You had to have heard the voice live to know how it shone - like a laser, straight and unfailingly true. The core of the sound was as tempered steel, the top could cleave the mightiest orchestral tuttis. When Birgit Nilsson's Brunnhilde strode into Wagner's Ring, you heard but still did not quite believe her exultant battle-cry Hojotoho]: no scooping to those repeated high Bs for this singer - she'd pluck them cleanly off the ledger lines and duly nail them to the back of the auditorium. No theatre was ever quite big enough for Nilsson and Strauss's Elektra, the Nordic chill in her tone-colour lent unsettling authenticity to Puccini's implacable ice-princess Turandot.
Like her great Scandinavian predecessor, Kirsten Flagstad, Nilsson used the minimum of vibrato to warm and enrich her tone - a national characteristic, no cover, no camouflage, just the unvarnished truth. Intonation needed to be perfect. She was the ultimate Wagner heroine, resilient and enduring. She was the reigning star at Bayreuth, the Wagner shrine, over 20 years of regular appearances. Her proud, impassioned Isolde was legendary: she sang it a staggering 209 times across the world, though one wonders if it can ever have blazed more incandescently than it did at Bayreuth in 1966 when Wieland Wagner's historic production was unveiled under Karl Bohm. The live recording of that event would be among any Wagnerian's desert island eight. Part of the thrill comes from knowing that what we hear was not manufactured in any studio but recorded as it happened in Wagner's own theatre.
That was important to Nilsson: the occasion, the public, the emotional and physical heat of those special performances where all the senses were engaged and the voice was exactly where she wanted it. For anyone who has ever wondered how it must feel to be inside a voice such as this, to be the source of Isolde's Liebestod at the moment of sublimation, Nilsson can illuminate: 'It's a lustful feeling . . . You feel it in your whole body. And when everything is functioning, when the support is there and the sound is in the right place and I hear those tones ringing in my head, then I know it's all working.'
It was certainly all working for those classic Bayreuth performances. Nothing, she says, could ever really compare with the all-consuming emotional experience of singing Isolde: the sheer length of the role, the fine line between love and hate that is at its heart - the ultimate challenge, tougher even than the more technically demanding Elektra.
So when did this extraordinary voice first show itself? To what extent are once-in-a-generation voices like this born rather than bred? Well, the voice was always there, it seems, a natural facility, though initially of a much darker complexion - at one point it was thought she might actually become a mezzo (perish the thought). Her mother was encouraging, her father less so. He had a future mapped out for her down on the family farm at Vastra Karup in southern Sweden. But she did make it to the big, bad city, and she did win a place at the Stockholm Royal Academy, and later the Royal Opera. Her baptism of fire began there. Leading sopranos were dropping like ninepins - hardly surprising if Nilsson's experience was anything to go by. Her very first role was Agathe in Weber's Der Freischutz (three days' preparation), her second was Verdi's Lady Macbeth under Fritz Busch (three weeks' preparation - an eternity in comparison); then came Sieglinde and Brunnhilde in Siegfried (both roles in 14 days). Character-forming or just plain madness? 'I wouldn't recommend it. But I had to do it. It was all I had; I took what I could get. And I got only leading roles] But I was lucky - the voice was more or less established. I never feared for the top notes. I was no machine, but my voice was reliable and healthy and I was very strong.' Indomitable. Tosca, Senta, Lisa in Tchaikovsky's Queen of Spades were among the roles that quickly followed. She had developed carnivorous tastes - the meaty and the dramatic; her voice timbre and temperament demanded it. 'I grew with each role, I learnt the hard way from every performance. I've always said that the best voice teacher is the stage.'
In Nilsson's case, it had to be. Voice teachers were almost her undoing. The Scottish tenor Joseph Hislop opened her ears to the 'ping' at the centre of every tone, but put all the pressure on her vocal cords. She was hurting. A subsequent teacher forced her top register and strove to develop a powerful chest voice - again intense pressure on the cords. She would have none of it. 'You end up with different registers and a hole in the middle. I've heard too many singers go this way. You can't keep stretching an elastic band - eventually it snaps.'
She didn't. She took her own voice in hand, learnt to trust her own instincts. Her new creed: think high, focus, feel. 'Nobody ever talked to me about support, about placing the sound in the cavities of the forehead; nobody ever explained to me that this was my sound-box, just like a violin or a piano. I needed guidance, but there was no one.' She still insists that hers was never a huge voice (tell that to the man at the back of the balcony), that its carrying power lay in the concentration of tone, the 'ping'. Without that, she says, a big voice is just 'hot air'. No doubt the young singers in her master classes hear all about the importance of the 'ping'.
Even now, at 75, you can still hear the stentorian ring in her speaking voice. She expresses concern that some young singers have come to expect a 'quick fix' from their training, rather like a visit to a beauty parlour; she worries about the pressure put upon them to make speedy, lucrative careers, about good lyric voices being pushed up to sing roles that they are plainly not ready, or in some cases not equipped, to sing 'just because the stage director thinks they have stage-presence and will look good for TV and video. The voice must come first. Can you imagine a violinist or a pianist saying - well, you know my fingers are a little stiff but I feel so much]'
Nilsson has so many tales to tell. You know you are in the presence of a little piece of musical history when she casually expresses disappointment at not having worked with Furtwangler, Bruno Walter, or Toscanini, or when she proudly lets slip that the first Cavaradossi to her Tosca was Beniamino Gigli. He was plainly a kindred spirit, a wholly natural, big-hearted talent who could give a performance and then sing all night around the dinner table. One suspects that he, like her, was not overly reverent. When you've sung as much Wagner as Nilsson has, you have to retain your sense of humour. The Nilsson anecdotes are legion: there was the time Brunnhilde greeted the dawn at the close of Siegfried only to find herself open to the elements in more ways than one, or the time that the score of the Germany vs Sweden football result was relayed via King Mark. Or her sound advice to the young soprano seeking tips on singing Isolde: 'My dear, be sure you have a comfortable pair of shoes.'
An evening with Birgit Nilsson: 7.30 tonight, Donmar Warehouse, Earlham St, Covent Garden, London WC2 (071-867 1150)
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