BERLIN — A previously unknown work by Johann Sebastian Bach has turned up in a crate of 18th-century birthday cards removed from a German library shortly before it was devastated by fire last year, researchers said Wednesday.

Experts say the work for soprano and string or keyboard accompaniment, composed for a German duke’s birthday, is the first new music from the renowned composer to surface in 30 years.

A researcher, Michael Maul, from the Bach Archiv foundation found the composition, dated October 1713, in May in the eastern city of Weimar. The Leipzig-based foundation said no doubt existed about the authenticity of the handwritten, two-page score.

“It is no major composition but an occasional work in the form of an exquisite and highly refined strophic aria, Bach’s only contribution to a musical genre popular in late 17th-century Germany,” the foundation’s director and a professor at Harvard University, Christoph Wolff, said.

Mr. Wolff said the work, written when Bach was 28, was among documents taken from the Duchess Anna Amalia library in Weimar for restoration before September’s devastating fire.

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

Mr. Maul, who has been combing church and government archives in eastern Germany since 2002 for clues about Bach’s life and work, said he was stunned to discover the work in the last of five crates of documents that had been in a room completely gutted by the fire.

He said the two pages were among several hundreds of poems and greetings written by officials and clerics to honor the 52nd birthday of the duke of Saxony-Weimar, whom Bach served as a court organist.

“If I hadn’t decided to go through them systematically, I would never have thought to look there,” Mr. Maul said.

Mr. Maul said it was the first Bach work to come to light since 1975, when a copy of the “Goldberg Variations” in a private collection was found to contain extra canons for piano in the composer’s own handwriting.

The last previously unknown vocal work by Bach to surface was in 1935, when the single-movement cantata fragment “Bekennen will ich seinen Namen” was discovered, the foundation said.

Bach composed the work for a solo soprano, to be accompanied by strings or a harpsichord, to mark the 52nd birthday of Duke Wilhelm Ernst of Saxony-Weimar, whom Bach then served as court organist, the foundation said.

The soprano was to sing a 12-stanza poem beginning with the duke’s motto “Everything with God and nothing without him” written by the theologian Johann Anton Mylius.

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.

Sir John 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.

Mr. Maul said hopes were high that the aria would be performed in Leipzig or Weimar to mark the first anniversary of the fire from which it had such a narrow escape.

The blaze destroyed about 50,000 historic books and damaged another 62,000. Restoration costs are estimated at between $61 million and $73 million.

The 16th-century rococo palace that houses the library reopened in February.

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

Mr. Maul said the foundation would exhibit the score once copyright issues have been cleared up.