BERLIN — Experts have discovered a previously unknown work by Johann Sebastian Bach in documents taken from a German library shortly before it was damaged by fire, researchers said yesterday.

It was believed to be the first new Bach work to surface in 30 years.

Researcher Michael Maul found the aria, dated October 1713, in May in Weimar, the Bach-Archiv foundation said on its Web site (www.bach-leipzig.de).

There was no doubt about the authenticity of the handwritten, two-page score, the Leipzig-based foundation said.

Maul said it was the first Bach work to come to light since 1975, when a copy of the "Goldberg Variations" was found to contain extra canons for piano.

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

The foundation's director, Christoph Wolff, said the work, written when Bach was 28, was among documents taken from the Duchess Anna Amalia library for restoration before September's fire.

"Otherwise the work would have been consumed by the flames, and we would never have known of its existence," Wolff said.

Bach composed the work at age 28 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.

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