The first string quartet I was invited to write poems for was Haydn's Op. 51, The Seven Last Words. Paul Barritt, the Hallé Orchestra's Permanent Guest Leader who also organizes Tring Chamber Music in Hertfordshire, had long ago played alongside one of my brothers in the National Youth Orchestra. He phoned out of the blue and commissioned me to write seven poems, to perform with his quartet in a chapel at an Easter concert. He said the problem with performing this piece was that Haydn wrote the movements to be played between sermons and all were slow: they need something in between. I was fascinated. I grew up playing viola in family string quartets, I played frequently in concerts until my mid-twenties but also sang a lot, including religious music in choirs. Even so, the Crucifixion was . . . daunting. I later wrote about tackling the task for The Guardian (www.theguardian.com/books/2014/apr/18/christ-last-words-ruth-padel-poetry-haydn).

I started by researching Haydn's own commission, from around 1785, for orchestral interludes to be played in the Chapel of the Santa Cueva in Cádiz in Holy Week, between sermons meditating on each of the Sayings. Haydn wrote seven movements and added an Introduction, and also an extra fast movement at the end, ‘Il Terremoto’, for the earthquake which rent the veil of the temple after Christ died. After talking with friends, I guessed he might also have thought of the famous earthquake of 1755, which, when he was twenty-three, devastated Lisbon and many other cities in Spain and Portugal, including Cádiz. It was so large it caused tsunamis from North Africa to Cornwall, and even in Ireland it partly destroyed the Spanish Arch section of Galway City and flooded the marketplace in Kinsale. By striking on a holy day, All Saints Day (1 November), and destroying nearly all the churches in Lisbon, it caused throughout Europe an equally violent emotional and spiritual aftershock – a geological challenge to faith. Voltaire mentioned it in Candide (1759), and his ‘Poème sur le désastre de Lisbonne’ (Poem on the Lisbon Disaster, 1756) attacks the notion that God really does organize everything for the best.

Haydn was a devout Catholic, and, of course, took the commission very seriously. As he wrote later:

Afterwards, he transcribed the work for string quartet as his Op. 51 (published in Vienna by Artaria in 1787).

I listened to it again and again, and discussed it with musician friends, especially David Waterman of the Endellion Quartet, who had often performed it. Meanwhile I thought about the emotional arc of the Sayings, which are simply seven sentences recorded in the Gospels. No one knows who put them together. Three come from Luke, three from John. One appears in both Mark and Matthew. They begin with the words of Forgiveness, Salvation (or Comfort) and Relationship, attending to the needs of other people: the torturers, the crowd, the good thief tortured with Jesus, Jesus’ mother, his friend. In the central word of Abandonment, ‘My God, why has Thou forsaken me?’, Jesus addresses his own emotional pain. Only then can he admit physical need in the Word of Distress. ‘I thirst’ sums up all need, all the bodily agonies he is enduring. After admitting his humanity in this way he comes to the Word of Triumph or Achievement, ‘It is done’. All these Sayings have been endlessly written about, but this one is generally taken to mean that Jesus has done what he was put on earth to do. He can now come home, as it were, to the word of Reunion: ‘Father, into Thy hands I commit my spirit’. Emotionally, the arc almost follows a classic therapeutic model, from displacing unbearable pain onto others, towards admitting one's own loss and need, then finding a sense of achievement and wholeness.

Musically, I pondered the expressive character of different movements. The pizzicato in ‘I thirst’ suggests the longed-for drops of water. In ‘Why hast Thou forsaken me?’ the first violin goes up into a huge riff alone, as it were, in space, in the dark. Like (you imagine) a body alone in extremis. Through tone-painting and surprising juxtapositions, Haydn evokes the struggle of Christ's last hours. On 8 April 1787 he wrote to his London publisher William Forster: ‘Each word is expressed by purely instrumental means in such a way as to make the most profound impression on even an inexperienced listener's soul’ (Geiringer and Geiringer, Haydn: A Creative Life in Music, 87). What could I do with this?

‘Go in fear of abstractions’, said Ezra Pound, in a famous essay of 1913, ‘A Few Don'ts by an Imagiste’ (Poetry 1/6, 201), whose advice for young poets is still profoundly useful. ‘The natural object is always the adequate symbol’ (201). Poems can turn vague and woolly on you if you go too abstract. They get at the universal most effectively through the particular. They prefer to imply the big concepts, or suggest them through focusing on concrete details. I took Haydn's own aims as a guide too, thinking of the performance and his hope of producing ‘the deepest impression on the soul’. I followed the music, and read the theology woven into and around the Sayings, but decided to focus on Christ as Son of Man, on the vulnerable human body, so the audience as they listened to each movement could imagine each moment in physical terms. I would let what was happening at a cellular level stand for all other processes of thought and feeling.

I also turned to the medics. I've always been interested in the history of medicine, and I discovered that in the eighteenth century, and ever since, physicians have speculated – in a lot of exceedingly concrete and rather gory detail – about what actually Christ died of. You can't put ‘crucifixion’ on a death certificate. Was it suffocation, blood loss, cardiovascular collapse?

That approach seemed to work. The poems became the heart of a collection called Learning to Make an Oud in Nazareth, about music and faith in all three religions of the Middle East, and several priests have asked if they could use them in Good Friday services. But in musical performance, they did something else. Twice in one day, matinee and evening performances in a very cold chapel, with frost on the ground outside, I read them with Paul Barritt's quartet. I loved performing with a quartet again, as a poet this time, not a player. Remembering what it was like playing in chapels, how numb your fingers could get, I lent the second violinist my jacket: she needed it more than I did. The quartet played beautifully and both audiences were very moved. Many people came up and said that hearing each poem beforehand made their experience of each movement much more intense.

I was delighted. I had hoped that the imagination, research and musical study I put into the poems might create something I could really offer the audience, but hadn't known it would work. They might have just found the poems intrusive, or annoying. People who love music don't always enjoy poetry. But maybe, like stained glass in medieval churches showing visual representations of biblical stories, an extra medium intensified the emotional effect. Later I did the poems with a student quartet in Little Gidding Church, locus of one of T. S. Eliot's most profound religious poems. And then, several times, with the Endellion String Quartet.

This worked so well, with the same warm audience response, that David Waterman and I wondered if we could embark on another project together. The Aspect Chamber Music Series (www.aspectmusic.net/) – which specializes in mixing words and exposition with chamber music and began in Notting Hill, but now operates also in New York – commissioned me to write and perform some poems for its next Beethoven concert. I would read my poems between the Endellion's performances of Op. 18 No. 6 and Op. 131.

This was a whole new order of challenge. I discovered there were thousands of Beethovens. Everyone had their own. Instead of the Passion, I had to address Beethoven's chaotic, iconically creative life. David Waterman lent me books – and I began to realize how very much primary material there was! Above all the music. But also conversation books, sketchbooks, diaries and letters – and the medical treatments Beethoven tried for his ears. As with the Passion, there was the question of what was happening physically. Not that the music was trying to convey this, as in Haydn. But the personal suffering, the physical fact of it and emotional distress it caused, underlay all the music Beethoven wrote from 1798 onwards.

I decided to focus on Beethoven's life between 1800, when he was thirty, going deaf, working on his first quartets, and 1826, the year before he died, when he wrote Op. 131. And also on his relation with the quartet form. Why was four so good to compose with? In poetry, too, there is always the perennial mystery of form – what form gives to content, how each enhances the other and how they work together. Whenever you start writing a new work, you face this all over again.

I started with the La Malinconia passage in Op. 18 No. 6, which in some ways foreshadows some of the late quartets. In homage to the seven movements of Op. 131, I wrote seven poems, for seven moments in the rough quarter-century between his composition of those two opuses (1798–1800 and 1825–1826). I wanted to bring out what I saw as a paradox: that Beethoven created disastrous human relationships, but wonderful musical relationships, which, wherever they venture harmonically, always end up with feelings of peace, joy and love. He was the great idealizer anyway, and I began to feel that what he idealized, above all, was the very concept of relationship. Something he could not achieve in life but achieved so magnificently, and joyfully, in music.

I also wanted to focus on the idea, and the drama, of four voices reacting to each other, replying, veering away, angry, peaceful, dissonant, harmonious. In playing quartets by Mozart, whose work seemed to me a many-stranded bridge between Haydn and Beethoven, I had always been struck by the way the drama, the interplay of melodic relationships between voices, reminded me of the musical and human relations between soprano, alto, tenor and bass in his operas.

On opera, I have worked in a different sort of way. I taught a course on women in opera at Princeton University. For BBC Radio 3, in a series of interval talks called ‘Close Encounters’ in which I sang the illustrative passages, I analysed various operas through the twists and turns of one particular scene in each of them. But I'm not trained in harmony; I write out of the experience of playing and singing. Thomas Adès helped me prepare for singing Mozart's ‘Dove sono’ at a friend's funeral by accompanying me in rehearsal. He also pointed out, in commenting on a draft of my discussion of that aria in a piece for The London Review of Books (www.lrb.co.uk/the-paper/v19/n02/ruth-padel/putting-the-words-into-women-s-mouths), that I focused on melodic shape rather than harmonic change.

In response to the idea of four voices, I found my seven Beethoven poems developing four different personae. One had an objective voice, like a narrative scene-setter, or a historian who knew about medicine and neurology. One (drawing on letters and memoirs) was a contemporary voice. Some visitor, a close friend or a beloved, a colleague or pupil; someone who loved, admired and worried about Beethoven. The third was a sort of lyrical myth-maker, who addressed Beethoven as ‘you’ and saw him as a hero, a courageous Orpheus going into a darkness where no musician had gone before. And finally, there was Beethoven himself, his own words drawn from letters and diaries.

The poems went down well in performance. Again, audience members came up and said how hearing them enriched their experience of the music. It surprised me more this time. Evoking the medical stages of crucifixion before hearing each of Haydn's seven Adagios was one thing. But I felt much more awe at the prospect of writing words to go before any of Beethoven's quartets. The music felt to me so passionate, so expressive of human feeling, that no words needed to be said. What was I doing? What did I want to get across, in poems? The human element, the person? But who is a composer – isn't the sound enough? Didn't Beethoven come across magnificently in his music anyway?

I realized that many people genuinely like to know more about the feelings and events behind music they hear. However much you love and know the music, and find your own feelings in it, something in you might also want to find the human truth of what the creator was experiencing as he or she wrote it. History and biography, journals and letters, filtered through the condensed articulation of thought which only poetry can give, seems to be able to offer that without intruding on the music. I felt I was privileged and lucky to discover this and it seemed to meet some need in me, too.

One editor who came to the concert wanted to publish the poems straight away, but I knew they were not ready. Poems can work really well on a stage but you must not be fooled by that: they may not be at all ready for publication. I put them away and left the whole project to mature in the dark, like wine, while I focused on other writing commitments.

But I did eventually sign a contract with my own publisher for a future biographical work of poetry. I had already done a biography of Charles Darwin in poems and thought maybe one on Beethoven might work. Meanwhile I did more concerts with the Endellions, at which David Waterman spoke beforehand, about musical aspects of the quartets, and I read poems between the different pieces. We put together an event in which they played Tchaikovsky's Third Quartet, in E flat minor, and Janáček's Intimate Letters. We based the event on the relation between a love affair, passionate love letters, and the composition of a string quartet. Between the two quartets I read a long poem, ‘Writing to Onegin’ (www.poetryinternational.org/pi/poem/13587/auto/0/0/Ruth-Padel/WRITING-TO-ONEGIN/en/tile), based on Pushkin's poem Eugene Onegin and the letter-writing scene in Tchaikovsky's opera. Another concert featured quartets written by Schubert as he faced the fact he was going to die young. It began with the Quartettsatz d703, an unfinished fragment written in 1820, when Schubert was twenty-three and embarking on what critics now see as his mature phase of composition. Three years after he wrote that, he was diagnosed with syphilis. ‘Think of a man’, he wrote to a friend on 31 March 1824, ‘whose health can never be restored, whose brightest hopes have come to nothing, to whom love and friendship are but torture’ (in Elizabeth Norman McKay, Franz Schubert: A Biography (Oxford: Clarendon, 1996), 192). Knowing his genius would not have the chance to flower over decades, he crammed his astonishing body of mature work into only a few years and turned first to the string quartet form to convey, rather as Beethoven did in the late quartets, despair at fast-approaching mortality, as well as his aching awareness of joy in the life he had to leave.

Writing poems for this made me think again about what form can do, how the pressures of a particular form help you write in one particular direction. Why did Eliot invoke the quartet form in his blank-verse meditations (published individually in 1940–1942) on our relationship with time, the universe and the divine, which he called Four Quartets (New York: Harcourt, 1943)? I have always wondered about that title. Ought it not to be simply Quartet? None of the single poems seem to be modelled on quartet form, separately. They form, as it were, a quartet together. But perhaps he simply wanted to emphasize the solidity of four. And we know he had Beethoven's late quartets in mind, however loosely, while writing them.

Astonishingly, Schubert wrote the A minor and D minor quartets, two of the most wonderful pieces in the whole chamber-music repertory, in just two months, February and March 1824. He wrote them knowing that not only death but probably insanity was coming. Together, they are an extraordinary testament to his capacity for celebrating life in the face of death, but they could not be more different. The delicately haunting A minor has the lyric resonance of his songs. ‘Death and the Maiden’, called after his song of the same title whose theme he used in the second movement, has the fury of a tornado. So I wrote and read poems on Schubert's year of 1824. They worked fine on stage, in the moment. But I felt they had come too easily in performance, and I wanted to think about them much more deeply and carefully, so I left them, too.

But finally, five and a half years after I wrote my first Beethoven poems, I was free of other commitments and set out for Bonn and Vienna to do the research, in depth, for a whole book of new poems. I intended it to address both Beethoven and Schubert's lives and music, based on work I had done for both composers. Schubert was still fresh in me, I didn't want to leave him out. I planned to call it The Divine Spark, after what Beethoven allegedly said Schubert possessed upon seeing some of the latter's manuscripts. In Bonn, I realized it was insane to pack both into one book. Beethoven was much too big. I had to leave Schubert for another day.

In deciding on a form for the whole book of Beethoven Variations, and indeed for the forms of individual poems, I went right back to the drawing board, rewrote and rethought everything. It seemed to work – this time on the page, too. I put Variations in the title because I was struck by the way Beethoven's great claim to fame, at first, as a keyboard player, was his astonishing powers of improvisation – and maybe also because of that moment in Heiligenstadt when, despite the deafness hanging over him, he wrote the Eroica Variations, which paved the way for his great transition from early to heroic styles. Rather like Schubert composing those quartets when facing the death sentence of syphilis, it was an act of creative defiance – and I somehow connected it, formally, with variation.

I was particularly pleased that musicians liked it. But I still have a date with Schubert. I don't know yet what form it will take, but there he is in front of me, barely five foot, shy, chubby and short-sighted. There were two sides to him, said his friends. The sunny, gregarious Schubert of the Schubertiads, and the dark private unknown self which was, perhaps, responsible for the syphilis. I think you hear that in the music. Either way he was, according to Liszt, the most poetic musician who ever lived.

You always write out of a wish to discover something new. Otherwise, why bother? I need to discover not only more about Schubert, and his music, but what draws me to write about the life behind the music and why I – and luckily other people, too – want to know about it.