First, what is a Seraphim? The following is a description of angels from "A Readers Guide to Orthodox Icons". Below is a picture of a mosaic encrusted vault in Cefalu Cathedral in Sicily showing many-winged Seraphim which was made by Byzantine artists from Constantinople for the Norman Kings of the island:
"The name given, seraphim, is Hebrew and means “burning ones” (plural; the singular form is seraph). They are the closest to the throne of God, and as such are flame-like, “For our God is a consuming fire.” (Heb 12:29); “His throne was a flame of fire” (Dan 7:9); “the appearance of the Lord was like a blazing fire” (Ex 24:17). The seraphim, then, are usually painted in icons in red, signifying this flame. The six wings are arranged in a particular way: two pointing down (covering the feet), two up (covering the face), and two outstretched (in order to fly). They are also frequently found surrounding images of Christ in glory. In the 14th century fresco by Theophanes, the Seraph is shown holding a flabellum or hagion ripidion (lit. “sacred fan”) upon which are the words “holy, holy, holy” in Greek: the same thrice-holy hymn used in Isiah’s description of these angels."
"As a side-note, these “sacred fans” are still ceremonially used in the Divine Liturgy today, to protect the body and blood of Christ and as a sign of honor (think of Cleopatra reclining whilst a flunky next to her waves a huge fan). Often these Liturgical fans have an image of a seraph on them, to remind us that God is indeed present in the chalice; the same God who is surrounded by the seraphim who fly about Him."
There were 16 Serfs and Cherubim in Hagia Sophia. All of them were enormous - 20 ft tall or more. 4 were in the nave pendentives and 12 were in the golden vaults of the South Gallery near the Deesis. The image above gives you an idea of what they would these heavenly creatures that serve and worship God would have looked like there. There were images of Cherubim on the Ark of the Covenant and throughout the sanctuary in the Jewish temple in Jerusalem, where the High Priest offered incense to them on high holy days. Thus images - even sculptures - of them have been sanctioned by God Himself since the beginning. Hagia Sophia was built to emulate Solomon's Temple as a shrine for the worship of God and the city of Constantinople an earthly substitute for the Heavenly Jerusalem.
In 2009 Aya Sophia Museum authorities announced they had removed the star covering the face of the left Seraph in the pendentive of the Cathedral of Holy Wisdom, which is now a museum. In 1348 part of the eastern arch of the church - along with a part of the dome - collapsed and damaged a large area on the floor of the church along with parts of the iconostasis and ambo. It is possible the altar was damaged as well. Although the city of Constantinople and its elite merchants and noble families were quite wealthy the Imperial Court of the time was not. The restoration of the church was an expensive project and an massive engineering effort that lasted for seven years. Funds had to be raised from foreign sources, including the Tsar of Russia. Once can imagine the perilous condition of the church just after the collapse and during the long restoration. Until the arch and dome had been stabilized and reconstructed any earthquake could have brought down the rest of the vaulting.
The restoration was not perfect and the eastern vaults and dome show the scars of the repairs, which appear irregular and damaged the symmetry of them. This is particularly noticeable - and disturbing - to modern visitors to the museum who don't know the circumstances of the repairs and the capabilities of the engineers of the time. Hagia Sophia has many irregularities that date from the construction of the church. Over the years various repairs and reconstructions added to these. Even during the original building of the church changes and revisions were were made. All of these added to the structural irregularities of the structure, some of which were repaired during the Fossati restoration, when several columns were moved.
Above: Here's a view of the pendentive before it was cleaned and the star removed. You can see large patches of Fossati plaster work to fill in lost areas of mosaic, especially on the left above the wing.
Above: Here you can see a mosaic setting bed in the on the north side of great eastern arch of Hagia Sophia with the remains of a standing Virgin and Child dating to 1354. The gold mosaic has been scraped off - the mosaic has been badly mutilated on purpose. When did it happen - it must have been some time after 1850 when the Fossatti brothers saw and recorded it. We also have the watercolors and drawings of Saltzenberg from the same period.
Below her are the traces of a mosaic portrait of the Byzantine Emperor John V Palaeologus dating from 1354. This mosaic was discovered in 1989, but has not been completely uncovered. John reigned from 1341 to 1391. In the top center of the arch was a representation of the Throne of God. Fragments of figures - a Virgin and Child with Saints Peter and Paul, were found by Fossati in the western arch as well, but they seem to have been totally lost. I found this text on the web along with the picture below:
In 1989, a huge scaffold was erected in the eastern arch of Hagia Sophia to repair loose plaster. The workers had noticed mosaic tesserae in the arch and Tauss was asked to examine them. Upon climbing to the top of the scaffold and examining the exposed mosaics, Tauss verified the identification of the mosaics, which depicted a Hetoimasia, the Virgin, St. John the Baptist, and John V Paleologus, who had funded the restoration of the building after a series of earthquakes in 1345. These mosaics had not been seen or documented since 1847, when Gaspare Fossati quickly sketched them before they were re-covered with plaster during the restoration campaign commissioned by Sultan Abdülmecid I. Tauss consulted with Cyril Mango, who was also in Istanbul at the time, and also with two conservators from the Austrian Academy of Science, Peter Berzobohaty and Claudia Podgorscheck, who were working on the Great Palace mosaics next door. They discussed technicalities and the possibility of funding and support for the conservation of the Paleologian mosaics in Hagia Sophia. Tauss suggested it was necessary to install an elevator to accommodate the work. It would work as a pulley lift and could take people and materials to the work area. The enormous estimated cost for the project was probably what kept it from ever happening. The mosaics in the east arch still haven’t been conserved and are barely visible to visitors because they are so high from the floor. There is no known publication about the 1989 re-discovery; Tauss said he would not publish his photographs of the mosaics without permission from the Directorate of the Hagia Sophia.
Above: You can see the traces of a mosaic of St. John the Baptist, below him would have been a portrait of John's wife Helena Kantakouzene. I never can quite figure out what the condition of these mosaics is. Is all that's left a setting bed?
Above: Fossati drawing of the mosaic of John V on the north side of the great eastern arch.
Above: Just behind the cable and above a patch of painted plaster is the ghostly image of the head of John V Palaeologus in Hagia Sophia, with a beard, crown and halo. he looks better here than in the Fossatti drawing.
The mosaic of the Seraphim was added in the final restoration of the damage of 1348, which took a long time to complete. The face is around 4 ft tall and cannot easily be seen from the floor of the church.
Above you can see the tips of the wings of the Seraphim on the right in a black and white image from 1960 taken by Robert van Nice. They are as nicely done as the one on the left side of the apse. I find it interesting that the Seraphims are not identical and have differently placed wing tips. The one on the right was not damaged in the 14th century earthquake and did not need to be replaced.In the image above you can see the tips of the Seraphim on the left, the one whose face was revealed. You can see the bad join between the 14th century repair and rest of the pendentive and dome cornice. Below is an image showing both Seraphims so you can compare the wings. There are pictures of the tips of the wings of at least one of the Seraphims on the opposite side of the church, so three of them have definitely survived in part.
Both Seraphims were intact in the 18th century and were visible, since none of the vault mosaics were covered until sometime sometime after that. Below is a reconstruction of the interior of Hagia Sophia showing the right seraphim.