In 1953, while singing Sieglinde at the Bayreuth Festspielhaus, the conductor Clemens Krauss was to forecast her future, suggesting her voice was actually a mezzo-soprano. Despite her great success as a soprano, she realized that her entire voice was constantly darkening in color. In 1955 she began a year of restudy with the celebrated baritone Giuseppe Danise. Her first two roles were Amneris in "Aida" and Laura in "La Gioconda". On February 15, 1956, she debuted as a mezzo-soprano at the Metropolitan in a brilliant portrayal of Marina in "Boris Godunov" under Dimitri Mitropoulos. October, 1957, was the beginning of a long career in London at the Royal Opera House. Her debut as Carmen was a success and, in time, she was heard as Amneris ("Aida"), Marina ("Boris Godunov"), Ulrica ("Un ballo in maschera"), the Nurse in "Die Frau ohne Schatten" and the Old Prioress in "Dialogues of the Carmelites". In the Zeffirelli–Giulini production of "Falstaff", her Mistress Quickly became the model for this role. Carmen, Klytemnestra ("Elektra"), Mistress Quickly and the Pique Dame ("The Queen of Spades") became her signature parts.