High-energy X-rays reveal the secrets of ancient Egyptian inks

An international team of scientists used high-energy X-rays to analyze 12 fragments from ancient Egyptian papyri and found lead compounds in both red and black inks used. According to their recent paper, published in the Proceedings of the National Academy of Sciences, this is evidence that these compounds were added not for pigmentation but for their fast-drying properties, to prevent the ink from smearing as people wrote. Painters in 15th-century Europe used a similar technique when developing oil paints, but this study suggests ancient Egyptians discovered it 1,400 years earlier. So the practice may have been much more widespread than previously assumed.

“Our analyses of the inks on the papyri fragments from the unique Tebtunis Temple Library revealed previously unknown compositions of red and black inks, particularly iron-based and lead-based compounds,” said co-author Thomas Christiansen, an Egyptologist from the University of Copenhagen.

As I've written previously, synchrotron radiation is a thin beam of very high-intensity X-rays generated within a particle accelerator. Electrons are fired into a linear accelerator to boost their speeds and then injected into a storage ring. They zoom through the ring at near-light speed as a series of magnets bend and focus the electrons. In the process, they give off X-rays, which can then be focused down beamlines. This is useful for analyzing structure because 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.

That's what makes synchrotron radiation particularly useful for analyzing art and other priceless artifacts, among other applications. Back in 2008, European scientists used synchrotron radiation to reconstruct the hidden portrait of a peasant woman painted by Vincent van Gogh. The artist (known for re-using his canvases) had painted over it when he created 1887's Patch of Grass. The synchrotron radiation excites the atoms on the canvas, which then emit X-rays of their own that can be picked up by a fluorescence detector. Each element in the painting has its own X-ray signature, so scientists can identify the distribution of each in the many layers of paint.

Last year, we reported on the work of a team of Dutch and French scientists who used high-energy X-rays to unlock Rembrandt's secret recipe for his famous impasto technique, believed to be lost to history. Impasto (translated as "dough" or "mixture") involves applying paint to the canvas in very thick layers. It's usually done with oil paint because of the thick consistency and slow drying time, although it's possible to add acrylic gels as a thickening agent to get a similar effect with acrylics. Rembrandt used it to represent folds in clothing or jewels, among other objects, in his paintings. The scientists discovered the presence of a mineral called plumbonacrite in the impasto layer—an uncommon element in paints from that period.

And earlier this year, we reported on the work of an international team of scientists who used this method to determine the cause of alarming signs of degradation to Edvard Munch's famous painting The Scream. Their analysis revealed that the damage is not the result of exposure to light, but humidity—specifically, from the breath of museum visitors, perhaps as they lean in to take a closer look at the master's brushstrokes.

Ink is history

This latest study builds on work over the past decade or so to investigate the invention and history of ink in ancient Egypt, Greece, and Rome. "Ink is history in the sense that ink has been used to inscribe a vast number of scripts and languages on various media over the course of more than 5,000 years," the authors wrote, with the earliest such examples dating back to Egypt, circa 3200 BCE. During this period, black ink was used to write the main body of a text, and red ink was used for highlighting headings, keywords, and so forth.

"By applying 21st century, state-of-the-art technology to reveal the hidden secrets of ancient ink technology, we are contributing to the unveiling of the origin of writing practices," said co-author Marine Cotte, a scientist at the ESRF.

These inks were typically made from soot and ocher, mixed with some kind of binder (usually gum Arabic), then suspended in animal glue, vegetable oil, or vinegar. Then the mixture would be dried and pressed into pellets so that scribes could easily carry the inks with them. When they needed to use it, they would mix the dried pellet with a bit of water, using the nib of a reed pen for the actual writing. In that sense, the colorants were more closely akin to paints, in that they would be classified as pigments rather than dyes.

Cotte, Christiansen, and their colleagues have previously studied the red, orange, and pink inks used on 11 surviving fragments from several manuscripts found in two small cellars in the so-called Tebtunis Temple Library, southwest of Cairo. That work revealed an unusual red ink based on a mixture of iron and lead compounds that had not been previously documented, although there is a reference in Pliny's Natural History to blending red ocher and lead white to make an orange-reddish pigment. It was generally used as a flesh tone by Egyptian painters between 30 BCE to 400 CE, according to the authors, but had not been identified in ancient Egyptian papyri until their study.

Ring around the ocher

For this latest study, the team was interested in analyzing the mineral compounds of the red and black inks from the temple papyri fragments, especially the specific iron and lead compounds. They used numerous synchrotron radiation techniques to probe the chemical composition, including micro X-ray fluorescence, micro X-ray diffraction, and micro-infrared spectroscopy. They found a complex mix of lead phosphates, potassium lead sulphates, lead carboxylates, and lead chlorides.

“The iron-based compounds in the red inks are most likely ocher—a natural earth pigment—because the iron was found together with aluminium and the mineral hematite, which occur in ocher," said co-author Sine Larsen, also of the University of Copenhagen, of the results. "The lead compounds appear in both the red and black inks, but since we did not identify any of the typical lead-based pigments used to color the ink, we suggest that this particular lead compound was used by the scribes to dry the ink rather than as a pigment.”

Cotte et al. believe that the temple priests likely did not make the inks themselves, given the complexity of the red ink in particular, which would have required some specialized knowledge, and the sheer amount of raw materials that would have been needed to make them.

The team also noted an unusual "coffee ring effect" in the red ink markings. The coffee ring effect occurs when a single liquid evaporates and the solids that had been dissolved in the liquid, like coffee grounds, form a telltale ring. It happens because the evaporation occurs faster at the edge than at the center. Any remaining liquid flows outward to the edge to fill in the gaps, dragging those solids with it. In this case, the red ocher pigment is present in coarse particles, which stayed in place while the more finely ground soluble lead compounds diffused into the papyrus cells to create a ring effect, making it appear (at the micrometer scale) as if the letters had been outlined.

"The advanced synchrotron-based microanalyses have provided us with invaluable knowledge of the preparation and composition of red and black inks in ancient Egypt and Rome 2,000 years ago," said Christiansen. "And our results are supported by contemporary evidence of ink production facilities in ancient Egypt from a magical spell inscribed on a Greek alchemical papyrus, which dates to the third century AD. It refers to a red ink that was prepared inside a workshop. This papyrus was found in Thebes, and it may well have belonged to a priestly library like the papyri studied here, thus providing insights into some of the chemical arts applied by Egyptian priests of the late Roman period."

DOI: PNAS, 2020. 10.1073/pnas.2004534117  (About DOIs).