May 1950 at London's Royal Albert Hall, Flagstad was almost 55 years old. Her voice by this point was darker, heavier, and more inflexible than when she had sung for Strauss at Bayreuth, and she was becoming reluctant to venture above the staff, as would be notoriously demonstrated in the recording of Tristan und Isolde two years later; the Strauss songs, particularly the Hesse settings, were thus not ideally suited to her resources, and she found herself tested to her limits. "Frühling" indeed, gave such trouble that Legge, in promoting the concert, was two days before the event advertising the Strauss as "three songs with orchestra". In the event, Flagstad rose to the occasion and included "Frühling" (with, however, excluding the highest note), and the close of "Im Abendrot" was followed by a respectful silence in memory of Strauss. The concert, which aside from the Strauss songs consisted of Wagner (including Isolde's Liebestod and Brünnhilde's Immolation), received favorable reviews; recordings of Flagstad's contributions were made from the radio broadcast, and are today commercially available. Flagstad added "September", "Beim Schlafengehen", and "Im Abendrot" to her repertoire, and recordings (technologically superior to those taken at the premiere) exist of these performed in concert; she did not, however, sing "Frühling" again. Retirement After her retirement from the stage, she continued to give concert performances and record – first for EMI setting down her definitive account of Isolde in the first commercially released account of Tristan und Isolde, and then for Decca Records. She even made some stereophonic recordings, including excerpts from Wagner's operas with Hans Knappertsbusch and Sir Georg Solti conducting the Vienna Philharmonic Orchestra. In 1958, she sang the part of Fricka in Wagner's Das Rheingold, the first instalment in Solti's complete stereophonic set of the Ring Cycle, released by Decca on LP and reel-to-reel tape. She also spent time mentoring young singers in her native country, including contralto Eva Gustavson. From around 1952, when she gave her Met farewell, until her death 10 years later, Flagstad's health steadily deteriorated. She was in and out of hospitals on an increasing basis both in the number and the length of her stays for a variety of ailments. She even joked with an interviewer in 1958 that Oslo hospital had become her home away from home. From 1958 to 1960, Flagstad was the first Director of the Norwegian National Opera. In her last years