Dating a composite ancient wooden artefact and its modifications. A case study

The recent restoration of the large wooden doors of the medieval St. Ambrogio Basilica in Milan gave the opportunity for a broad project of scientific examination, including dating with radiocarbon and dendrochronology. The two doors represented a puzzle for art historians, because of the non-documented modifications occurred during their history. In fact, they are composed of various wooden elements, some of them surely modified with the 1750 jubilee restoration, assembled into three layers. The large number of these elements suggested not to restrict the samples only to the main carved scenes, but to extend them to frames and backboards, in order to obtain a consistent set of data and a more reliable chronology. Analytical campaign oriented to dating, coordinated by Quartana Restauri and LAM (University of Bergamo), took into account the different characteristics of woods, needing 24 samples carefully extracted from hidden areas to be radiocarbon dated by AMS (Accelerator Mass Spectrometry) at CEDAD (Centre for Dating and Diagnostics, University of Salento, Lecce, Italy) laboratories, and two of them dated at LABEC (Laboratory of nuclear techniques for cultural heritage, University of Florence and INFN, Italy) for comparison. The results obtained for the vertical woods of the back, belonging to Renaissance period, suggested further studies employing dendrochronological analyses based on high-resolution photographs, carried out by CUDaM (University of Milano-Bicocca). ED-XRF (Energy Dispersive X-ray Fluorescence) analyses were also carried on, by LAM, with portable instruments, in order to study the composition of the residues of polychromy, where present