Revelatory is an overworked word but precisely the right one for this new release. Sibelius’s reputation rests almost wholly on his orchestral output of tone poems and symphonies, but in his youth – before the opportunities for larger composition presented themselves – he cultivated chamber forms. The early string quartets have long been known but none of the piano trios aside from the delightful 1888 Loviisa Trio (named from the Finnish resort where the Sibelius family holidayed that year). It alone of the works here has been recorded before, in splendid accounts from the Tapiola Trio listed above and by Yoshiko Arai, Seppo Kimanen and Juhani Lagerspetz (Ondine, nla). The present account is fully their measure, but BIS’s sound – engineered by Uli Schneider – is by some way superior.

The fascinating couplings here are diverse fragments from Sibelius’s student output. Folke Gräsbeck’s useful notes suggest that some shorter pieces might be studies for better-known works, such as the song-cum-melodrama Näcken (the A flat Allegretto), or orchestral Scène de ballet and Karelia Intermezzo (the E flat Allegretto, completed almost seamlessly by Jaakko Kuusisto). The most remarkable, though, is the D minor Allegro (1889), in a completion by Kalevi Aho, and – the Alla Marcia aside – the latest piece here. Its six-minute span really does hint at the structural mastery to come.

However, the major find has to be the Korpo Trio (1887). Named after yet another youthful holiday venue, this is Sibelius’s largest chamber work apart from the G minor Piano Quintet, running for nearly 36 minutes. Its structure is unusual for a devotee of Haydn and Mozart, though less so of Beethoven, with a central span of more than 15 minutes and substantial outer movements. An early, apprentice work undoubtedly with barely a hint of the personal fingerprints familiar from his mature scores, the Korpo Trio yet tantalises with the prospects of what the symphonic master might have achieved had he applied his genius to more intimate forms. Chamber music making as it should be done.