Devotion by Design, National Gallery, London

As we gaze on the sweet-faced Madonnas and suffering saints who people the Renaissance galleries of museums, it is easy to forget that these are images in exile. Not only were most originally painted for churches, but many were also part of larger paintings whose other elements are either lost or on display in isolation elsewhere.

Now, in an instance of the ill wind that prompts cash-strapped museums to focus on their permanent collections rather than mount costly temporary shows, the National Gallery has decided to winnow its holding of early Italian altarpieces into a display that highlights their neglected origins.

The result is an exhibition rich in revelation. Who knew, for example, that a small, detached panel by the 15th-century Tuscan painter Lorenzo Monaco showing the baptism of Christ could be traced back to its roots – as part of an altarpiece by Agnolo Gaddi that now hangs in the Louvre – through the gilt rosettes punched below its frame? Or that Piero della Francesca, arguably Italy’s greatest mid-century master, created an old-fashioned polyptych – where saints were confined to divided panels against plain gold grounds – for his local Augustine order because they demanded he use a wooden frame rejected by another painter 28 years earlier?

The practice of hanging a painting above an altar became widespread during the 13th century. By then, the Catholic Church had accepted the doctrine of transubstantiation – the transformation of the bread and wine into the body and blood of Christ – that is one of Catholicism’s most ineffable mysteries. It’s possible that altarpieces were intended to aid the enormous leap of faith required by the Eucharist – the ritual of consecration that the priest performed at the altar with his back to the congregation.

Liturgy demanded that the priest raise the consecrated host above his head so the faithful could witness it. One can imagine the effect of seeing the holy wafer mirrored by say, the chubby foot of a Christ Child, as in Benozzo Gozzoli’s “The Altarpiece of the Purification”, or in front of a bunch of grapes – symbol of wine and therefore of Christ’s blood – in Carlo Crivelli’s “Madonna of the Swallow”. Such images would help make this most abstract of concepts human and visible.

Altarpieces also served more practical purposes. For example, a predella – the strip of small paintings that ran below the main images of polyptychs – could also serve as a tabernacle to hold the consecrated host. A charming image of the Last Supper by Ercole De’ Roberti, showing Christ raising a snow-white disc to explain the Eucharist to his disciples, was revealed to bear the outline of a keyhole – a sign that it originally served this purpose.

Even more engaging is the notion of a painting as a ground plan. The work of the 14th-century artist Andrea di Bonaiuto da Firenze, a horizontal painting showing the Madonna and Child flanked by 10 saints charts the dedications of the chapels in the Church of Santa Maria Novella in Florence.

One of this show’s most valuable contributions is a copy of the contract drawn up before Benozzo Gozzoli embarked on his “Altarpiece of the Purification”. The extraordinary detail of its requirements, which not only name each saint to be included but also their position within the group flanking the Madonna and Child, bear witness to the complex network of patronage behind its creation. Ostensibly commissioned by a confraternity (a lay institution of young men), the location of this painting within the grounds of the Dominican convent of San Marco and the clergy’s close rapport with the ruling Medici family demanded that it satisfied myriad different interests.

In such a case, the painter’s art lay less in using his imagination to invent a story than in transforming a ready-made mise-en-scène into an image of organic harmony. Gozzoli does not disappoint: his ivory-skinned Madonna glows like a sacred queen at the apex of a pyramid of worshippers, while the carpet of flowers below her throne is a reminder that God’s work embraces the natural world.

The arrival of the Renaissance changed religious painting radically. As the 15th century unfolded, painters discovered the technical skill and the intellectual liberty to relinquish the hieratic polyptych in favour of a new style of altarpiece known as the pala. Painted on a single panel, it showed the vivid biblical stories – annunciations, crucifixions, depositions – that had previously been relegated to the predella paintings, whose diminutive size often rendered them invisible to all but the priest.

Aided by the new found technique of perspective, artists were able to render the rocky landscapes, pine-dotted gardens and ecclesiastical interiors of the terrestrial world with extraordinary naturalism. For a congregation who were still largely illiterate, the effect of seeing the Christian story told through these vivid, humanist narratives must have been intensely powerful.

Two paintings set up here to chart this evolution show that the process was far from linear. Created in the early 15th century, Giovanni del Ponte’s “Ascension of St John the Evangelist” has the divided panels and pinnacle-crowned frame of a polyptych yet the main image, with the saint kneeling on a carpet of clouds as Christ tugs him skywards, is full of lively narrative.

Even more unorthodox is the pala displayed in contrast. A homage to St Jerome, whose life of learning and suffering made him a favourite painterly subject, it shows the saint in his trademark pose with a stone in his hand – ready to beat his breast – and the lion at his side. Yet his image is separated from the saints around him by a gilt frame so that Jerome is simultaneously portrayed as an unattainable icon and as a vital protagonist of Christian history.

Behind most of these works lies a sorry tale of dismemberment. In the late 16th century, many paintings were dismantled when the rood screens – which separated the nave from the chancel – they decorated were removed on the orders of the Council of Trent (1545-63) to make churches more accessible to the laity. In the second half of the 18th and first half of the 19th century, the suppression of religious institutions across Italy saw thousands of artworks cut up and sold on a market that was increasingly hungry for Italian sacred paintings.

Unscrupulous dealers also assembled entire altarpieces out of panels that had never been together in the first place. A polyptych of the baptism of Christ by the 14th-century painter Niccolò di Pietro Gerini on show here was bought by the National Gallery from two Florentine art merchants in the 19th century. At that time, however, it also boasted three pinnacles painted by a different artist, Giovanni da Milano, which the dealers probably added to boost its value. They have now been removed and are on display separately in the museum.

No one who has visited Venice or Florence would disagree that the finest way to experience an altarpiece is in the church for which it was created. In an attempt to capture the effect, the curators here have actually arranged one gallery to resemble the interior of a church, complete with sacred music, candles and crucifix. It is the perfect setting in which to give thanks for an illuminating encounter with these exquisite painterly orphans.

Until October 2, The National Gallery

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