Experts have discovered a previously unknown work by Johann Sebastian Bach (search) in a German library, a research foundation devoted to the composer said Wednesday.

Historians found the aria in May in the Anna Amalia Library in the eastern city of Weimar, the Bach Archiv foundation (search) said on its Web site.

There was no doubt about the authenticity of the handwritten, two-page score, dated October 1713, Leipzig-based the foundation said. It was the first unknown vocal work by Bach to surface since the discovery of the single-movement cantata fragment "Bekennen will ich seinen Namen" (BWV 200) in 1935, the foundation said.

"The find is a well-rounded composition -- not a major work, but a casual piece of superior quality," the foundation said.

Bach composed the work for a soprano, to be accompanied by strings or a harpsichord, to mark the 52nd birthday of the duke of Saxony-Weimar, for whom he worked as a court organist, the foundation said.

A solo soprano was to sing a 12-verse poem beginning with the duke's motto "Everything with God and nothing without him" written by Johann Anton Mylius, it said.

The work was Bach's only known strophic aria, in which several stanzas are set to the same music, and the precise date made it valuable to researchers studying the development of the German composer's style, the foundation said.

It was not clear if it was played at the time, but the foundation said English conductor Sir John Eliot Gardiner is preparing to record it.

Gardiner last month received a medal in recognition of his performance of Bach music from the Saxony city of Leipzig, where Bach was cantor of St. Thomas Church for 27 years.

Germany's Baerenreiter publishing house plans to publish the composition in the fall.

The library where the manuscript was found in a 16th-century rococo palace that reopened in February, five months after a suffering extensive damage in a fire.