A CONCERT by Gustav Leonhardt was not like any other. He approached his harpsichord with the air of a mortician, slightly flexing his long, delicate hands. As he played he sat bolt upright, gaunt and aquiline, unsmiling in his crisp, perfect suit, with his elbows held close to his sides. No unnecessary gesture, no hint of emotion: senza baldanza, as a composer might have marked it. He did not have the look of a man on a mission. But he was.

Mr Leonhardt's life-work was to persuade the world how beautiful the harpsichord was, and how the harpsichord repertoire should be played. When he first fell in love with it, in the shape of the fairly bad instrument his parents bought for their house at Graveland in the Netherlands, he recognised it as the king of keyboards. Organs were noble characters, and he played church organ for years. Virginals were pleasing; he wrote a book on Flemish examples. But fortepianos were awful, the sound muffling all over the place when the hammer hit the keys, which put him off playing his beloved Mozart; and modern grands were unspeakable. None had that direct pluck of plectrum on string for which he loved the harpsichord—though that mechanism was also fearsomely exacting, even “diabolical”, and that was why he did not smile as he played.

It would also have been vulgar. Mr Leonhardt was ever on the watch for that, whether in the form of electric lighting, or showy articulation, or hotel breakfast buffets, or Beethoven's Ninth. (“That ‘Ode to Joy', talk about vulgarity! And the text! Completely puerile!”) His own manners were exquisitely courteous; he seemed to have stepped from the past, and even a shockingly fast drive in his Alfa Romeo might end with Mr Leonhardt, lost, finding his way home not by sat-nav but the stars.

When he began to study harpsichord seriously, at Basel in 1950, the instrument had been neglected, or overlaid with Romantic sweetness, for decades. He intended to restore it to the simple, original sound, “salt rather than sugar”, that Johann Sebastian Bach had written for. If people found that sound too thin for modern halls, and the pitch disturbingly low, too bad; their ears would just have to get used to it. And after a while, they did.

It meant hard work for him. He began by tirelessly hand-copying hundreds of original scores in the Vienna Library, when he was meant to be studying conducting (but he scorned conducting, thinking it “the easiest way out” in music, with never a wrong note to worry about). He continued by making a definitive recording in 1953 of Bach's “Art of Fugue”, and publishing an impassioned argument that the piece had been written for solo harpsichord rather than ensemble. That stirred up interest in pre-Romantic music, though still not enough to fill a room when his little consort played Biber's unpublished “Fidicinium sacro-profanum”, or other treasures he had unearthed. He thought of those as his catacomb days. Fairly quickly, however, listeners warmed to Byrd and Frescobaldi, Rameau and Ritter; his own recordings, especially with Nikolaus Harnoncourt of all Bach's Cantatas, fanned the flame; and the early-music movement has flourished ever since.

What would Bach do?

Mr Leonhardt's own standards of “authenticity” were severe, as befitted a man whose brick house in Amsterdam had shelves, tiles and floorboards unaltered from its last updating, in 1750. It certainly did not mean just stringing a modern violin with gut and buying an old bow, while keeping the modern bridge. It did not mean playing a harpsichord strung with modern steel, or fussy ornamentation as a piece was played. For years he laboured to find the most authentic replica harpsichord; his favourite, by Martin Skowroneck of Bremen, which had pride of place in his huge drawing room, was made of 18th-century woods. That may have been why it sounded better than any other, but he could not exactly tell.

The search for authenticity often ended in a mystery. It was never just a matter of getting the instruments right. Who knew how Bach, the greatest musical genius who had ever lived, had played? There was no phrasing, no indication of loud or soft, on scores from his time. That run of notes—legato or non legato? Equal or unequal? Nobody knew. And on top of that came the mysteries of performance. Did Bach lead from the harpsichord or the violin? Did he like the acoustics in St Thomas's in Leipzig, or did he hate them? When Mr Leonhardt taught in Vienna and Amsterdam, never taking more than five pupils at a time (many of them becoming distinguished harpsichordists in turn), he never imposed a method on them; he simply listened to their playing, heard what they lacked, and worked on it. Their approach was up to them. Music could not be authentic, he often said, in the way a poem or a painting was. You could never know exactly what was in the composer's head.

Nonetheless, when he was asked in the mid-1960s to play the part of his favourite composer in “The Chronicle of Anna Magdalena Bach”, and was filmed in a long periwig and a frock-coat forging elegantly through the fifth Brandenberg Concerto, many listeners may well have felt that this was as near the mind of Bach as any man could reasonably get.