At the entrance to Anselm Kiefer’s forthcoming exhibition at the Royal Academy visitors will encounter a typically paradoxical Kiefer object: a giant pile of lead books, sprouting wings. When I asked Kiefer to explain this strange object, he immediately — and characteristically — began talking about alchemy.

Lead, of course, was the material from which alchemists hoped to make gold. ‘But at the beginning,’ Kiefer explained, ‘it wasn’t just a materialistic idea, it was a spiritual one: to transform matter into a higher spiritual state.’ So, I suggested, in a way all art is alchemy: transforming one substance — paint and canvas, for example — into something else entirely. ‘Yes, certainly,’ Kiefer replied.

At 69 Kiefer is widely regarded as one of the most important artists alive, or, to put it another way, a master at the alchemy of metamorphosing all manner of items into something more interesting, and sometimes much more valuable than gold: contemporary art. He is one of a succession of notable artists who emerged from Germany in the 1950s and 1960s, beginning with Joseph Beuys, who was a mentor to the young Kiefer, and including Gerhard Richter and Georg Baselitz. Unlike Beuys, however, who was essentially a performance and installation artist, Kiefer’s work often takes more tangible and traditional forms: painting, sculpture, books, architecture — plus unclassifiable mixtures of one or more of the above.

He deals with epic subject matter. Often his paintings depict a landscape simultaneously ancient and modern, covered with ruins that might be the result of either war damage or the ravages of time: mighty ruined structures reminiscent of pyramids or ziggurats, halls like those of Wagnerian heroes and gloomy funereal vaults derived from the architecture of the Third Reich. From such pictures, lead model aircraft and warships may dangle, and metallic wheat or sunflowers sprout. The impression is of some lost civilisation.

‘I like vanished things,’ he explains, ‘because I like things that are ruined. That’s a starting point for me.’ Indeed, it was the setting in which his life began. Kiefer was born in March 1945, as the bombs were falling on Germany. He grew up playing in the shattered remains of buildings amid a defeated and discredited culture.

He is, despite the range of media in which he makes work, at heart a painter. ‘A photograph,’ he insists, ‘is only the instant the shutter was open, while a painting doesn’t only show a moment; it presents a history. It’s a living thing. It changes, it has depth.’ His paintings tend to be sober in hue, as if the colours were under a blanket of mist. ‘I like to hide my colours; if you go close to one of my paintings, you see all the reds, violets and greens. But I like them to be hidden under the grey, and I prefer misty landscape because it’s more enigmatic, more veiled.’ He admires Caspar David Friedrich, ‘the master of fog’.

As an artist Kiefer is a remote descendant of Friedrich, the great exponent of German romantic landscape. You might call him a ‘post-catastrophic romantic’; that is, an artist working in the aftermath of the Nazi apocalypse. His work is crammed with literary, religious and poetic ideas.

‘I have written since I was small and there was always an internal battle in me: whether to become a writer or a painter. For a long time it was not clear if I would become a writer or an artist; now it is clear.’ But his mind continues to be full of poetry and he often writes words on his pictures. These are not, however, the explanation of what you see on the wall, more a commentary. ‘The work comes first. To begin with you go into the colour, and do something unconsciously. Then you have an object to interrogate, and you have a conversation with it.’

Clearly, in terms of subject matter and sheer productivity, Kiefer is more a maximalist than a minimalist. When a few years ago he moved his centre of operations from the South of France to Paris, the transportation of his art and effects required no fewer than 110 pantechnicons.

At Barjac, in the département of Gard, where he lived and worked in and around an abandoned silk factory for two decades, an enormous amount still remains even after all those lorries have taken their loads away; so much that when I visited it last May with a group from the RA it took us the best part of a day to see it, and even then we didn’t quite finish.

There you wander past tottering concrete towers, which resemble a cross between the brutalist architecture of the 1970s and the medieval skyscrapers of San Gimignano, from which metal sunflowers bloom. There are underground chambers, one resembling a handcrafted version of the ancient Egyptian temple of Karnak, and 50 or so gallery spaces, all installed with pieces of Kiefer’s art, covering 35 hectares of land.

Conversations with Kiefer tend to be as all-encompassing as his work and also — unlike his work — surprisingly jolly. Despite the weight of history, intellectual complexity and horrors of Nazism that his art carries, he gives the impression of not taking himself too seriously. At Barjac, the discussion shifted unexpectedly from medieval scholasticism to chocolate cake. Kiefer originally hails from the Black Forest, and his cousin had sent a delicious Schokoladenkuchen for tea (which we ate with the artist and a BBC film crew).

He grew up as a Catholic — he served as an altar boy — in one of the frontier zones of Europe: the point of south-western Germany where the Latin and Teutonic worlds meet. His parents hid with him in the forest during daytime air-raids. ‘They put everything they thought was important into a little cart, and spent the day among the trees.’

Later in his boyhood, the Kiefer family moved to a spot close to the Rhine, again in a thickly wooded area. He describes the forest as ‘an enigmatic place. As a kid, I would go in there and I didn’t know where I was going to come out. Also, you know, when the cuckoo sings in the distance it gives a very poetic feeling of space.

‘In the spring when the river overflowed its banks, we had the water of the Rhine in our cellar. So in a way the border between France and Germany was going through our house. It was fantastic, there was no frontier anymore, the water was everywhere. That was what made me start to reflect about borders.’

Kiefer likes things that flow from one state to another. A few years ago, when he had an exhibition in Paris, he had the idea of building a high tower with, on the top, a lead-smelting device that would drop molten metal to the ground, so that ‘when the lead arrived at the bottom, it would not be solid but not liquid anymore either, between the two states’. Unfortunately, he was prevented from doing this for ‘bureaucratic reasons’ — so he did it anyway on his estate at Barjac.

The lead is a reference to alchemy, as we have seen. But it is also one of Kiefer’s favourite substances: he really likes the stuff, which is why our discussion next moved to plumbing. ‘When I started to use lead, which was 40 years or so ago, in the early 1970s, I had an old house in the forest and there was a problem with the piping. I found, when I came to do the repairs, that I had to do them with lead. I was so fascinated by it as a material, although at that time I knew nothing about it.’ The fascination was partly with its visual appearance: its sombreness, its greyness. But he also appreciates its mutability, the way it is easy to melt and bend.

He likes things that mutate, and that includes art. ‘Paintings change in two ways. They change naturally, with time, and they change intellectually, because the artist is only half of the process, the other half is the spectator and every spectator creates his own painting.’ So, I asked, will 50 per cent of the exhibition at the Royal Academy really be the creation of the visitors? ‘Absolutely!’