The oath of the title is the promise made by one young lady (soprano) to another (mezzo) that out of gratitude for saving a parent's life she would come to her assistance if need and opportunity were to arise. Five years later, though originally living in different countries and with no mutual connection, they find themselves in the same city and in love with the same man (as the way is in these affairs). Need and opportunity duly arise, the soprano does the decent thing and, though she dies, the lovers live, presumably happy ever after. The story is adapted from Hugo's Angelo, tyran de Padoue which, further adapted and transported to Venice, became a source for Ponchielli's La Gioconda.

La Gioconda, we observe, survived in the popular repertoire, whereas Il giuramento did not. Hearing it now, in this shortened concert version, we once again have to raise the question “why?”: that is, why the judgement of what we dignify with the name of history (for 150-odd years is not really very long) should have reached its verdict with such certainty and pronounced its sentence with such severity. The arbiter, it seems, is the errand-boy. La Gioconda has tunes which (in the days of whistling errand-boys) the errand-boy could whistle. Il giuramento is not short of melody but, like most of Mercadante's work, lacks the catchy,whistlable type. Otherwise it has most of what we want: it is beautifully scored for both voices and instruments, it is effectively constructed and it establishes the three principal characters (through their music) as credible people placed in a painfully intense relationship.

Such (or something of the kind) must have been the belief of Gerd Albrecht, who championed Mercadante in the early days of the present revival. He conducted a concert performance of Il giuramento in Berlin in 1974, and then in 1979 at Vienna gave three similar performances, the first of which was recorded. And it very nearly didn't happen. The tenor Peter Dvorsky fell ill at the last moment and it seemed impossible to find a substitute at four days' notice. But Plácido Domingo was in Vienna, recording, with just one afternoon free to rehearse a score he had seen for the first time the day before. In the event, as Marcel Prawy wrote: “While all his colleagues had to follow their parts from their music, Domingo glanced at his score only from out of the corner of his eye and sang and acted with a freedom that suggested he had been performing it all his life.”

He also sings with gorgeous tone, refinement of style and emotional conviction, well matched by the two women. Agnes Baltsa is on top form, the purity, warmth and (at that time) evenness of her voice delightful to the ear and hugely appreciated by the audience. In the Gioconda part, Mara Zampieri impresses, as she did at Covent Garden in those years, as a singer of marked individuality, her timbre sometimes tense with a fast flicker-vibrato, her manner sometimes almost disconcertingly indrawn. For many reasons, then, this was a night to remember and a performance to preserve.