These four sets, issued by EMI to mark the 25th anniversary of Maria Callas’s death, are important historical documents on various counts. None has been issued before in an official format. All come from live performances at La Scala in the mid-1950s when the diva was at the height of her powers. The three that Callas also recorded in the studio here receive from her, in live circumstances, more vital performances. Finally, the four cover the main aspects of her career – in turn, classical, bel canto, Verdi and verismo work, thereby boxing the compass of her amazing achievement as a singing actress of unique powers. The main, indeed perhaps the only, drawback, is the intrusive distortion at climactic moments on the three earlier sets.

It was Callas’s particular genius to find exactly the appropriate mode of expression for every role she tackled. So here we have Callas the avenging tigress (Medea), the shy, vulnerable sleepwalker (Amina), the distressed, tragic heroine (Maddalena) and the tormented, guilty wife (Amelia). Within those parameters there are a hundred different individual inflections to reflect the emotion of the moment: indeed, as John Steane points out in one of his illuminating notes, it is often a small aside that reveals as much about the character she is portraying as a big set-piece. That said, it is the shattering outburst of Act 3 that makes her Medea such a terrifying experience, the ingenuous contemplation of true love her Amina so touching, the urgent, searing narrative, ‘La mamma morta’ her Maddalena so human, and the appeal before her husband her Amelia so heartfelt. These are readings of the central solos that have seldom, if ever been surpassed.

The first two of these performances, chronologically speaking, were, to an extent, fortuitous events. De Sabata was intended as the conductor of Medea but fell ill. Serafin wasn’t available, so Callas asked for Bernstein, of whom she had heard good things. The results are revelatory. Then for January 1955 Trovatore, rather than Andrea Chénier, had been planned at La Scala, but Mario Del Monaco wanted the Giordano opera to replace the Verdi, probably because he felt there he would be the undisputed star. He also thought Callas might not went to learnMaddalena in a short space of time. Of course, being the determined woman she was, she did so – to electrifying effect, proving her tenor wrong on both counts.

Bernstein, in his first operatic assignment anywhere, breathes new life into Cherubini’s score, in spite of some rude cuts, and finds much more immediate drama in it than did an ageing Serafin in Callas’s commercial recording. In consequence she is in absolutely terrific form, the very incarnation of a woman who has been crossed in love. By a similar token, Votto imbues Chénier with a dramatic pulse that turns what can be a desultory work into a quasi real-life drama, bristling with pertinent incident.

Later in 1955, Bernstein and Callas again joined forces for an utterly charming account of Sonnambula, another reading that quite puts in the shade the studio version. The conductor’s feeling for rhythm and colour and Callas’s subtle responses to her fragile role are everywhere felt.

In the case of Ballo the context of an evening in the theatre makes this a more arresting, vivid interpretation on all sides than its studio counterpart of a year earlier, with Gianandrea Gavazzeni galvanising his fine cast to great things. As the late John Ardoin put it in his study of Callas’s recordings: ‘The La Scala performance is sung with more vivid colours, with accents more etched and a general intensification of Verdi’s drama.’ Here the sound picture is appreciably superior to that on the earlier sets.

And it isn’t only Callas and the conductors who make these sets so unmissable. We hear in all four performance a house at the peak of its form. Take the soprano’s tenor partners. La Scala produces just the apt singer for each role. Gino Penno has the right heroic, spinto sound for Jason. Del Monaco, often denigrated for his bellowing on his Decca sets, provides many shades of tone and feeling as well as the beef for Chénier. Cesare Valletti – a still underrated artist – is as sensitive and involved an Elvino as any on disc – and I do not forget Pavarotti – and he duets to perfection with Callas, Bernstein giving them the space to phrase as only they can phrase. Finally, the irresistible Di Stefano is the soul of vital declamation as Riccardo. What an estimable quartet!

In Ballo, Ettore Bastianini’s forthright Renato, the only role he sang in London and one of his best in an all too short career, and Giulietta Simionato’s classic Ulrica are huge assets. Other roles are filled with house singers of the day, of whom I would single out Aldo Protti’s deeply eloquent, magnificently sung Gérard (where is there his equal today?), Giuseppe Modesti as a warm Creon and Rodolfo, and Lucia Danieli, who makes so much of the tiny part of old Madelon in Chénier.

So, all in all, here is a feast not only for Callas aficionados but for anyone interested in the history of Italian opera at its most potent.