'Invisible ink' on antique Nile papyrus revealed by multiple methods

For more than a century, numerous metal crates and cardboard boxes have sat in storage at the Egyptian Museum and Papyrus Collection Berlin, all of which were excavated by Otto Rubensohn from 1906 to 1908 from an island called Elephantine on the River Nile in the south of Egypt, near the city of Aswan. Eighty percent of the texts on the papyrus in these containers have yet to be studied, and this can hardly be done using conventional methods anymore. Thousands of years ago, the Egyptians would carefully roll up or fold together letters, contracts and amulets to a tiny size so that they would take up the least possible space. In order to read them, the papyri would have to be just as carefully unfolded again. "Today, however, much of this papyrus has aged considerably, so the valuable texts can easily crumble if we try to unfold or unroll them," Prof. Dr. Heinz-Eberhard Mahnke of Helmholtz-Zentrum Berlin and Freie Universität Berlin describes the greatest obstacle facing the Egyptologists, who are eager to unearth the scientific treasures waiting in the boxes and crates in the Berlin Egyptian Museum.

Testing the fragile papyrus with nondestructive methods

The physicist at Helmholtz-Zentrum Berlin knew from many years of research how to analyse the fragile papyrus without destroying it: shining a beam of X-ray light on the specimen causes the atoms in the papyrus to become excited and send back X-rays of their own, much like an echo. Because the respective elements exhibit different X-ray fluorescence behaviour, the researchers can distinguish the atoms in the sample by the energy of the radiation they return. The scientists therefore long ago developed laboratory equipment that uses this X-ray fluorescence to analyse sensitive specimens without destroying them.

Scholars in ancient Egypt typically wrote with a black soot ink made from charred pieces of wood or bone and which consisted mainly of elemental carbon. "For certain purposes, however, the ancient Egyptians also used coloured inks containing elements such as iron, copper, mercury or lead," Heinz-Eberhard Mahnke explains. If the ancient Egyptian scribes had used such a "metal ink" to inscribe the part that now appears blank on the Elephantine papyrus, then X-ray fluorescence should be able to reveal traces of those metals. Indeed, using the equipment in their laboratory, the researchers were able to detect lead in the blank patch of papyrus.

Revealing sharper details at BESSY II with "absorption edge radiography"

In fact, they even managed to discern characters, albeit as a blurry image. To capture a much sharper image, they studied it with X-ray radiography at BESSY II, where the synchrotron radiation illuminates the specimen with many X-ray photons of high coherence. Using "absorption edge radiography" at the BAMline station of BESSY II, they were able to increase the brightness of this technique for the sample studied, and thus better distinguish the characters written on the papyrus from the structure of the ancient paper. So far, it has not been possible to translate the character, but it could conceivably depict a deity.

Composition of the invisible ink resolved in the Rathgen laboratory

The analysis at BESSY II did not identify the kind of leaded ink the ancient scribes used to write these characters on the papyrus. Only by using a "Fourier-transform infrared spectrometer" could the scientists of the Rathgen Research Laboratory Berlin finally identify the substance as lead carboxylate, which is in fact colourless. But why would the ancient scribe have wanted to write on the papyrus with this kind of "invisible ink"? "We suspect the characters may originally have been written in bright minium (red lead) or perhaps coal-black galena (lead glance)," says Heinz-Eberhard Mahnke, summarising the researchers' deliberations.

If such inks are exposed to sunlight for too long, the energy of the light can trigger chemical reactions that alter the colours. Even many modern dyes similarly fade over time in the bright sunlight. It is therefore easily conceivable that, over thousands of years, the bright red minium or jet black galena would transform into the invisible lead carboxylate, only to mystify researchers as a conspicuously blank space on the papyrus fragment.

Method developed to study folded papyri without contact

With their investigation, Dr. Tobias Arlt of Technische Universität Berlin, Prof. Dr. Heinz-Eberhard Mahnke and their colleagues have pushed the door wide open for future studies to decipher texts even on finely folded or rolled papyri from the Egyptian Museum without having to unfold them and risk destroying the precious finds. The researchers namely developed a new technique for virtually opening the valuable papyri on the computer without ever touching them.

The Elephantine project funded by the European Research Council, ERC, and headed by Prof. Dr. Verena Lepper (Stiftung Preußischer Kulturbesitz-Staatliche Museen zu Berlin) is thus well on its way to studying many more of the hidden treasures in the collection of papyrus in Berlin and other parts of the world, and thus to learning more about Ancient Egypt.