Here I am sandwiched between a violinist, a viola player and a baby, discussing Schoenberg’s Verklärte Nacht. The violinist – Corina Belcea – and viola player – Krzysztof Chorzelski – both belong to the Belcea Quartet, which has recently recorded the work. The baby is Belcea’s six-month-old son and he spends most of the conversation gazing at me with a singular intensity, as though he somehow shares Schoenberg’s expressionist vision. It certainly makes an interesting counterpoint to our interview.

Not least given Verklärte Nacht’s subject matter, revolving, as it does, around a couple and a baby. But here the parallels end. In the narrative – based on Richard Dehmel’s poem of the same name – a couple are wrestling with a dark secret: the woman is carrying another man’s child, and the whole drama is concentrated on the man’s ability to forgive her. Chorzelski and Belcea are not a couple at all, only colleagues. Nor does Belcea seem the type for impassioned revelations of any sort. In fact she leaves most of the talking to Chorzelski, who finds it easy to get enthused about this seminal work. ‘It talks about things that were not talked about at the time: sex, extramarital love, betrayal,’ he says excitedly, ‘but instead of condemnation there is forgiveness here, and a lack of judgement; it strains against social convention.’

And it strains against tonality, which teeters throughout on the brink of collapse. ‘You can see that the composer is aware of the fact that the language at his disposal is no longer sufficient. And there are passages where it actually does explode,’ says Chorzelski. He wafts his hand over the section marked Lebhaft bewegt: ‘The chromaticism here is so advanced that the music here is basically atonal.’

But while this 1899 tone-poem looks ahead to a future date, when Schoenberg would lead colleagues down an avenue called ‘12-tone’, it is clearly still a product of late German Romanticism. Schoenberg wrote it upon meeting his future wife, Mathilde Zemlinsky, and poured into it a level of sensuality befitting even Wagner. In fact, the language is very much in the mould of Wagner’s Tristan und Isolde; there is even a direct quote from the opera at one point.

That sense of excess is, for the Belcea Quartet musicians, both the appeal and the challenge. ‘The writing is very symphonic; and the fact that Schoenberg decided to expand the score into an orchestral version in 1917 suggests that he was well aware of that,’ says Chorzelski. ‘A lot of the music is very much like a wall of sound.’ But while he admits that nothing can convey this quite like an orchestra, he is keen to point out the advantages of the original chamber version: ‘We have the flexibility to recreate both a symphonic sound when necessary and a more filigree texture when it’s called for.’

What they couldn’t always do was devote themselves to fine-tuning. ‘It’s not always possible when recording a sextet with two extra colleagues who are joining us for a short period, one of them for the first time,’ Belcea emphasises. But, as she maintains, this music does not constantly demand pinpoint precision. ‘In a string quartet you can usually tell if something is not together or in tune. With this piece certain textures are used just to provide colour and it’s harder to pinpoint exactly what’s going on, so you just let the wall of sound wash over you.’

Chorzelski takes this line of thought even further, arguing that Schoenberg’s markings should not all be taken literally: ‘There is one passage, for example, where he writessul tasto for us but sul ponticello in the orchestral part.’ He continues: ‘It’s an indication that things were open to interpretation. What’s important is trying to discover the intention underneath it all.’

There are moments, however, when we are left guessing. A case in point occurs near the beginning, where Schoenberg inserts a comma in the first violin and first viola parts (bar 13) Chorzelski hazards a guess as to what it might mean. ‘It feels like a held breath. And yet, as a listener, you don’t tend to hear it, because the voices playing the chords don’t participate in the held breath.’

So what is Chorzelski’s analysis? ‘Perhaps Schoenberg was aiming for a general feeling of hesitancy; the hesitancy of a woman with a very heavy heart about to make a confession.’ But the question remains, how explicit should this particular marking be to the listener?

More explicit, at any rate, than many of the directions that follow. For there are moments where the dynamic instructions come across as self-defeating. Take the passage at letter Q, where all the parts are marked fortissimo. ‘If everyone plays fortissimo, it sounds like a washing machine on spin cycle,’ says Chorzelski. ‘You have to find a way of playing that allows the theme to travel through the first violin, first viola, first cello and be heard clearly.’

At the same time, the intensity of the music has to come through – no small ask, according to Chorzelski: ‘I think our ears are numbed by the amount of music and different styles that we are constantly exposed to. So it’s easy to become complacent and convey something that was meant to shock in a way that is not shocking.’

Perhaps the most famous example of this is the ‘non-existent’ inverted ninth chord (10 bars after letter C). By now we’re desensitised to it, having heard this piece so many times. Yet when first heard, it caused a furore among the great and good of Vienna, who pronounced it unidentifiable, and therefore impermissible.

Being aware of the back story proved useful to the chamber group, who tried their utmost to maximise the chord’s shock impact. But it wasn’t easy, according to Belcea: ‘Since it’s on a diminuendo, you can’t just go for force and play as loud as you can. You have to find a depth that mirrors the tension.’

This may seem like a paradoxical demand, but then Verklärte Nacht is full of those: maintaining clarity alongside spontaneity; and, perhaps more significantly, maintaining momentum in spite of the abrupt fluctuations in mood. The solution? An ability to let the music run its own course. As Chorzelski maintains, it’s important to perfect the points of transition, such as the end of the woman’s declaration (just before letter K), and the sudden transition from E flat minor to D major at the beginning of the Sehr breit und langsam section (11 bars before letter M), where her lover begins to declare his acceptance. But it’s also essential to play through those transitions uninterrupted, which is why live material, taken from concert performance, forms the backbone of this recording. Chorzelski concludes: ‘I feel this piece is different from others we have recorded in the sense that we have to let go of a certain wish for control. And provided that you have colleagues that you are happy with, you accept the results that come without your interference.’

The historical view

Arnold Schoenberg – The composer on his use of the ‘non-existent’ inverted ninth chord

‘There is no such thing as an inversion of a ninth chord; therefore there is no such thing as a performance of it, for one cannot perform something that does not exist. So I had to wait for several years.’

Richard Dehmel – A letter to Schoenberg after hearing the Viennese premiere, 1902

‘I had intended to follow the motives of my text in your composition, but soon forgot to do so, I was so enthralled by the music.’

Charles Hazlewood – On BBC Radio 3’s Discovering Music, 2014

‘This piece has one foot firmly in the soil of that full-blown decadent Romanticism and the other foot reaching out very uncertainly over the precipice into the 20th century, the century of wars.’