For three centuries it was hidden in an old shoebox, concealed beneath a couple of blank pages. But Tuesday music experts across the world were hailing the discovery of a previously unknown work by the German composer and genius of the Baroque era, Johann Sebastian Bach. The work, for a soprano and harpsichord, was written in October 1713 as a birthday present for Bach’s patron, Duke Wilhelm Ernst of Saxe-Weimar.

Bach, then the court organist in Weimar, penned the composition to go with a 12-stanza poem dedicated to the duke, but its existence was swiftly forgotten. The manuscript was apparently swept away into a box, together with numerous other poems and letters written to celebrate the duke’s 52nd birthday.

The library in Weimar where the music was stored for several centuries recently burned down, but by chance, the box containing the score had already been removed. Two weeks ago a member of the Bach Archive in Leipzig, Germany, Michael Maul, stumbled on the composition while looking through material relevant to Bach’s tempestuous but thinly documented life. The box contained more than 100 poems and verses, together with a mysterious “strophic aria.”

Proof that the work was genuine came when experts compared the hand-penned manuscript with Bach’s writing.

“After Michael and I had identified it as Bach’s, we opened a very expensive bottle of Champagne,” Peter Wollny, head of research at the Bach Archive, in Leipzig, said Tuesday. “Michael came back from Weimar two weeks ago and said he had found something interesting. We got the microfilm of the score last week. We compared it with Bach’s known compositions — and bingo.”

He added: “The last time anything by Bach was discovered was 80 years ago. So far we’ve only heard it on the computer. But it’s a charming little work, written for one singer — a soprano — and a harpsichord. There’s a little postlude at the end for a string ensemble — two violins, a viola and a cello. It takes just four or five minutes to play.”

British conductor Sir John Eliot Gardiner, who has been asked by the Bach archive to perform and record the aria, said: “It’s so exciting. Maul has been sleuthing away, looking at the records in Weimar, which is something of a forgotten town in terms of Bach’s history.”

Gardiner believes the aria is likely to be part of a longer cantata. “It is absolutely beautiful. So many of Bach’s cantatas went missing after he died. His son Wilhelm Friedemann Bach was pretty profligate with his father’s stuff. He sold manuscripts off, lost them, used them as fire lighters. So when something like this turns up it is wonderful.

“It’s a reflective, meditative, soothing piece, as Bach’s church music so often is. It’s not going to set the world alight — enough of Bach’s music from this early to mid-period has survived to give us a sense of his musical personality at that time — but it’s just great to have this because every one of his cantatas and arias is on a completely different level from all of his contemporaries.”

Gardiner plans to record the piece before Christmas and perform it at London’s Cadogan Hall on Dec. 18.

Wollny Tuesday said that the composition sheds fresh light on Bach’s enigmatic early career in Weimar, a small town in central Germany, which was later made famous by Goethe but at the time boasted just 5,000 inhabitants.

Born in 1685 into a highly musical family, Bach worked as court organist in Weimar from 1708 to 1717. He was also a member of the town’s chamber orchestra, which he led from 1714. During this period he was rapidly becoming famous, not just for his compositions but as Germany’s greatest organist. His storming performances frightened the organ builders.

“We hardly know anything about Bach from this period because very little has survived,” Wollny said Tuesday. “There are very few compositions. It fills a black hole in his artistic career. It also tells us a great deal about his musical and vocal style during the Weimar period.”

So delighted was the Bach Archive by its discovery that it Tuesday flew in professor Christoph Wolff, the world’s leading expert on Bach, from Harvard University. Wolff said he was convinced the work was genuine, and described it as “an exquisite and highly refined strophic aria, Bach’s only contribution to a musical genre popular in late 17th century Germany.” Other stunning Bach discoveries could follow, he predicted.

The box where the manuscript was discovered, said Wollny, was only removed from the library because the researcher, a bookbinder, was interested in the rare marble paper on which the work was written. “Otherwise it would have been burned down in the fire. Nobody would have known that it existed,” Wollny said.

There has been no previous record of, or reference to, the composition, which has the words “aria,” “soprano solo” and “ritornello” written at the top. The last time an undiscovered piece by Bach turned up was when a cantata was found, but the work was a mere fragment.

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