The paper uses the example of historical paper research in Vienna around 1900 in order to analyze the dynamics of scientific cooperation between the natural sciences and the humanities. It focuses on the Vienna-based plant physiologist Julius Wiesner (1838–1916), who from 1884 to 1911 studied medieval paper manuscripts under the microscope in productive cooperation with paleographers, archaeologists and orientalists (Josef Karabacek, Marc Aurel Stein, Rudolf Hoernle). The paper examines why these cooperations succeeded and how they developed over time. Here we (...) distinguish between two forms of cooperation: while Wiesner initially worked only reactively, in a “closed cooperation”, he later entered into “open cooperations”, in which both parties defined research questions and methods. This form of cooperation proved particularly successful, but at the same time was especially demanding because, in addition to contributing one’s own skills, it required considerable “interlocking expertise” (Andersen 2016). This was favored because the historical auxiliary sciences were in a phase of upheaval. Wiesner contributed his knowledge of technical microscopy and developed a genuine interest in historical questions, while the humanists were prepared to open themselves to scientific processes and ultimately acknowledged Wiesner as a historian of paper in his own right. (shrink)