A team of German scientists has used a combination of cutting-edge physics techniques to virtually "unfold" an ancient Egyptian papyrus, part of an extensive collection housed in the Berlin Egyptian Museum. Their analysis revealed that a seemingly blank patch on the papyrus actually contained characters written in what had become "invisible ink" after centuries of exposure to light.

Most of the papyri in the collection were excavated around 1906 by an archaeologist named Otto Rubensohn, on Elephantine Island, near the city of Aswan. They've been gathering dust in storage for much of the ensuing decades, and because they are so fragile, more than 80% of the text within remains undeciphered. “Today, much of this papyrus has aged considerably, so the valuable texts can easily crumble if we try to unfold or unroll them,” said co-author Heinz-Eberhard Mahnke of Helmholtz-Zentrum Berlin and Freie Universität Berlin. That makes noninvasive imaging methods essential to the project.

In 2016, an international team of scientists developed a method for "virtually unrolling" a badly damaged ancient scroll found on the western shore of the Dead Sea, revealing the first few verses from the book of Leviticus. The so-called En Gedi scroll was recovered from the ark of an ancient synagogue destroyed by fire around 600 CE. To the naked eye, it resembled a small lump of charcoal, so fragile that there was no safe way to analyze the contents. The team's approach combined digital scanning with micro-computed tomography—a noninvasive technique often used for cancer imaging—with segmentation to digitally create pages, augmented with texturing and flattening techniques. Then they developed software (Volume Cartography) to virtually unroll the scroll.

"Much of this papyrus has aged considerably, so the valuable texts can easily crumble if we try to unfold or unroll them."

The artifacts analyzed by the German team were made of a different material, and hence a different approach was needed to uncover the hidden text. "The En Gedi scroll is parchment," said Mahnke. "We are dealing with papyrus." Papyrus is produced from the stems of the plant in two layers, perpendicular in the direction of the fiber, he noted. The varying thickness of the base writing material can make it harder to identify script. Also, the papyri he and his colleagues studied were folded along orthogonal lines, rather than rolled up, so that the objects took up the least amount of space. "Mathematically, a scroll is a roll, topologically identical whether it's a nicely rolled cylinder or heavily distorted," said Mahnke. A folded papyrus doesn't have that advantage.

So Mahnke and his colleagues used the synchrotron radiation source at the BESSY II facility (Berlin Electron Storage Ring Society for Synchrotron Radiation) in Berlin for their experiments, building on work earlier this year demonstrating proof of principle on a mock-up sample. Synchrotron radiation differs from conventional X-rays in that it's a thin beam of very high-intensity X-rays generated within a particle accelerator. Electrons are fired into a linear accelerator (linac), get a speed boost in a small synchrotron, and are injected into a storage ring, where they zoom along at near-light speed. A series of magnets bend and focus the electrons, and in the process, they give off X-rays, which can then be focused down beam lines.

H-E Mahnke et al./Journal of Cultural Heritage

H-E Mahnke et al./Journal of Cultural Heritage

H-E Mahnke et al./Journal of Cultural Heritage

T. Arlt et al./Journal of Cultural Heritage

T. Arlt et al./Journal of Cultural Heritage

Secrets revealed

That makes it ideal for non-invasive imaging, since in general, the shorter the wavelength used (and the higher the energy of the light), the finer the details one can image and/or analyze. And the technique can be used to image fragile archaeological artifacts without damaging them. Shine that high-energy X-ray beam onto a fragile papyrus and the photons excite the atoms, emitting echoing X-rays in response. Those echoes will fluoresce in different ways depending on the atoms present in the sample.

According to Mahnke, it was common for ancient Egyptian scholars to use a black ink made from charred bits of wood or bone—in other words, carbon-based inks. But they would also used colored inks, which contain traces of metallic elements such as iron, copper, mercury, or lead. The X-ray fluorescence produced when imaging the samples should therefore indicate whether those metallic elements are present.

For instance, the German team found traces of lead in the blank patch of papyrus and were able to discern faint, blurry characters. Infrared spectrometry identified the ink as a colorless lead carboxylate. However, "We suspect the characters may have originally been written in bright minium (red lead) or perhaps coal-black galena," said Mahnke. Over the centuries, those inks would have slowly transformed into invisible lead carboxylate, leaving a seeming blank space on the papyrus.

In order to enhance the brightness to gain better contrast between the papyrus and the characters, the team combined absorption edge radiography and tomography to further illuminate the sample. That gave them a clearer image of the characters, although nobody has yet been able to translate them. Analysis of another papyrus revealed the Coptic word for "Lord."

The best part: the German researchers were able to virtually open the fragile papyri without ever touching them. It should now be possible to use these techniques to study the remaining papyri in the Elephantine Island collections. "The greater challenge is the revealing of text written in carbon ink, which shows practically no contrast in absorption tomography," said Mahnke. "One needs to find out subtle differences to make a distinction between the organic (carbon based) material and the ink (also carbon-based) [that is] good enough for a contrast."

DOI: Journal of Cultural Heritage, 2019. 10.1016/j.culher.2019.07.007 (About DOIs).