In 1823, Antonio Salieri - a venerable court composer in Vienna, Gluck's protege and also the respected tutor of Beethoven and Schubert - allegedly accused himself of poisoning Mozart, and tried to slash his own throat in remorse.
As he was suffering from dementia, there was no reason to believe his distraught confession. What motive could he have had for committing the crime? During Mozart's lifetime, Salieri was Emperor Joseph II's perk-laden favourite, with nothing to fear from any bumptious, undiplomatic upstart. But the phantasmal offence soon acquired a mythic inevitability. In 1830, five years after Salieri's death, Alexander Pushkin wrote a so-called "little tragedy" which - more tersely and teasingly than Peter Shaffer's play Amadeus - reimagines the relationship between the two composers. "Mozart, you are a god and know it not," says Pushkin's Salieri. "But," he adds, "I know it." That knowledge causes him only grief. Mozart's gifts have been squandered on a frivolous idler, while Salieri himself remains a mediocre note-spinner.
Unable to accept this divine injustice, Pushkin's Salieri invites Mozart to supper at an inn and slips poison into his glass. Mozart drains it, innocently toasting himself and Salieri as sons of harmony, and promptly composes the Requiem that will serve as his own obsequies. The murderer consoles himself by arguing that his act of deicide might be, at last, the work of genius which he believed to be beyond his capacity. Pushkin's brief scene ends with a rhetorical question through which Salieri alludes to an earlier rumour about creative sacrilege: didn't Michelangelo kill the model whom he employed to pose as the crucified Christ, to ensure that the dead body would look more lifelike?
Shaffer's Salieri, paraphrasing his namesake in Pushkin, says that "God needed Mozart to let himself into the world". Mozart, consequently, is Christ, a deity temporarily attired in human flesh, so Salieri represents ungrateful mankind, slaughtering the Saviour. The real Salieri also spoke in such exalted terms, but his intentions were pious, not satanic. In one of his letters he quotes St Augustine, who suggested that music is a communion between heaven and earth: it enables God to eavesdrop on our souls and to take pleasure in their burnished goodness. Salieri retained the Orphic faith of Renaissance composers, who heard in music the consonance of the universe itself, with its chiming crystal spheres, and - as when Orpheus subdued the brutes by playing his lyre - he called its harmonic grace "a pleasure that tames man". These touchingly devout declarations were unearthed by the musicologist Claudio Osele while helping his companion Cecilia Bartoli to prepare her new and revelatory recital of arias by the composer who was first slandered and then, even more woundingly, forgotten. So what does Salieri's music tell us about the state of his own soul?
The first track on Bartoli's disc is unfortunately infernal: a stormy tirade from the opera La secchia rapita, in which her voice is shredded by leaps across abysmal intervals. Here she resembles one of the harpies who torment the deranged Salieri in Amadeus. Once past this trial, she persuasively supports Osele's comparison between Salieri and Canova, the sculptor who was his contemporary. At best, both possess the grave, noble calm of neoclassicism, and some of the operatic characters on Bartoli's recording sound like statues that can express emotion without disturbing their pedestalled poise. The crusader Rinaldo in Armida prays for sleep, which he calls a sweet simulacrum of death. A beatific tune on the flute supplies him with its forgetful mercy, while Bartoli's whispered tone and time-suspending trills create a dreamy inertia and allow Rinaldo to lapse into peace and silence. A miserable, jilted wife in an aria from La scuola de' gelosi tranquillises herself by singing. The repetitions of a rondo allow her to regain her optimism, but the mood remains stoically subdued, refusing to show off with exultant high notes.
Osele emphasises the longevity of Salieri, and therefore the stylistic distance he travelled during his career. He was born only six years before Mozart (which means that he was hardly the grumpy elder imagined by Shaffer) and died at the age of 74; his music began by adhering to the enlightened agenda of Gluck - whose sculptural characters, according to Mozart, sound as if they shat clods of marble - but reached ahead towards the wildness and agitation of his pupils Beethoven and Schubert. Often he derided conventions he had outgrown. In La secchia rapita, another wife with a philandering husband enlists a blustering battery of drums, horns and trumpets to prove her own fidelity, but in doing so mocks the attitudinising of Gluck's heroines by comparing herself to Penelope, Lucretia, Artemisia and a variety of archetypal chaste spouses. The sarcastic curdling of Bartoli's diction makes clear her scepticism about this charade; she belongs to the earthier company of those whom she calls "donne triviali".
The 18th century's rigorous division between opera seria and opera buffa, aristocratic tragedy and plebeian comedy, has broken down, and in an aria from La cifra Bartoli riotously reorchestrates this chaotic world. A peasant girl, volunteering to marry Milord Fielding, bossily separates bucolic instruments from their more urbane, courtly counterparts: harpsichords and spinets abandon their decorous tinkling, and are drowned by the uproar of castanets, clarinets and timpani. A lament by another character in the same opera illustrates Bartoli's versatility but, more importantly, gives a tantalising sample of Salieri's range. Here the orchestra evokes the bereft Scottish wilderness through which the dismayed heroine wanders, and the shakes and shudders of the vocal line are a graph of her despair. The role was written for Adriana Ferrarese del Bene, for whose gymnastic voice Mozart designed the role of Fior-diligi in CosI fan tutte; on this evidence, it does not sound unworthy of her.
This summer, Bampton Classical Opera in the Cotswolds staged Salieri's Falstaff, which managed not to be annihilated by comparison with Verdi's benign, autumnal version of The Merry Wives of Windsor. Salieri, who emphasises Falstaff's money-grubbing and does without the lyrical intercession of the young lovers invented by Verdi, is actually truer to Shakespeare's mercenary play. Salieri once more sympathises with the underlings, and - like Mozart in Le nozze di Figaro - looks at the intrigue from the point of view of the weary servants.
It is a pity that Bartoli, who relishes Salieri's occasional zaniness, did not include Mrs Ford's pidgin aria in her recital: turning up at the inn disguised as a gormless Fraulein, she sings Falstaff's praises in a hotchpotch of German and Italian. The episode allows Salieri to ridicule the garbled Esperanto of opera, and to glance wryly at his own shuttling between Italy, Vienna and Paris. Still, for the time being, Bartoli has done enough for him. Romantic writers often declared that Mozart's music was synonymous with heaven. Shaffer's invidious play deprived Salieri of its supernatural comfort, and left him writhing in a remorseful hell; Bartoli, by giving him back his voice, has restored him at least to the outer suburbs of the celestial city.
The Salieri Album will be released on 29 September. Cecilia Bartoli sings arias by Salieri in Manchester on 5 October, Birmingham on 7 October, and at the Barbican, London on 22 December