Cool art techniques

Via the New York Times, this neat bit of spectroscopy: 

The year was 1791, and while Marie Antoinette may not have had the favor of the people of France, she did have a pen pal. Her confidant, Axel von Fersen, was a Swedish count, and one of the French queen’s close friends.

Between the summers of 1791 and 1792, though the queen was kept under close surveillance after a botched escape attempt, she still managed to sneak letters to the Count of Fersen. He copied the letters, which are now held in the French national archives. But between the time the letters were written and the time they arrived at the archives, some mysterious actor censored the letters, scrawling out words and lines with tightly looped circles of ink.

The content of the censored lines — and the identity of the fastidious scribbler — eluded historians for nearly 150 years. In a paper published on Friday in the journal Science Advances, scientists have now revealed the redacted content of eight of the censored letters between Marie Antoinette and the Count of Fersen. The researchers used a technique called X-ray fluorescence spectroscopy, which can detect the chemical signatures of different inks without damaging documents.

...The method that prevailed was X-ray fluorescence spectroscopy, or XRF, which differentiated the chemical signature of the ink used by the original author and the ink used by the censor. The initial XRF scans revealed that both texts had been etched with metal-gall ink, a common ink made with iron sulfate. “But the iron sulfate is not pure most of the time,” Dr. Michelin said. “It contains other metallic elements, like copper and zinc. With that slight difference, we can differentiate the inks.”

In some letters, copper was present only in the original ink, so isolating the element on its own would remove the censor. “So just with the map of the copper, I can read the text,” Dr. Michelin said...

 It seems that art science always has the best stories.