Medievalist art historian at the University in Bern, 1934-1968. Hahnloser's father was a physician with an appreciation for modern art. The young man grew up knowing the modern Swiss artists of his day, including Pierre Bonnard, Vuillard, Felix Vallotton, Giovanni Giacometti, and Ferdinand Hodler. He studied art history at the university in Basle under Friedrich Rintelen. Beginning in 1921 he entered the university in Vienna working under Julius von Schlosser. This group of Schlosser students--all classmates--would later become the major art historians of their time: Hans Sedlmayr, Otto Pächt, Charles de Tolnay, E. H. Gombrich and Ernst Kris. Schlosser's personality, his strength, philosophical/historic analysis of sources and outlook became formative for Hahnloser (Betthausen/Metzler). After graduation in 1926 and military service, he returned to Vienna as a privatdozent and Schlosser's assistant. The two became close friends; Hahnloser, a gifted violinist, toured Europe with Schlosser as cellist as part of a string quartet. In 1933 at the 13th International Congress of the History of Art, Stockholm, he traveled with a group of medievalists including Richard Hamann, Kenneth John Conant and Paul Frankl, lead by Johnny Roosval, to see the discovery of the only gothic church still with its wooden arch scaffolding remaining (Frankl). Hahnloser took the photos. In 1934 at the death of Bern art historian Arthur Weese Hahnloser was called to succeed him. The following year he published his dissertation research on the "sketchbook" (guild book) of Villard de Honnecourt (1220-1230). A masterful philological and art-historical examination of the book, it was the first German-language translation. Before and during World War II, when travel was limited, Hahnloser devoted himself to the history of early Swiss art. He organized the excavation of the Cluniac cloister in Rüeggisberg bei Bern (completed 1947). He served as Rector of the University for the 1956-1957 year. As president of the Association for Swiss Art History (Gesellschaft für schweizerische Kunstgeschichte) in 1957, he inaugurated and update of Hans Jennys' 1934 guide to art in Switzerland, now called the Kunstführer durch die Schweiz. A noted scholar on medieval glass, he led the Gesellschaft to commission the issuing of the Corpus vitrearum Medii Aevi (the corpus cataloging for medieval stained glass) in Switzerland. The first volume on the treasure of San Marco in Venice appeared in 1965. He retired emeritus in 1968. Hahnloser was ever fascinated with the conceptual image as the dominant impulse for medieval art. As he had began with his dissertation on Villard de Honnecourt, he returned to this. His students included Marcel Georges Roethlisberger.
Hahnloser's methodology was a complex combination of Vienna-School (Schlosser) documents analysis and visual analysis. Hahnloser's use of the Gedankenbild (mental construct) can be traced to the concept of Gedächtnisbild (image of memory) of the Viennese archaeologist Emanuel Löwy. He also employed a psychoanalytic art history. In 1960 Frankl termed Hahnloser's work on Villard the most comprehensive treatment of the topic. Throughout his life he contributed reminiscences on the artists he knew. He left a personal art collection of many of these artists in addition to works on paper by van Gogh and others.
- and Vaillant, Annette, and Cassou, Jean, and Cogniat, Raymond. Bonnard: ou, Le bonheur de voir. Neuchatel: Éditions Ides et Calendes, 1965, English, Bonnard; with a Dialogue Between Jean Cassou and Raymond Cogniat. Greenwich, CT: New York Graphic Society, 1966;
- and Volbach, Wolfgang. Il Tesoro di San Marco. 2 vols. Florence: Sansoni, 1965-1971;
- "Gedankenbild im Mittelalter und seine Anfänge in der Spätantike." Actes du XVIIme Congrès international d'histoire de l'art (1955): 592;
- Das Gedankenbild im Mittelalter und seine Anfänge in der Spätantike. Rome: Accademia nazionale dei Lincei,1968 pp. 256-266;
- [art collection:] "Werke aus der Sammlung Hahnloser." [special issue of] Du no. 11 (November 1956)