyry columns in the Palatine Chapel in Aachen, and the colonnade of the basilica of Santa Maria in Trastevere. Spolia in the medieval Islamic world include the columns in the hypostyle mosques of Kairouan and Cordoba.
Although the modern literature on spolia is primarily concerned with these and other medieval examples, the practice is common and there is probably no period of art history in which evidence for "spoliation" could not be found.
Interpretations of spolia generally alternate between the "ideological" and the "pragmatic." Ideological readings might describe the re-use of art and architectural elements from former empires or dynasties as triumphant (that is, literally as the display of "spoils" or "booty" of the conquered) or as revivalist (proclaiming the renovation of past imperial glories). Pragmatic readings emphasize the utility of re-used materials: if there is a good supply of old marble columns available, for example, there is no need to produce new ones. The two approaches are not mutually exclusive, and there is certainly no one approach that can account for all instances of spoliation, as each instance must be evaluated within its particular historical context.
Spolia had apotropaic spiritual value. Clive Foss has noted that in the fifth century crosses were inscribed on the stones of pagan buildings, as at Ankara, where crosses were inscribed on the walls of the temple of Roma and Augustus. Clive Foss suggests that the purpose of this was to ward off the daimones that lurked in stones that had been consecrated to pagan usage.
Liz James extends Foss's observation in noting that statues, laid on their sides and facing outwards, were carefully incorporated in Ankara's city walls in the Seventh century, at a time when spolia were also being built into city walls in Miletus, Sardis, Ephesus and Pergamum: "laying a statue on its side places it and the power it represents under control. It is a way of acquiring the power of rival gods for one's own benefit," Liz James observes. "Inscribing a cross works similarly, sealing the object for Christian purposes".