Seven centuries ago a poet penned the most ecstatic art review ever written. Francesco Petrarca, known as Petrarch, had commissioned the Sienese artist Simone Martini to paint a portrait of his beloved, Laura. The result was so marvellous, he wrote, that if all the famous artists of ancient Greece “competed for a thousand years they wouldn’t have seen a tiny bit of the beauty that’s conquered my heart”.
Petrarch’s rave review has it right. Conquering the heart is what Martini and other 14th-century painters from Siena do in the National Gallery’s devastatingly exact, epochal exhibition about the moment western art came alive. Simone’s painting of Laura is lost but you see why he was the artist for the job. He is so expressive, so tender, exploding any idea of medieval art as remote.
There is a melting intimacy to the soft pale faces from his Palazzo Pubblico altarpiece that are displayed as a row of portraits. Saint Anselm is a dreamy knight with a flag in his hand. Saint Luke, with his sketchbook and stylus at the ready as he gazes passionately at the Virgin Mary, may be a self-portrait. The Virgin herself is rapt as she looks at, or mystically past us. Even strapping baby Jesus in her arms is struck by sublime, face-misting thoughts under his mop of golden hair.

Isn’t such sensitive portraiture meant to have entered European art much later, in the age of Botticelli, Petrus Christus and Leonardo da Vinci? The Renaissance, this exhibition shows, was all there in microcosm in the creative explosion that took place in a small hilltop city 700 years ago. The Siena of this exhibition has nothing much to do with the tourist Siena of the Palio horse race. Instead you walk into a dark space where gold-mounted paintings are picked out by intense lighting, and are suddenly plunged into a world of inner feeling.
The love between mother and child peels away your cynicism in a small, heart-opening panel that’s the first thing to hit your eyes – a little painting by Duccio, done some time between 1290 and 1300. Around it are examples of what a worshipper in an Italian church would usually have seen at this time: stiffly posed images of the Virgin by Byzantine artists and their imitators. Duccio’s Virgin and Child bursts out of those conventions. Instead of sitting coldly enthroned in his mother’s arms, Jesus, dressed like a little adult but with baby hands and feet, reaches up to touch his mother’s veil, pulling it aside to reveal her face.
Duccio has a touching, truthful eye for how a real baby might touch mummy’s face. This is the start of an intimate journey as three generations of Sienese artists compete to be still more alive to motherhood, culminating with Ambrogio Lorenzetti’s Madonna del Latte in which a life-sized baby looks at you intensely with a huge, gloating eye as he sucks on Mary’s naked breast, holding it in his little hand, letting us know her life-giving milk is all for him.
Once again, though, Simone Martini adds a genius twist. His painting Christ Discovered in the Temple, lent by Liverpool’s Walker Art Gallery, brings acute psychology to a story of family tension. The teenage Christ has wandered off in the Temple, panicking his parents. Joseph has just brought him back. Jesus looks sulky and confrontational. His father’s purple face indicates he is just about controlling himself. Mary sits calmly but her words, written in Latin on the book in her lap, roughly translate as, “Son, why do you treat us this way?”
All this emotional power comes from a mastery of space. Perspective was invented in Florence during the Renaissance, right? Not entirely. The artists of early 14th-century Siena did not yet comprehend the systematic, “single-point” perspective Leonardo would use. Instead they play with partial perspective effects to create fairytale vistas of walled cities, merchant ships and the soaring interiors of gothic cathedrals.

This playful mapping is at its most magical in the masterpiece at the heart of this show. Duccio’s Maestà was a massive altarpiece broken up in the 18th century. The National Gallery owns three panels from a layer of narrative scenes at the bottom, known as a predella. It has borrowed accompanying panels to resurrect this artistic miracle.
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You follow the row of paintings like a comic strip: Jesus defeats the devil, heals a blind man, raises Lazarus from the dead. Yet what grabs you is Duccio’s spatial conjuring. As the devil tempts Christ to throw himself from the highest point of a temple, we see an amazingly solid, polygonal building filling the wooden panel, even seeming to bulge out of it. Christ confronts Satan on a marble balcony rendered in eye-fooling depth. You can see inside where gothic arches and a particoloured pavement recede with hypnotic reality. It’s not just visual trickery. Space creates emotion. The solidity of real objects means that when the devil dares Christ to jump, there is danger.
Poetry again is key. Duccio was the contemporary of Dante whose Divine Comedy visualises Hell, Purgatory and Paradise as real places with the same spatial precision that makes the Maestà so wondrous. Like Dante, you’ll be guided through other worlds by this exhibition. O Siena!
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Siena: The Rise of Painting 1300-1350 is at the National Gallery, London, from 8 March to 22 June