Fires in buildings housing historical collections of books and papers have occurred throughout history and all over the world. The impact on written cultural heritage has frequently been disastrous and its preservation in libraries and archives is still an open issue. The application of traditional conservation materials and techniques on charred paper has so far been unsatisfactory. As a consequence, a large number of paper manuscripts harmed by fire have been left untouched since the devastations, excluded from access and even from simple handling due to their extreme fragility and to the tendency to fragmentation. This contribution presents the results of a research project aimed at developing an innovative green chemistry conservation treatment for the strengthening of burnt paper, with a specific focus on handmade rag paper, which was the support for writing from the 13th to the 19th century in the western Latin world. The research work involved the Institut de Recherche et d’Histoire des Textes – IRHT, CNRS (Paris), the Centre de Recherche sur la Conservation – MNHN, CNRS (Paris) and the Italian University of Padua. It advanced through progressive steps through the study and characterization of historic papers damaged by fire, to the production of burnt samples to be used in the experimental part, to the development of an innovative treatment for the reinforcing of the charred paper areas and the performance of some test applications in loco carried out in library. The proposed methodology makes use of a nanocomposite formulation highly compatible with the burnt paper matrix and based on nanocelluloses, both alone and combined with polysiloxanes. When applied with an airbrush, it limits the fragmentation and loss of material, as assessed by the critical folding angle measurements carried out on handmade samples. Physico-chemical characteristics of the treated paper samples evaluated using FTIR, SEM-EDS and XRD ensured the persistence of the original cellulosic material, as expected from the minute amount of nanocomposite introduced. The tests carried on charred historic written documents showed that the readability of the textual information 'hidden' in the blackened areas was perfectly accessible using multispectral imaging techniques, confirming that this conservation treatment can effectively contribute to securing the written cultural heritage victim of fire, allowing cautious and safe access to the text for the purpose of their reproduction for the benefit of the wider scholarly community.