Antonio del Pollaiuolo and Piero del Pollaiuolo, 'The Martyrdom of Saint Sebastian', completed 1475

A pale, clean-shaven young man, naked but for a loincloth, is bound to a tree. He gazes towards heaven, seemingly unaffected by the arrows which pierce his body.

This is Saint Sebastian, a Roman soldier who secretly converted to Christianity. When his faith was discovered, he was executed. He miraculously survived being shot by arrows, so was beaten to death. Saint Sebastian was an enormously popular ‘plague saint’. Plague repeatedly ravaged central Italy in the fifteenth century, and the wounds of the arrows were thought to be like the swellings it caused.

This painting was made in the mid-1470s by two brothers, Antonio and Piero del Pollaiuolo, for the private chapel (the Oratory of Saint Sebastian) of the banker and oligarch Antonio Pucci next to the courtyard of Santissima Annunciata in Florence. A relic – an arm bone believed to be Sebastian’s – was kept on the altar in front of the painting.

The location had a direct impact on the scale of the picture which, at just under three metres high and over two metres wide, was an exceptional size for a private chapel. The Pucci chapel was very large – it even had its own entrance from the street – and the altarpiece needed to be clear from over 16 metres away. It was designed to make an impact at a distance, although it would only have been on view on special occasions. Unlike medieval polyptychs, which had many panels, the format of this painting – it is a pala, with a single large pictorial space – and its scale allowed the artists to show off their talent at linear perspective and the important artistic concept of disegno. The monumental figures of Sebastian and his tormentors make a giant triangle in the foreground, with the archers‘ arms and legs pressing up against the sides of the painting. Behind them the winding river draws our eyes into the blue distance.

The Medici were the ruling family of Florence, and Antonio Pucci was one of Lorenzo de’ Medici’s right-hand men. As such, he was keen to imitate the opulence and modernity of the art the Medici themselves funded. The painting’s mix of classical and Christian imagery was typical of the Florentine fascination with works all‘antica, which the Medici promoted. Behind Sebastian’s martyrdom, knights on horseback gaze up in horror at the saint. Although they are in contemporary armour, they are meant to be Roman soldiers. One carries a red banner with the initials SPQR, which stand for senatus populusque romanus (’the senate and people of Rome'). On the left is a crumbling triumphal arch, perhaps symbolising the pagan culture which Christianity had conquered, decorated with roundels showing the Pucci emblem – the head of a Black African man, in profile. On its sides we see previous episodes of Sebastian’s life: a battle below, and his trial before the emperor above.

It’s very difficult to work out which brother painted what. The muscular archers around Sebastian – they seem to be frozen in motion – are probably by Antonio: their solidity and strength recall his paintings of the Labours of Hercules in the Medici palace (known from copies). The archers are arranged in pairs and have three basic poses, and are pivoted so that we see them from different angles; Antonio, who was a sculptor, might have made statuettes to use as models. He was, like Andrea Mantegna, intensely interested in anatomical exactness and in showing the human figure in dramatic, dynamic poses. Surviving drawings by him include a Saint Sebastian (Museum Kunstpalast, Düsseldorf) and an archer (Kupferstichkabinett, Berlin) which are related to this altarpiece. The figure of Saint Sebastian forms a vivid contrast to these straining, grimacing, unshaven archers. This is partly meant to emphasise the difference between him and them, but his soft, long-limbed body with its clumsily articulated hips and oddly flexible legs, is also closer to Piero’s style.

The idea for the panoramic landscape with its meandering river in a wide valley came from the Pistoia Santa Trinità Altarpiece and ultimately from Netherlandish painting. But the Pollaiuolos used oil paint rather than the tempera, which was usual for Italian painting at the time. This allowed them to achieve the delicate changes in tone and gradual blending of the colours fading to the blue hills on the horizon. A similar landscape appears in Piero’s Apollo and Daphne.

Look closely and you can see everyday life going on oblivious to the martyrdom: boats on the river, a party of people making their way along the road on the bank, scattered farms. There’s a walled city – clearly Florence, with the outlines of Brunelleschi’s dome and Giotto’s campanile dimly visible – in the distance. The whole scene has been transposed from classical Rome to the Arno valley. For Christian viewers at the time, the saints lived in the here and now, as well as in heaven.