Arcanum is the first recording by the duo of Armenian-American violist Kim Kashkashian, one of the central voices of ECM New Series, and Russian-born composer and pianist Lera Auerbach. It sets two works for viola and piano side by side. One is Auerbach's transcription of Dmitri Shostakovich's 24 Preludes op. 34, a cycle of short, often playful piano pieces through all the major and minor keys. The other is Arcanum, Auerbach's own darker sonata, written for Kashkashian and dedicated to her.

The album was recorded in October 2013 at Radio Studio DRS in Zürich, with Manfred Eicher producing and Peter Laenger as engineer. It appeared three years later. Auerbach made the transcription in 2010, on a commission for the Great Lakes Chamber Music Festival. She composed the sonata in 2013. Instead of programme notes, the booklet carries a conversation with both musicians and an essay by Anselm Cybinski. Explaining what drew her to Kashkashian, Auerbach speaks of "a quality of life-or-death-intensity to her performing, which is rare and wonderful".

Shostakovich wrote the preludes in 1933, in the line of Bach and Chopin, and most last between one and two minutes. In the duo version they become small chamber scenes: a waltz, a funeral march, a nocturne, a humoresque. The Largo no. 17 is a good example, its spare material divided between viola and piano. Arcanum means "mysterious knowledge", and Auerbach links it to an inner voice that guides us without words. Its four movements carry Latin titles. Advenio opens in very sparse writing, Cinis (ashes) moves like a stylised funeral march, and Postremo and Adempte follow, about 25 minutes in all.

Joshua Kosman of the San Francisco Chronicle called the transcription "a wizardly reimagining of its source", adding that the two musicians play "with urgency and finely tuned balance". In the German press, Werner Theurich of Spiegel Online heard Kashkashian bring out the links between folk roots and tonal avant-garde in the sonata. Steve Arloff of MusicWeb International (2017) singled out Postremo as his favourite movement.

CD edition of ECM New Series 2375, released on 30 September 2016.