Clarkson argued that "foreign marks" and "foreign matter" are important to understand the book's physical history and the social history in which the book resided and that this evidence of use should be preserved.[28] In 1968, the International Centre for the Study of the Preservation and Restoration of Cultural Property organized an international seminar on the subject of conservation of library materials in Rome.[29] In 1969, the first university-level conservation conference occurred at the University of Chicago where they published Deterioration and Preservation of Library Materials.[29]