The Apse Mosaic at Santa Maria in Trastevere, Detail: Isaiah
This is a detail from the triumphal arch (see below). Usually Isaiah and the phrase on the scroll are features of Nativity images, because the words on the scroll translate as "Behold the virgin shall conceive and bear a son." But in this case Isaiah and Jeremiah flank the apse mosaic of Christ and Mary enthroned in Heaven. The nativity pictured below Isaiah's portrait is that of Mary herself, the first in a series of panels on her life that continues below the apse mosaic and onto the right side of the triumphal arch.
As usual, Isaiah is pictured as an old man with a gray beard. As in his portrait at San Clemente, from a century or so earlier, his garb is quasi-antique, with an amply flowing mantle over a tunic with decorative bands at the collar and hem.
Extra-biblical traditions had it that Isaiah and Jeremiah were both put to death for their preaching.1 Unless simply decorative, the palm trees that stand beside them may refer to their martyrdoms and/or to Psalm 91:13, "The just one shall flourish like the palm tree: he shall grow up like the cedar of Libanus." The notion of flourishing is certainly suggested by the panels below their names, in which two youngsters display a kantharos wine cup or bowl surrounded by an array of fruits and flowers. The kantharos and the birds flanking it have both been symbols of immortality since classical times. (See my page on these symbols.)
I am not sure of the meaning of the T-L ligature on the tail of Isaiah's mantle. Letter-like imprints are also found on the mantles of saints in 6th-century mosaics such as those in the Mausoleum of Galla Placidia (example) and the procession of martyrs in Ravenna's Sant'Apollinare Nuovo.
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Photographed at the basilica by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.