Many of the social architectures of the late nineteenth century are made of iron and glass structures: several cities in united Italy welcomed ferrous alloys for the construction of theatres and urban galleries, reco¬gnising their technical-structural potential for covering large spans. Among these was Rome, which, despite its distance from the great nineteenth-century commercial movements, experimented with the use of ferrous materials not only for new constructions, but also for the creation of spaces for collective use on existing buildings. In fact, two singular episodes can be traced back to the Roman context: the covering of the Corea Amphitheatre erected on the remains of the ancient Mausoleum of Augustus and that of the winter garden annexed to the Palazzo per l’Esposizione di Belle Arti, built when the latter was still under construction. The experiences, although united by the use of metal materials, are profoundly different: the first one works on a highly stratified palimpsest and the experimentation with metal materials is brought back to a technical code that resolves the coverage of the large span by reducing the conditions of interaction with the pre-existence; the second one relates with a just built con¬struction to provide it with a modern space that shows the construction opportunities of iron and glass structures and the articulation of new and possible formal codes.