The Stratigraphy of the Palimpsest
The wall to the right of the apsesemicylindrical niche in the back wall of the presbytery covered by a semidome (known as the palimpsestTerm normally used to indicate ancient manuscripts on parchment in which a new writing was superimposed on the old one, which has previously been scraped off. In this case it stands for the presence of different superimposed layers of painting from different periods, which are visible contemporaneously due to the partial loss of one or more of them wall) shows the richest stratigraphy found in the church.
Seven layers from different periods are superimposed, out of which five are painted and one preserves traces of colour.
The first layer in contact with the brick wall consists of residues of a bedding plasterMortar used for the adhesion of marble slabs and/or mosaics for an opus sectileDecoration composed of thin, polychrome marble slabs which form an inlay work (marble inlay) decoration. This decoration, probably from late antiquity (4th - 5th centuries A.D.), is also visible on the wall to the left of the apse, on the lateral walls of the presbytery and in other areas of the church.
The second layer is a plaster painted with a two-tone border, probably from before the building was transformed into a church. Only a few fragments are preserved at about 3/4 of the height of the wall. Apparently this layer is not preserved in any other part of the church.
The third layer is the fragment of Maria Regina, the enthroned Virgin adored by an angel on the right. The painting, in a severe, late-antique style, is dated to the 6th century. The painting was clearly made before the apse was opened, in that the panel would originally have extended to the left, with the Virgin as centre of a symmetric composition, with two flanking angels. It may be the only fragment of this layer in the church.
The fourth layer consists of two fragments making up an Annunciation, one with part of the face of the Virgin, the other with the so-called Fair Angel. This pictorial phase, dated to the first half of the 7th century is characterized by a strongly impressionistic style of HellenisticFrom Hellenism: period of diffusion of Greek culture and civilization in the Mediterranean Basin between 4th and 1st century B.C. tradition. It is thought that some small fragments of strongly deteriorated painted plaster at the corresponding position on the left side of the apse and the white plaster with traces of paint (fifth layer) are part of the same pictorial phase.
The fifth layer consists of a white plaster with only faint traces of colour visible close to the corner of the apse, in which it is present in extensive areas. The same layer appears in the conch of the apse where coloured decoration appears more frequently, though fragmentarily (two figures with yellow haloes surrounded by red contours and a red circle). These fragments suggest a full-scale composition in this layer.
The sixth layer is represented by the fragments of the cycle painted during the pontificate of Martin I (649-653), which probably covered the whole presbyterySpace reserved for the clergy in Christian churches, situated in the central part of the church, behind the altar at least up to the height of the apse. Preserved are two Church Fathers holding forth scrolls, which have a counterpart on the other side of the apse. The dating is based on the texts on the scrolls, which were cited at the Lateran Council in 649. The imitation marble panel (dado) below is attributed to the same pictoral phase, as are many other paintings in the church.
The seventh layer consists of fragments of a scheme from the pontificate of John VII (705-707). A small fragment with the head of a Church Father is preserved close by fragments of layers three and four. Higher up on the two sides of the apse and above, in the huge lunetteSemicircular surface area of a wall , in this case, resulting from the conjunction of the wall and a barrel vault, it seems to be the only painted layer, and that all earlier layers were removed in preparation for this decoration. The pictorial cycle of John VII was left as the last decoration of the apse and side walls of the presbytery. Only the apse itself was re-painted for the last time during the pontificate of Paul I (757-767 A.D.).