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The treatment of the rubble from the fire at the cathedral of Notre-Dame de Paris was the subject of an unprecedented protocol, drafted jointly by several departments of the ministère de la Culture and implemented by these same teams, assisted by compagnons (workers, foremen, forklift operators, automatic machine operators) present on the safety site. The aim of this approach, which was implemented immediately after the disaster, was to identify, from amongst all the collapsed materials on the floor and on the vaults, those that could be recovered, both for restoration purposes and for long-term scientific research - thus passing from the status of debris to that of vestiges. A sampling protocol, using robotic equipment on the ground and rope access technicians on the vaults, was thus organised to document and locate each material recovered (photographs, orthophotographs, geolocation): material specialists and archaeologists took turns for nearly twenty-four (24) months, in a context of lead pollution and according to an extremely constrained schedule, to carry out this sorting and draw up an initial inventory. At the end of this first phase of the scientific project, nearly ten thousand (10,000) pieces of timber, six hundred and fifty (650) pallets of stone items and three hundred and fifty (350) pallets of metal items are now stored in warehouses rented by the project management team, under the responsibility of the ministère de la Culture, and are currently the subject of a detailed inventory. All of the vestiges collected will thus be able to be used for the restoration of the building and, through the many research programmes already underway, contribute to a renewed knowledge of the cathedral of Notre-Dame de Paris. [ABSTRACT FROM AUTHOR]

• Description of method used for dry-mounting the collapsed transverse arch of the nave of Notre-Dame de Paris. • Archeological analysis of voussoirs composing the transverse arch. • Archeological and geological examination of the vaults of the upper nave of Notre-Dame de Paris (tool marks, centering method) and proposed dating. During the fire in April 2019, part of the central vaulting of the nave of Notre-Dame de Paris collapsed beneath the impact of the fallen spire. The blocks composing the transverse arch were decontaminated, then subjected to archeological examination prior to attempting to dry-mount them (i.e., without mortar) on the ground. The purpose was to collect as much information as possible about the arch itself (its state of conservation, geometry, fashioning of the blocks, assembly, etc.), to consider the possibility of putting the fallen stones back in their original places, and more broadly to improve knowledge about mediaeval cathedral vaulting. This article describes the method used during this mounting, which took place in winter 2020/21. Five criteria of differing kind and importance guided decisions on where to place the voussoirs, which way round to position them, and which to associate with which. Three were taken into consideration from the outset, with the interpretation of two of these changing over time: the non-standardized dimension of the voussoirs (their thickness), the presence of a cross mark on one of the joint faces, and the presence of rectangular notches on the extrados of the blocks for housing the end of a centering system for constructing the vault compartments. Two other criteria were incorporated as work progressed, pertaining to the order in which the blocks were collected after the fire, and the geological facies of the stones, thus introducing the idea of degree of confidence. By the end of this operation, only four blocks had been placed with certainty. Nevertheless, the criteria thereby defined and their weighting have been used as a starting point for other investigation methods. In terms of archaeology, the observations carried out on this arch in these exceptional conditions suggest we should reconsider the dating of the nave vaulting, while throwing light more broadly on the relative chronology of building works on this landmark monument of Gothic architecture. [ABSTRACT FROM AUTHOR]