The photography collection of the German documentation centre for art history, Deutsches Dokumentationszentrum für Kunstgeschichte – Bildarchiv Foto Marburg (DDK), comprises more than 2.8 million objects. These include roughly 500,000 glass negatives, of which nearly 3,000 were made by the first Hessian state conservator Ludwig Bickell between 1870 and 1901. These early glass plates show a large number of retouchings and traces of their production and usage. This makes them rich sources for photographic and art historical or cultural historical research as well as conservation studies.