Fantastique! Many thanks loureviews, that's brilliant. It was an article by Will Crutchfield called 'The Story of a Voice' about Callas in the NY Times that started off my quest. I can't give you the URL at present because of my email problems and the NY Times search failed to find it but I guess it was from the same era as 'Vocal Burnout at the Opera'. I particularly like 'Vocal Burnout' because it is all about singers that I am familiar with. He singles out Renata Scotto and Beverley Sills for special treatment and I am certainly well acquainted with those divas both from live performances and from recordings. Anyway, I will study what Mr Crutchfield has to say about overwork ruining voices and see how I can equate that with his hypothesis that it was not only overwork that finished Callas but a faulty instrument to start with.
And of course some of the singers he cites developed serious wobbles but continued to sing for many years. I remember an amazing 'Frau ohne Schatten' in Munich in the mid 1960s when Hans Hotter sang the Spirit Messenger and Martha Mödl was the nurse and they both had such enormous vocal wobbles that in their duet I was expecting the sound waves to cancel each other out in some physical way, but no, they stood there, wobbling ferociously at each other and both collected enormous applause!