Through all these researches into an art that would lead to immaterialisation, Werner Ruhnau [German architect] and I came together in the architecture of the air. He was hindered by the last obstacle that even a Mies van der Rohe [also a German architect, famous for his reflecting glass skyscrapers in the U.S.] hadn't been able to overcome: the roof, the screen that separates us from the sky, from the blue sky. And I was hindered by the screen that the tangible blue on the canvas constitutes, which deprives man of a constant vision of the horizon.

In 1958; p. 45 - "Yves Klein, 1928 – 1962, Selected Writings"