By Javier C. Hernández

Deep in the vault of the Morgan Library & Museum in Manhattan on a late-spring day, the curator Robinson McClellan was sorting through a collection of cultural memorabilia. There were postcards signed by Picasso, a vintage photograph of a French actress and letters from Brahms and Tchaikovsky.

When McClellan came across Item No. 147, he froze:

“I thought, ‘What’s going on here? What could this be?’” McClellan said. “I didn’t recognize the music.”

McClellan, who is also a composer, snapped a photo of the manuscript and played it at home on a digital piano. Could it really be Chopin? He had his doubts: The work was unusually volcanic, opening with quiet, dissonant notes that erupt into crashing chords. He sent a photograph to Jeffrey Kallberg, a leading Chopin scholar at the University of Pennsylvania.

“My jaw dropped,” Kallberg said. “I knew I had never seen this before.”

After testing the manuscript’s paper and ink, analyzing its handwriting and musical style, and consulting outside experts, the Morgan has come to a momentous conclusion: The work is likely an unknown waltz by Frédéric Chopin, the great fantasist of the Romantic era, the first such discovery in more than half a century.