Complex sentence: Kurtzman experimented with drawing on blue-lined paper; the blue lines were not supposed to reproduce visibly when the book was printed, but they were unintentionally reinforced by the gray wash used in the artwork.
Split into simpler sentences: Kurtzman experimented with drawing on blue-lined paper. The blue lines were not supposed to reproduce visibly when the book was printed, but instead they were unintentionally reinforced by the gray wash used in the artwork.