In 1924, he joined the Berlin Secession; by this time he was developing an increasingly realistic style of painting that used thin glazes of oil paint over a tempera underpainting, in the manner of the old masters. His 1923 painting "The Trench", which depicted dismembered and decomposed bodies of soldiers after a battle, caused such a furor that the Wallraf-Richartz Museum hid the painting behind a curtain. In 1925 the then-mayor of Cologne, Konrad Adenauer, canceled the purchase of the painting and forced the director of the museum to resign.