The paper presents the generation of an interactive cataloging system for the Courthouse of Milan (Palazzo di Giustizia) and more than one hundred artworks stored in the building. The courthouse is an “unknown museum” in the city and represents a masterpiece of the architectural and figurative culture in the '30s. The online system developed in the project aims at solving a twofold task. First, it must present the "unknown museum" to citizens through a simple and effective online website. Second, it must serve as a repository for technical information not available to the public and only limited to the specialists in conservation. This second section includes catalog forms produced according to specific national standards for artworks, which require a variety of information such as size, material, artist, state of conservation, and description of previous restorations and interventions. The catalog, supported by the development of a preventive conservation plan, meets the need to identify all artworks and their conditions, planning interventions, and keeping a record of restoration activities.