When the Danish painter Christen Købke set out to depict the sprawling architectural mass of Frederiksborg castle, a dark genius seemed to possess him. The castle was a national landmark, and in the Romantic age was being rediscovered by writers and artists as a relic of Denmark's glorious history: here was a great relic of the Danish renaissance, now long past. Like the devil tempting him to fly, this genius urged him to go up, up – higher, higher – into one of the soaring towers of the castle, to look down on its black rooftop and over the still landscape beyond. Look, look, said the devil, look into that sky. How empty it is – how infinite!  Købke drew the scene from the loftiest heights of the building and then, back in the studio, painted from memory, painted it exactly. Like a brutally cropped photograph, his view from Frederiksborg's high towers takes in a roof as abstract as a bar placed across the canvas, a red rectangle of a chimney, the spire of a tower and the woods over the silver water. Yet this fills only the lower third of the canvas. Above there is nothing but air, the immense space of an illuminated sky. It seems to be pressing down into nothingness.  Købke's painting Roof Ridge of Frederiksborg Castle, with View of Lake, Town and Forest (c1834–5) is a disconcerting masterpiece of Scandinavian art. Its empty sky, its melancholy attention to the unvisited heights of a building, can be seen as a precursor of the chilled fjord scenes painted by the Norwegian artist Edvard Munch half a century later.  Comparisons with Munch might not be the first thing to strike a visitor to this new exhibition at the National Gallery in London, Købke's first outside Denmark. Initially, this feels like an introduction to Copenhagen's own Jane Austen: a sensible, modest artist who patiently worked his little bit of ivory, portraying the people around him in calm, finely observed pictures of bourgeois life. Are the curators attempting to overturn the cliched view that Scandinavian art is, um, exciting? If you thought artists in the north were all about revelations of the sublime and encounters with the abyss – from the eerie Romanticism of Caspar David Friedrich to the modern light art of Olafur Eliasson – then huh, what do you know? Here is a Danish artist of the Romantic age, regarded by Danes as the greatest of his time, and he is so sensible, looking at his work is like going to church.  But this is not the whole story, not by any means. Købke's quietness is filled with strange imaginative wanderings. His eye seems drawn to the interstitial, the neglected, the silently waiting. Even if there was nothing Romantic about his art, the brevity of his biography would qualify him as one of this breed: the son of a prosperous Copenhagen baker, he was born in 1810, and dead by the time he was 37. A lot of his life was spent in the Citadel, a vast fortress built to defend Copenhagen's harbour, and in his time used as a prison; his father was the prison baker. From his paintings, you would never guess its military origins, or that it housed prisoners locally known as "slaves". Or would you?  A sky tinged with blood  Købke is drawn to the fortress's gatehouses and drawbridges, which he paints with a hypnotic sense of time slowed to a snail's pace. For instance, his 1837 picture The Northern Drawbridge of the Citadel in Copenhagen, concentrates on the red wooden structure that suspends the bridge over mirroring still water: ice-cold water, surely. People stop on that bridge and beside the moat, staring or talking quietly. Above and beyond, we see that empty sky again: it is tinged with salmon pink, as if blood were running from the bridge's frame into the ether. At first glance so placid, this painting lures you into a frozen moment, so that you share the introspection of the people in it; you, too, are passing time by the bridge, gloomily. It reminds me of Munch's paintings of young women gazing into Oslofjord, to the extent that I wonder if Munch saw this work. It resembles a Van Gogh painting of a similar drawbridge structure near Arles. Købke, like Van Gogh, appears attracted to the eccentricity of the framework, which becomes troubling and uneasy.  The difference between Købke and these later heroes of the northern vision is that he revelled in a precise academic style. At the time, Copenhagen's Royal Academy of Arts was one of the most highly regarded schools in Europe when it came to drawing and painting in the classical tradition. And Købke was a good pupil. His paintings testify to his belief in the Greek style, in close study of the human figure – all the rules of academic art that European painters were to rebel against 50 years later. Købke's painting of a male nude is in a tradition that goes back to Michelangelo and Raphael. His 1830 painting View of the Plaster Cast Collection at Charlottenborg shows a curator wiping dust off a pedestal beneath a cast of a Greek hero fighting a centaur from the Parthenon.  And yet, look again at this painting. The wan contemplative spirit of Scandinavian art once again creeps in, over the shelves, infusing the silver light. Why show a man dusting? It is a pessimistic detail, and reminds us of the existence of dust. The custodian is engaged in the daily battle to keep these fragments white and gleaming. Suddenly we are there, in this empty gallery, on a freezing morning, watching this man dust antiquities in the gelid, vodka light.  Back at the castle, Købke stands by the lake and watches evening redden the sky. The mass of red walls and spired towers is reflected in the water precisely. Dark shadows glare from the palace windows. Strands of cloud hang in the emptiness. Købke is a patient, careful artist, but what he records, so accurately, is a world whose routines seem poised on the edge of infinite mystery. He is a craftsman of the abyss.
Danish master Christian Købke painted empty skies, intimate portraits and melancholy landscapes. Jonathan Jones gets lost in a world of infinite mystery