Our admiration for the great singers of the past is based on gramophone records. But what do we know about the precise circumstances in which they were made? Undoubtedly performances by tenors like Max Lorenz and Lauritz Melchior were stellar events ... although I am inclined to doubt whether both these singers had such tremendous voices as is claimed for them today.
Christian Thielemann 
Perhaps only a German mind can so effortlessly misrepresent obvious facts and then raise doubts as to whether there are any such facts at all. (This, after all, is the principal characteristic of German philosophy from Kant to Heidegger.) Coming from a highly experienced and internationally acclaimed opera conductor, Thielemann's doubt is worse than frivolous. It is delusional. There is no gentler way to describe his state of denial in the face of readily available and utterly compelling evidence that Lauritz Melchior's legendary status as a Wagnerian Heldentenor is fully justified by the unmatched glories of his singing.
So lets take a quick look at the evidence.
Starting with Melchior's well-known commercial recordings (1927 - 1935), there is nothing mysterious about "the precise circumstances in which they were made." The limitations of very early electrical recording technology did not allow any kind of post-recording mixing and editing. Most importantly, it was not possible to make a weak voice soar effortlessly over orchestral fortes without placing the orchestra a mile away from the microphone. And this is not what is heard in Melchior's two 1928 arias from Lohengrin, recorded with the Berlin State Opera Orchestra under Leo Blech. The balance of these recordings is remarkably natural and realistic, and the sound (unlike that of overfiltered and glassy travesties issued by EMI years ago) is thrillingly full and vivid for its time despite some amount of surface noise. Whether in Hochtes Vertrau'n from the Act 3 duet or in O Elsa! Nun ein Jahr from the opera's finale, the voice makes it immediately clear why Melchior's singing has been likened to, among other things, a burst of sunlight through the clouds.
Second, the veracity of these commercial recordings is fully backed by Melchior's live recordings from the Met, again made in the pre-tape era when "live" meant just that, as opposed to only that the singers on stage were "not dead" (while the recordings were stitched together from multiple performances, dress rehearsals, and/or post-performance patching). It is little short of astonishing to hear the 53-year-old Melchior in a (remarkably well recorded) 1943 Met broadcast of Lohengrin and to realize that his voice - fifteen years after he made the above commercial recordings (which is longer than the entire careers of some Heldentenors), and after having delivered hundreds of Lohengrins, Siegmunds, and Tristans on the Met's stage - had retained the youthful, virile, rounded, and overwhelmingly powerful golden clarion sound it had back in 1928. Starting with Mein Herr und Konig and continuing with In fernem Land to the opera's end, Melchior's is the only voice in the history of recorded sound that makes me believe (even with a Gigli-like sob in his Lebwohl) that I am in the presence of a true Wagnerian hero. And his baritonal cries of Wälse! in Ein Schwert verhieß mir from a 1941 Met broadcast of Die Walküre upgrade my impression to that of being in the presence of a superhero. No wonder some music critics who had attended many Melchior performances at the Met and lived long enough to hear quite a few later Heldentenors described him as "not only the greatest of his time but also, in all probability, the greatest Wagnerian tenor who ever lived."
In the end I think that Thielemann's delusional state of denial deserves a modicum of sympathy, for it is likely nothing more than a psychological defense mechanism. As one of today's opera conductors who specialize in Wagner, he has to put up with the disembodied and almost inaudible cooing of Klaus Florian Vogt, the careful and laborious yet barely audible crooning of Andreas Schager, the paranasal and only slightly more audible mooing of Jonas Kaufmann, the cavernous and sour off-pitch grumbling of Robert Gambill, the wobbly bleating of Ian Storey, or the very loud and shrill screaming of Peter Seiffert. Even if Thielemann is lucky to work with a Heldentenor like Torsten Kerl or Robert Dean Smith, it must be hard for him not to think that these singers, good as they are by today's standards, are only pale approximations of the Wagnerian vocal glory heard at the Met from the mid-1920s until Melchior's retirement in 1950. Which is where self-delusion comes to the rescue. If you convince yourself that there were no "tremendous voices" in the past, it will be a lot less painful for you to make a living by conducting Wagner operas with less than stellar or even laughably inadequate voices.
1. Christian Thielemann, My Life with Wagner, Pegasus Books, New York, p.118, italics mine.
2. Jens Malte Fischer, 'Sprechgesang or Bel Canto: Toward a History of Singing Wagner', in Wagner Handbook, Harvard U. Press 1992.
3. This is one of many delightful quips in Manuela Hoelterhoff's often hilarious look at the world of opera in her book Cinderella and Company, Alfred A. Knopf, 1998.
4. Harold C. Schonberg (1915-2003), "Are these the all-time great voices", New York Times, Oct.16, 1983.