Shortly after Irina Shachneva graduated from conservatory in 1985 in the Russian city of Gorky (now called Nizhny Novgorod), her conducting professor gave her one last assignment.

“He came to me with a small black box full of negatives,” Shachneva recalled, “and he said to me, ‘I feel like no one will perform this, but you will.’ I said, ‘Perform what? What is this?’ He said, ‘You’ll find it in this box.’ ”

Shachneva had been entrusted with the score of “Liturgy,” an hour-long choral work by the Russian composer Konstantin Shvedov. The piece dates from 1913, but was kept hidden through the years of the Stalinist regime and beyond.

Now Shachneva is finally poised to fulfill her professor’s charge.Slavyanka, the San Francisco-based Russian chorus that she has led since 2012, will give the first U.S. performance of Shvedov’s 13-movement setting of the Divine Liturgy of St. John Chrysostom, the principal prayer service of the Russian Orthodox Church, on Friday, May 19, at Berkeley’s St. Mark’s Church.

“This is beautiful music, in a unique compositional style I’ve never been familiar with before,” Shachneva said. “I fell deeply in love with it. I was really kind of surprised.”

If even a Russian-born choral conductor can train without being exposed to Shvedov’s music, she’s hardly alone. Shvedov is a shadowy figure in the musical annals, unknown even to the compilers of the authoritative 20-volume New Grove Dictionary of Music and Musicians.

“I have to be honest and say that I didn’t really know about him before this,” said Harlow Robinson, a leading scholar of Russian and Soviet music who was brought in as a consultant on the project. “He’s one of the many composers who left Russia around the time of the revolution and just kind of got lost.”

Shvedov生于1886年,让他约half a generation younger than such late Romantic icons as Sergei Rachmaninoff and Alexander Scriabin. Shvedov immigrated to the United States, where he died in 1954 without leaving much of a musical legacy.

But those who’ve sung his “Liturgy” are convinced that there are treasures here to be unearthed.

“It’s a different sound from most Russian liturgical writing,” said Paul Andrews, the chorus’ co-founder and former director who still sings with the group. “The chords move in ways that you don’t necessarily expect, and the way the individual voices operate gives the music a shimmering quality.”

For a piece like this to have gone underground, only to surface in time for a centennial performance, is not as surprising as it might seem. Throughout the Soviet era, any public expression of religious feeling was a dangerous undertaking, best kept under wraps.

“The Bolshevik regime was founded on atheistic principles and detested the Orthodox Church,” said Robinson. “They destroyed hundreds and thousands of churches. They shot priests and nuns in mass killings. And liturgical music was effectively banned for public performance until the 1980s.”

For Shachneva, a physical copy of the score for a liturgical work feels like an echo of the Orthodox icons her grandmother kept stashed beneath the mattress. Her professor, she said, had inherited the score from his own father, a priest with a deep interest in music.

“The reason why people of this generation kept this music in negatives was because of the history of fear in Stalin’s time,” she explained. “If somebody comes to your house and finds church music, you could be arrested and even pay with your life.”

Religious content wasn’t the only thing working against Shvedov’s “Liturgy.” There’s also the composer’s decision to leave the USSR, which made him a traitor in the eyes of the regime.

“一旦有人从苏联移民,they were sort of wiped out of musical history,” Robinson said. “That was even true of important figures like Rachmaninoff. They just became a persona non grata.”

The silence surrounding such figures means that even the provenance of a score can be mysterious or completely unknown. The photographic negatives given to Shachneva by her professor suggest that a printed edition of the “Liturgy” once existed. But for performances of the work, which had its modern-day premiere in 1988 in Moscow, singers had to transcribe the music by hand.

“Shvedov’s name was totally deleted,” Shachneva said. “The score may have been put on the very last shelf of the libraries that nobody can actually reach, or even know that this composer existed.”

The revival of Shvedov’s choral work on its 100th anniversary could be considered timely. But it also comes at an awkward time for any expression of interest in Russian cultural history.

The Russian invasion of Ukraine, which has lasted more than a year, has prompted protests about performances of any Russian music, even works dating from long before the current political makeup of the country.

“We talked about this quite a bit in our choir, because we are people with all kinds of perspectives,” said Andrews. “Ultimately we put together a statement that expressed our view on the matter.”

That statement reads, in part, “Like other Americans, we are horrified by the war in Ukraine, call urgently for its end, and have raised money to offer help to victims. But art has something more important that it can do. … Even in these agonizing times, we are called to be brothers and sisters with people halfway around the world in the common human family.”

Robinson draws a distinction between artists of the past and contemporary performers such asconductor Valery Gergievor soprano Anna Netrebko, who have publicly aligned themselves with Russian President Vladimir Putin.

“I can understand why there would be unease in having them perform,” Robinson said. “But I think punishing Tchaikovsky, Mussorgsky or people who actually suffered under Soviet oppression, is ridiculous.”

Reach Joshua Kosman:jkosman@sfchronicle.com; Twitter:@JoshuaKosman