Between the Lines of the Herculaneum Papyri using X-Ray Imaging Techniques

A photographic montage showing a calcinated Herculaneum papyrus scroll on a Greek scriptures background. Scrolling Back the Past at Herculaneum

Once a chic resort on the Bay of Naples, Herculaneum was favoured by the finest of Roman’s elite society, who spent the hot Italian summers there… until a catastrophe struck one afternoon in 79 AD.  The Villa dei Papiri, excavated centuries later, was found to contain the only library to have survived from the Classical World – a unique cultural treasure, which the eruption of Mount Vesuvius nearly destroyed, and yet preserved all at once.  How do you read what is essentially a charred book? 

 

The Herculaneum Papyri

Unlike Pompeii, the deep layers of volcanic material, which covered Herculaneum preserved wooden and other organic-based objects such as roofs, beds, doors, food, and even some 300 skeletons.

The Herculaneum papyri are part of a complete library, containing hundreds of such papyrus scrolls.  This library is an inestimable treasure and the only library passed on to us from Antiquity.

A map collage showing the Herculaneum "Villa dei Papiri" where the ancient scrolls were found, and its geographic location in Southern Italy.
Villa dei Papiri, Ercolano, Campania, Italy

Around the mid-18th century, the architectural remains of Pompeii and Herculaneum were discovered by Rocque Joaquin de Alcubierre (1702-1780), a military engineer in the Spanish Army.  A library was discovered in the remains of the small city of Herculaneum, left preserved by layers of pyroclastic material.

Between 1752 and 1754, the relatively small collection of 2,000 Classic scrolls were retrieved from the ruins in Southern Italy.  About 600 of those remain as yet unopened.

However, due to the pyroclastic events associated with the eruption, the scrolls were carbonised and locked-up, rendering the writing on the scrolls unreadable.

Ancient artefacts of this type are often discovered at archaeological sites.  Nevertheless, it is not always possible to unroll the scrolls and read their contents.

 

Physical Unravelling

A quite sophisticated mechanical method, devised by Antonio Piaggio in 1754, for unrolling the best preserved scrolls – applicable to their final, interior parts that make up less than a third of their original length.

The fragments pulled off by Piaggio’s machine were fragile and hard to read.  In normal light, they appear as black as burnt newspaper.

On some pieces, the eye can make out nothing.  On others, by working with microscopes and continually moving the fragments to catch the light in different ways, a few letters can be made out.

Those types of methods continued to be used until the beginning of the 20th century on those scrolls that could, at least in principle, withstand the physical stress imposed by the process.    Meanwhile, the fragments fell apart…

The physical unrolling can be difficult for several reasons, such as scroll fragility, stuck sheets or even the presence of parasites and worms that have destroyed the paper.

The classical approach is to carefully remove one by one each layer of papyrus and recompose this kind of puzzle over a flat plane. This is an invasive method and it is no longer recommended by the recent trend of the restoration theory.

More recently, the writing in rolled up Herculaneum papyri was successfully explored by different non-destructive methods that involve playing with light – one of the latest is X-ray phase-contrast tomography.

So, what are X-rays?

 

The first X-ray picture ever taken by William Röntgen of his wife's hand and wedding ring.
Röntgen’s first X-ray. Wilhelm Röntgen took this radiograph of his wife’s left hand on December 22, 1895, shortly after discovering X-rays. The picture revealed her wedding ring and denser bones on a special photographic plate covered in barium platinocyanide. Source: umhs-sk.org

Röntgen’s X-Rays

Along with plenty others of humankind’s revolutionary breakthroughs, the story of X-ray technology is based on a purely accidental discovery.

In 1895, Wilhelm Röntgen was studying cathode radiation, which occurs when an electrical charge is applied to two metal plates inside a glass tube filled with rarefied gas, when he noticed a faint light on nearby light-sensitive screens.

In itself, this response was not so surprising – fluorescent material normally glows in reaction to electromagnetic radiation – but Röntgen’s tube was surrounded by heavy black cardboard screen.  Röntgen assumed this would have blocked off most of the radiation.

Further investigations revealed that this was caused by a penetrating, previously unknown type of radiation, which Röntgen called ‘X-rays‘.

Wilhelm Röntgen’s remarkable discovery precipitated one of the most important medical advancements in human history.  From then on, X-ray technology would allow doctors see straight through human tissue to examine broken bones, and it would quickly become a powerful tool for physical experiments, investigating the body’s interior and saving lives.

 

X-Ray Technicals

Most X-rays have a wavelength ranging from 0.01 to 10 nanometres, corresponding to energies in the range 100 electronvolts to 100 keV.  X-ray wavelengths are shorter than those of UV rays, and typically longer than those of gamma rays.

A schematic diagram illustrating the wavelength and photon energy ranges of X-ray and photographic illustrations of the possible applications: X-ray crystallography, mammography, medical CT and airport security scanners.
X-rays are part of the electromagnetic spectrum, with wavelengths shorter than visible light. Different applications use different parts of the X-ray spectrum. Source: Wikipedia

 

X-rays with photon energies above 5-10 keV (below 0.2-0.1 nm in wavelength) are called hard X-rays, while those with lower energy are called soft X-rays.  Due to their penetrating ability, hard X-rays are widely used to image the inside of objects, e.g., in medical radiography and airport security.

 

Recent X-Ray Imaging Techniques

X-Ray Computed Tomography

One way to scan the interior of the brittle scrolls, without unrolling them, is to use X-ray computed tomography (XCT) – a technique with broad application, such as creating cross-sectional images (like virtual slices) of the brain.

Two photographs showing the resolution of two Greek words on a fragment of papyrus.
Two words read on a hidden layer of the fragment. Source: Mocella, V. et al. 2015

This method is based on a property of X-rays, which are absorbed differently depending on the materials that the beam passes through.  Objects can be distinguished and visualized by measuring the variations in the X-ray absorption.

For example, XCT can tell apart the skull from the brain and identify tumours.  However, in the scroll’s case, difference in the absorption between the ink and papyrus is minimal and difficult to measure using the conventional method.

The task of reading the surviving scrolls has fallen to scientists using technology such as the European synchrotron, which produces X-rays 100 billion times brighter than the X-rays used in hospitals.

Last year, physicists used the 3D X-ray imaging technique to decipher writing in the scrolls.

 

Two photographs of some very damaged fragments of papyrus.
A section of On the Good King by Philodemus of Gadara, seen in normal light… (top) … And the same scroll seen in multi-spectral infrared light. There are four columns of text, with many gaps. The first column begins: “… hold power… think… but in both circumstances ‘one wise counsel conquers many hands’, and usually affairs succeed by this means, both without weapons and with a moderate force. This did not escape the poet, but he himself called Nestor ‘bulwark of the Achaeans’ because he was most experienced among them…” (bottom) Source: Mocella, V. et al. (2015)

X-Ray Phase-Contrast Tomography

The writings in carbonised Herculaneum scrolls, covered and preserved by the pyroclastic events in 79 AD, was recently revealed using X-ray phase-contrast tomography, without the need of unrolling the sensitive scrolls.  Unfortunately, some of the text is difficult to read due to the interference of the papyrus fibres crossing the written text vertically and horizontally.

X-ray phase-contrast tomography can reveal various letters hidden inside the precious papyri without unrolling them.

Until now, it was thought that the ink used for the manuscripts was carbon-based…

 

X-Ray Fluorescence

Recently, lead was found as an elemental constituent in the writing, rendering the text more clearly readable when monitoring the lead X-ray fluorescence signal.

Two photographs of the Greek alphabets deciphered inside intact rolled up Herculaneum scrolls using the new technique.
Greek Alphabets detected Inside intact rolled-up Herculaneum Scroll Source: Mocella, V. et al. (2015)

Several hypotheses were postulated for the origin and state of lead in the papyrus writing.  Multi-scale X-ray fluorescence micro-imaging, Monte Carlo quantification and X-ray absorption micro-spectroscopy experiments are used to provide additional information on the ink composition, in an attempt to determine the origin of the lead in the Herculaneum scrolls and validate the postulated hypotheses.

 

Never Previously

A photograph showing a very damaged fragment of one of the unrolled Herculaneum papyri.
Herculaneum papyrus unrolled and damaged.

Previously, it was attempted to read these scrolls by mechanically unrolling them. These attempts, however, usually led to the irretrievable loss of large parts of the text due to the brittle nature of the carbonised scrolls.

Different opening techniques, all less effective, have been tried over the years, until the so-called ‘Oslo method’ was applied in the 1980s on two Herculaneum scrolls now in Paris with problematic results, since the method required the rolls to be picked apart into small pieces.

It all started to change around two decades ago.

 

Adjusting the Light

In 1999, scientists from Brigham Young University in the US examined the papyrus using infra-red light.  Deep in the infra-red range, at a wavelength of 700-900 nanometres, it was possible to achieve a good contrast between the paper and the ink using optical interference.  Letters began to jump out of the ancient papyrus.  Instead of black ink on black paper, it was now possible to see black lines on a pale grey background.

Scholars’ ability to reassemble the texts improved massively.  All further attempts to unroll the papyri or to separate their layers mechanically had been abandoned until now to preserve their physical integrity and the possibility of reading them as continuous texts one day, because an excessive percentage of these ancient texts was irretrievably lost by the application of such methods.

Multi-Spectral Imaging (MSI), initially developed to explore the surface of the Earth and other planets from space, was adapted to read and record faded or burnt manuscripts.  The method has potential for investigating the degraded ornamental surfaces of other artefacts.  Chabries et al. (2003) showed how MSI achieved new readings from carbonised and damaged fragments of papyrus scrolls from Herculaneum, Petra and the Judean Desert.

In 2008, a further advance was made through multi-spectral imaging.  Instead of taking a single “mono-spectral” image of a papyrus fragment under infra-red light (typically around 800 nanometres), the new technology takes 16 different images of each fragment at different light levels, before creating a composite image.

But with X-ray methods, archaeologists are now jumping ahead.

 

An infographic explaining the different steps of the latest technique being used to read the Herculaneum papyri without damaging them. Step 1: Ancient scroll carbonised by hot gas from a volcanic eruption. Step 2: Scroll placed in a protective container in a powerful CT scanner. Step 3: X-rays reveal the structure of the scroll without unwrapping it. Step 4: Scientists are still trying to distinguish the ink from the paper.

Virtual Unrolling

In recent years, the virtual restoration of ancient papyri has become an important research challenge, because papyrus degradation is often very serious, so physical analysis can damage the artefact irreversibly.

In 2015, Allegra et al. addressed the problem of virtual unrolling to read papyrus scroll by avoiding a dangerous physical unrolling.  To this aim, the scientists proposed a virtual restoration method based on software manipulation of X-ray tomographic images.  Testing of the proposed approach involves the production of a realistic papyrus model being made using the ancient method and pigments compatible with the Egyptian use.

A stack of 259 slices, obtained through X-Ray Tomography device, was processed in order to obtain a digital unrolled papyrus that is quite similar to the hypothetical unrolled sheet.

And so, what about the composition of the ink itself?  Because, until now, it was thought the ink used for the manuscripts was carbon-based

But physicists also found that the papyrus contained high levels of lead, which could have been intentionally used in the ink.  The discovery pushes back the date for the first use of metallic ink by several centuries, since the commonly-held scientific belief once was that the Romans introduced metal in the ink in the fourth century.

The task of reading the surviving scrolls now relies on scientists using technology, such as the European synchrotron – which produces X-rays 100 billion times brighter than the X-rays used in hospitals.

As demonstrated by the works of Mocella, V. et al. (2015)  and Tack, P. et al. (2016), the writing in the Herculaneum scrolls can now be, at least partially, recovered by non-destructive X-ray imaging methods, without the future need of unrolling and potentially damaging the precious scrolls.

The Herculaneum papyri can now effectively be read from the inside.