A volcano erupts – or a floodgate opens – at the outset of Boulez’s Second Sonata. I dive right in with Tamara Stefanovich. We hear a D-A perfect fifth, then a D sharp-G sharp fourth. The ear may not hear the second interval as nested within the first, because they are pitched in different octaves. Then a four-note semiquaver hammering downwards, like Beethoven’s Fifth.

Two motifs, eight notes, one phrase: this is the ‘first subject’ of a sonata that sets out to blow up sonata form – by which I mean both the large-scale shape of the Classical-era sonata and the dramatic single-movement form outlined by the vocabulary of exposition, development, recapitulation and coda. Does sonata form need blowing up?

Boulez wrote the Second Sonata in 1948. He was 23 years old. He and his contemporaries had witnessed their elders visit unimaginable horror on the world around them. The Second Sonata is ‘a manifesto of the generation,’ remarks Stefanovich, ‘not just from him. A manifesto that was needed at that time.’

In a booklet introduction to her new recording on Pentatone, Stefanovich recalls her first encounter with the Sonata, living in Belgrade in the midst of the Balkan wars. ‘I was in my 20s,’ she says to me on a video call. ‘Having played the big Brahms and Rachmaninov pieces and Ligeti Études, and thinking that I was now equipped to deal with Boulez, and then finding out that I was pushed to my limits – it was a good but brutal awakening.’

Boulez’s artistic manifesto of the 1940s could be summed up as ‘Organised Delirium’, which Stefanovich has adopted as the album title (shared by Caroline Potter’s recent study of the composer, 10/24). Passage after passage of late Beethoven springs to mind from the phrase, especially in the quartets. If Boulez pays his respects to any figure from the past in the Second Sonata, it is Beethoven, who famously paid little respect to anyone.

Analysts have drawn threads between the Second Sonata and the Hammerklavier, such as the fugue or anti-fugue in the later work’s finale. Stefanovich alights on the prominence of the trill as an expressive device in the first movement. ‘A trill was always a kind of embellishment, an accessory, a prolongation of sound. Boulez uses it this way, but he puts a new spin on it. The trill halts momentum, it becomes a hysterical vibration in a confined space. Each time you hear the trill in this movement, it’s a kind of alarm bell to say that something new is coming.’

This second subject is chordal, ‘like Greek columns, hard to move,’ and the movement as a whole shifts rapidly between liquid and static states, reminding Stefanovich of Bartók: ‘He does this a lot in the piano concertos.’ In Boulez (Debussy, too), extremes of precision and expression do not operate in contradistinction. ‘The vocabulary we need for Boulez is a new vocabulary,’ says the pianist. ‘We’re in the middle of the 20th century, with all these new ideas about physics, industrialism, DNA. They had new ways of looking inside an organism and defining it, from the smallest cells to bigger ones.’

These ideas arise from the decades Stefanovich has lived with the Second Sonata, and from the hundreds upon hundreds of hours required to conquer its complexities, but they are also inspired by working with Boulez himself. ‘When I went to see him, I only had a pink pen, so everything he said to me is written down in pink.’ She holds up to the screen a score liberally annotated with private insights that qualify if not sometimes challenge printed injunctions from the 20-something composer: ‘Absolutely avoid, especially in slow sections, what could be termed “expressive nuances”.’

There is no place in the Sonata for the rubato that is taken as read in Chopin, not always to the music’s advantage. The three voices of the second subject are marked ‘absolutely equal’ – impossible to achieve in practice, and not even desirable when taken literally. ‘As Liszt said, no chord is ever equal. I played this section to Boulez in a very square way, and he said, “Plus plastique”! When we worked together, he was conducting the whole time. We talked a lot about the drama of a phrase, and about breathing. Where to breathe with excitement, and where is the sigh of relief?’

If only because there are fewer notes to follow, the long slow movement yields some of that relief. Boulez dabs colour across the full canvas of the piano: you could call the technique pointillist but Stefanovich steers me away from a search for continuity. ‘It’s like a memory game. Messiaen does something like this in Cantéyodjayâ [written the year after the Sonata]. Boulez says something, then something else. Then comes back, and adds one word more.’

The notes still belong together, of course, and such writing tests a pianist’s legato technique to the limit. ‘You just need to stretch your thinking,’ says Stefanovich, ‘and to have an extremely slow release of the note, to land on them softly, so you create this illusion that they are really connected. As if you were swimming in two different swimming pools. One is in Berlin, the other is in Paris. It has to be like in a dream, where you say to yourself that you can do this incredible thing. That’s what I miss when I hear new-music specialists play this piece: the mystery of it, and even the fun.’

In the middle of the slow movement, a storm breaks out from nowhere. It reminds Stefanovich of the same point in Schubert’s A major Sonata, D959: ‘It doesn’t make any sense. It’s just planted there. You’re walking along a path, you look up and there is a huge mountain in front of you, and you have no idea where it came from.’

The brief Scherzo offers the most straightforward way in for new listeners to the Second Sonata – like an action painting, as Stefanovich says, but also clearly divided into sections and shapes like traditional scherzos and trios. The opening quotes the nested fifth-fourth of the Sonata’s beginning, now in the same register, and plays with a cascading variant of the four-semiquaver motif. ‘The same characters are walking across the stage, now with less baggage.’ She points to the third line (the beginning of the first ‘trio’), among the most relaxed music in the whole Sonata, which Boulez compared to getting out of a pool with water dripping from your fingers. The sense of a world expanding, even at speed, intensifies through the following pages. ‘It’s like Ravel’s “Scarbo” on drugs!’ To the ear, or to this ear, the passage is a hyper-sensual, even erotic anticipation of the labyrinthine polyphonies of the finale.

The monumental scale of this finale counterbalances the first movement in direct emulation of the Hammerklavier Sonata. Boulez, however, intentionally ‘destroys’ his fugue subject by presenting it in a register so dark and murky that picking out individual notes is almost impossible. Given his imprecations elsewhere in the score, I wonder aloud if the marking of ‘Très librement’ is a hollow laugh in the face of the performer. For an answer, Stefanovich returns to Schubert. ‘It’s like when you listen to the best Schubert singers. They all sing the same phrase, because the poetry is there to direct them. They can only stretch it so far, but it’s the micro-inflection that makes the whole thing.’

It is in the finale that Boulez’s love of literature, and the poetry of his time, emerges most clearly in the strongly directed rhetoric of phrase after phrase. At the movement’s climax, however, Boulez infamously directs the pianist ‘to destroy the sound’ (‘Pulveriser le son’) in an explosion of notes as though he really has blown up his own piece. But to Stefanovich, Boulez compared the moment to putting your hand in a beehive: the notes fly out and disperse.

All that is left is the last page, a slow epilogue, ‘as if we only have enough strength left to say it was like this. These were the motifs. And, you see, he ends with a B. It couldn’t be more perfect.’