The finest of the works Aaron Copland composed in his American vein is the ballet Appalachian Spring, written in 1943-44 for Martha Graham. The work's premiere took place in the Coolidge Auditorium of the Library of Congress, so Copland found himself limited to just 13 instrumentalists, the maximum number that would fit and still leave room for the dance.
That enforced economy helped him produce a score of remarkable tenderness and austere beauty—precisely the qualities of the human spirit that Graham's ballet sought to evoke. In 1945, Copland devised a suite from the ballet scored for full orchestra, the form in which the music is best known today.
In this performance, conductor Michael Tilson Thomas has done something quite unusual. He's included an extra ten minutes of music that was part of the original ballet — music which Copland took out when he originally compiled the suite, saying it was mainly of choreographic interest. It's never been published, it only exists in manuscript and it's very, very interesting. So here, we have a full orchestral version of Appalachian Spring and it's really a corker; it's a wonderful performance.
Tilson Thomas is a generation further down the line, and yet he knew Copland, and he grew up with this music. He's developed the orchestra in such a way that it has a very lean sound — lean and clean with transparent textures. And that works beautifully in this music because it's one of the characteristics Copland was striving for when he developed this American idiom of the 1940's. He wanted lighter, clearer, more open textures, and an orchestra like the San Francisco Symphony puts those across ideally.