2D Macro-XRF to reveal redacted sections of French queen Marie-Antoinette secret correspondence with Swedish count Axel von Fersen

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.