During the French Revolution, Marie-Antoinette, queen of France and wife of Louis the XVIth, maintained a highly secret correspondence with the Swedish count Axel von Fersen, her close friend and rumored lover. An unidentified censor later redacted certain sections of the exchanged letters. This presumably sensitive content has been puzzling historians for almost 150 years. We report on the methodology that successfully unraveled this historical mystery. X-ray fluorescence spectroscopy was used in macro scanning mode on the redacted sections. Specific data processing was applied to improve the legibility of the hidden writings (elemental ratios, statistical data reduction, multimodal images fusion, unmixing procedure; image treatments). This methodology successfully revealed the redacted contents of 8 letters, shedding new lights on Marie-Antoinette and Fersen relationship but also on the author of the redactions. It will also be of great interest for other historical and forensic cases involving the disentanglement of superimposed multi-elemental materials.

2D Macro-XRF scans of the letters were carried out with a Bruker M6 Jetstream. The instrument consists of a measuring head made of a Rh-target X-ray tube and a 30 mm² XFlash silicon drift detector; mounted on a XY-motorized stage. The X-rays are focused through polycapillary optics, which allows to set the beam size by changing the working distance. Pixel size of the elemental maps - determined by the step size - was the same as the beam size and set between 50 and 180 µm, depending on the required resolution. Voltage and current in the X-ray tube were set to 50 kV and 600 µA, respectively; dwell times varied between 100 ms and 3 s per pixel. A first phase of the project consisted in optimizing the acquisition parameters (beam size, dwell time per pixel) in order to allow the text to be read while minimizing analysis time. For this purpose, one word from each letter was analyzed under different conditions. The redacted passages of each letter were then fully analyzed with this choice of parameters. In certain cases, the initial dataset quality had to be improved from a spatial (pixel size) and spectral (dwell time) standpoint, requiring much longer (up to 30 times) acquisition durations. 95 days of analysis were necessary for this project. Data were collected and treated with the Bruker M6 software. Additional data analyses and image treatments were carried out with ENVI (Excelis) and ImageJ softwares.

Point analyses were performed with the multi-point option of the M6 Jetstream (dwell time of 200 s, beam size 100 µm).