This venture saw them perform and record 198 of the 200-or-so surviving sacred cantatas in more than 50 churches associated - some of them very closely - with the composer and his music.
The works, wherever possible, were presented on the date in the church calendar for which, week by week, Bach had routinely composed them. The recordings, originally commissioned by Deutsche Grammophon, and then, when the German company disgracefully dropped out of the project, issued under Gardiner's own label, turned out to be a pilgrim's progress of the most illuminating sort.
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Bach's cantatas, more than his Passions and B minor Mass, still need champions. Gardiner, though not unrivalled, has proved the most eloquent of them, writing his own vivid essays to accompany the release of each pair of discs and making fascinating films of his experiences en route. To these far from routine performances he has now added a hefty 629-page book, flamboyantly named Music In The Castle Of Heaven and subtitled A Portrait Of Johann Sebastian Bach, in which the cantatas figure prominently, at the conspicuous expense, it must be said, of the more famous instrumental works.
But the vocal masterpieces, topped by the Passions and the Mass, are the ones Gardiner loves the best, and about which he has consistently enthralling things to say. People who write books about Bach often begin by declaring how little is known about him - the same used to be said about Mozart - but by using the music itself as a constant source of information, Gardiner brings the baroque era's greatest composer sharply into focus. Indeed, readers who choose Gardiner as their guide have no reason to fear that, even with scarcely a mention of the Brandenburg Concertos, they are going to be shortchanged; they will find that, on page after page, they are bombarded with eyebrow-raising information they did not already know. There is, as Gardiner reveals, no shortage of this. The result is a Bachian banquet.
It has all depended, of course, on how successfully Gardiner tracked down the things he wanted to serve up, and he was lucky, for a start, in the amount of material that is now coming to light in archives in what used to be East Germany. Although, biographically speaking, he takes a bit of time to wade through what he knows of Bach's youth and pre-history, once he gets fully into his stride he delivers one fresh and compelling perception after another. Above all, he lets the music speak for itself - which is what it should do in all good music books - and, in so doing, he keeps our attention riveted to what it is saying.
This is what gives Gardiner's vast portrait its special drive and vivacity. His personal involvement with his subject began early in his childhood, when one of the few authenticated oil paintings of the composer, his expression famously grumpy and quizzical, hung on the stairs of the Dorset farmhouse where Gardiner was brought up. Strange though it may seem, the painting - today frequently reproduced on record covers - had been entrusted to the Gardiners for safe-keeping during the Second World War, inspiring the conductor, now aged 70, to start the first chapter of his book with the startling words: "I grew up under the Cantor's gaze." Later, given the chance to become a pupil of the great Nadia Boulanger in Paris, he found the experience so stifling he ultimately abandoned it in favour of the assistant conductorship of the BBC Northern Symphony Orchestra in Manchester. His increasingly active life as a practical musician had begun.
If the early pages of the book are sometimes more self-portrait than Bach portrait, this does not impede Gardiner's quest to unveil a Bach sometimes strikingly at odds with the one usually offered to us. The result is a sort of musical detective story in which the notes of a cantata supply the clues that Gardiner scrupulously examines. In the course of his research he quotes, among modern scholars, all the right names - two of them, John Butt and Peter Williams, with Scottish links.
No other Bach biography, in my experience, provides such clear evidence of the tensions between the composer and his enemies among Leipzig's city fathers, by whom he was employed at the height of his career as Cantor, or musical director, of the historic Thomaskirche. Although to be told about these tensions is nothing new, to have them identified through the music of the cantatas makes you think you are in Bach's fierce, contemptuous presence as the performers deliver his attack on his opponents.
How else, Gardiner asks, are we able to grasp what happens in the course of Cantata No 178, "which exhibits such sustained defiance that one asks whether there is a submerged story here - of Bach operating in a hostile environment, of his ongoing conflict with the Leipzig authorities suddenly reaching boiling point". How much more satisfying, he continues, it must have been "to channel all that frustration and vituperative energy into his music, and then to watch as it rained down from the choir loft on to his chosen targets below".
Gardiner also depicts the copyists' weekly grind, in the days before blotting paper, as they prepared manuscripts in the composer's watchful presence for the forthcoming Sunday performance - poor Kuhnau, the best of the scribes, on one occasion receiving a smack across the knuckles for accidentally misspelling Bach's name on a title page, the spilled ink still visible on the paper (one of the countless arresting reproductions in Gardiner's book).
Yet Gardiner, as his Cantata Pilgrimage revealed, does not lack some Bachian asperity of his own. When he and his performers set off for St Magnus Cathedral, Orkney, the most northerly destination on their tour, a day's flight delay caused them to miss one of their two vital rehearsals there, as well as an opportunity to see the sights. Michael Tumelty, reviewing the festival for The Herald, did not fail to notice their ill-humour. Accusing them of behaving like prima donnas, he dismissed their concert as a dud. Gardiner, in his sleeve-note for the resultant recording, described the Kirkwall audience as "resistant to the music's charms" but admitted that "perhaps the fault lay with us travel-affected pilgrims".
Six months into his 59-concert tour, it seems he had suffered a frisson of Bachian impatience. But maybe, in a pilgrimage that took more time to perform and record the cantatas than Bach had needed to compose them, his response was understandable. His glowing, patiently written book makes full amends.