Solving a French Revolution mystery with chemistry

This exercise is a part of Educator Guide: Marie Antoinette’s Letters Are Uncensored by X-rays / View Guide

Directions for teachers: Ask students to read the online Science News article “Ink analysis reveals Marie Antoinette’s letters’ hidden words and who censored them,” which details how scientists used chemistry to unravel a mystery from the French Revolution, and answer the questions below. A version of the article, “Marie Antoinette’s letters are uncensored by X-rays,” appears in the November 6, 2021 issue of Science News.

1. Who was Marie Antoinette? Who did she write secret letters to while imprisoned during the French Revolution?

Marie Antoinette was a queen of France. She wrote letters to Swedish Count Axel von Fersen.

2. What mysteries surround the letters?

Three mysteries surround the letters, which have some words that are scribbled out with ink: What are the redacted words, who redacted them and what might they reveal about the relationship between Marie Antoinette and Count Axel von Fersen?

3. What technology helped scientists reveal the scribbled-out words? How does the technology work?

Scientists used X-ray fluorescence spectroscopy, or XRF. The technology works by shooting an X-ray beam at a sample to cause electrons within the sample’s atoms to jump to higher energy levels. That energy boost causes the sample to release its own X-rays, which scientists can use to figure out the sample’s elemental makeup.

4. How did the technology reveal the scribbled-out words? What are the words?

Using XRF, scientists determined that the ink Marie Antoinette used to write the letters and the ink used to redact the letters were made from iron sulfate. But each ink had distinctive ratios of copper-to-iron and zinc-to-iron. By scanning the letters pixel by pixel and mapping each pixel’s elemental differences  onto a gray scale, the scientists revealed the hidden words, including “beloved,” “madly” and “tender friend.”

5. Who crossed out the words, and how did scientists identify the person?

Count Axel von Fersen redacted the letters himself. The ratios of certain elements in the ink used to redact the letters matches the elemental ratios in the ink von Fersen used to make copies of other letters Marie Antoinette wrote him.

6. What is one mystery that remains unsolved?

Though the letters are now uncensored, it is still unclear whether the nature of the relationship between Marie Antoinette and von Fersen was romantic or political.