To Amsterdam. Where the sun is fiercely yellow, again, and the sky is intensely blue. Which is ridiculous. Because this is Amsterdam, for pity’s sake, not Provence. But for some reason the Fates have decided that whenever they send me to Amsterdam they must also ensure that the weather is meteorologically Mediterranean. When I was a student, I was forced to sleep rough here for an entire summer, slobbing out with hundreds of other dropouts in the Vondelpark, and not once did it cloud over or rain. Most of the time, this enforced sunniness is a gift from the gods. But when your task is to search for turbulent new truths about Vincent van Gogh and to explore again the darker stretches of his psyche, a sunny Dutch capital is the last thing you need. Bring on the rain. Give me clouds and misery. Help me out here!

I’m dropped off outside the strikingly concrete Van Gogh Museum, another place that feels completely wrong. Van Gogh should have a museum dedicated to him that is tottering and potty, somewhere visibly wonky that reflects the explosive nature of his genius and hints at the instability of his mind. Instead, the greatest collection of his paintings, his letters, the things he collected, is housed in a severe 1970s building inside which a tax-collecting agency ought to be located. It’s a bunker. It’s too tidy! But it is from here that the Van Gogh industry is run.

After his stupidly early death — 37 years was all he had — his works, letters and possessions passed to his family, who eventually gave them to the state on permanent loan. You will remember, because it is an important ingredient of his myth, that during his life Vincent hardly sold a picture. So the state got a lot. Yet the moment his life and work fell into the hands of his Dutch national guardians, they turned the hoses on him.

The Van Gogh Museum, you see, does not like the turbulent Van Gogh myth that so excites the rest of us. Neither do the surviving members of the family, who still have a considerable influence. The Lust for Life image of Vincent as an ear-cutting madman of genius offends their sense of rightness a little, and their sense of Dutchness a lot. They want to reconfigure him as someone sensible and recognisably local: disciplined, hard-working, logical, systematic, a straight-sided kind of a guy. I am here to find out if they are right.

Because something momentous is about to occur in the Van Gogh galaxy. His history is about to be retold at its Big Bang level. The world is about to have unveiled for it, with much international trumpeting, a gigantic new translation of his complete letters. Unedited. Uncensored. The truth. Ah yes, the letters. As if being the painter of scores of masterpieces that scorch themselves onto your retina were not enough, Vincent was also a marvellous writer. More than 900 missives by him are known to survive, most of them in the collection of the Van Gogh Museum, where they form — let me take a deep breath here — the greatest cache of writing about art left behind by any artist. They really do. There are other artists, of course, who wrote good letters, and other artists who were fine writers. Have you read Michelangelo’s sonnets? Superb. But Van Gogh differs from the rest in the sheer copiousness of his outpourings.

Between September 1872, when he wrote the first letter to his brother to have survived, and July 1890, when he wrote his last — found in his pocket, covered in bloodstains, after he shot himself by a chateau wall near Auvers-sur-Oise — Van Gogh was an obsessive correspondent. Most of these letters were to Theo, his younger brother, an art dealer, who supported Vincent financially and fraternally for the whole of his career. Without Theo, Vincent could not have happened: there would have been nothing to live on and nobody to talk to.

Their extraordinary correspondence — intense, relentless, gossipy, wheedly, utterly fascinating — is the main ingredient of the letters. Theo died a few months after Vincent. From syphilis. They are buried side by side in Auvers in a grave covered with Vincent’s favourite plant: ivy. (The sunflowers are our preferred taste, not his.)

This ivy, supposed to symbolise the brothers’ “clingy” relationship, was planted at the request of Theo’s widow, Johanna van Gogh-Bonger, the unsung heroine of the Van Gogh story. Jo is the one who collected the letters after his death and had them edited and published in 1914. They caused an immediate sensation and were quickly recognised as superior efforts of writing. Indeed, the letters have had almost as powerful an impact on the Van Gogh myth as his paintings. But Jo Bonger was as proper as she was energetic and warm-hearted. She was happy for the world to read the stirring and creative passages of her husband’s correspondence with his brother, but she didn’t want us to know everything. That would have been vulgar. So little sections here and there, where delicate family matters were aired, or immodest boy-to-boy suggestions made, were kept private. Censored.The new edition has put them all back. Indeed, it has put everything back. And then piled all manner of new stuff on top. The letters that have turned up more recently have been added. Endless footnotes have been gathered. Every quotation has been tracked down and discussed. Every artistic mention is illustrated. Every mark that Vincent made on paper has been reinvestigated, re-transcribed, reanalysed, then thought about again. The result is a six-volume monster that weighs roughly the same as Mike Tyson and will surely become an instant favourite on Desert Island Discs. Because this is now a book of unfinishable depths and lengths.

Two of its creators greet me outside the concrete bunker and lead me inside to the museum restaurant, where they ply me with delicious herring sandwiches in a very obvious attempt to get me onside. It works. I remember these sandwiches from the many days I spent here making a TV documentary on Van Gogh that involved going everywhere he went. I got the commission because I’d heard that the Van Gogh Museum had spent 10 years re-transcribing and re-translating the letters, and was about to publish the revolutionary results. That was five years ago. Which makes 15 years in all. A labour of love, or a task without end?

Hans Luijten, who has developed multiple sclerosis while working on the giant new edition and now limps through the Van Gogh Museum with worryingly fragile steps, describes the seemingly endless process as “a kaleidoscope that kept changing patterns”. Hans was responsible for re-examining the religious material in the letters. Poor man! When he was 22, Van Gogh gave up his first job as an art dealer and turned instead to God. He was living in London at the time, and I myself have stood in the pulpit in Isleworth from which he delivered his first sermon as a preacher. It was all about the road of life being long and tough, and couched in suitably interminable terms. Hans has tracked down all the biblical quotations with which Van Gogh stuffed his early letters, and set out to clarify their meaning. His efforts are truly heroic.

The other editor plying me with herring, Leo Jansen, whose job was to investigate all of Vincent’s artistic and literary sources, is surely being more accurate when he describes the 15-year odyssey as “like drops of oil hitting a watery surface”. As soon as the oil hit the water, it started to spread in every direction. Every mark had to be re-examined. Every sentence reconsidered. The overall aim was to be as accurate as a Dutchman could be. When Vincent misspelt a word, the misspelling was kept.

If Vincent didn’t finish a sentence, neither did Leo or Hans or Nienke Bakker, the third member of the chain gang, who joins us later and who was responsible for the Arles section of the letters. The new Vincent they were hoping to discover for us was methodical, determined, thoughtful — “more interesting as a man who knows what he is doing than as a mad genius, because everybody can be a mad genius”.

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