The study, led by Sowmeya Sathiyamani and Claudia Colini of the Centre for the Study of Manuscript Cultures at Universität Hamburg, together with colleagues from Radboud University and several European institutions, examined fifty-six papyrus fragments bearing sixty-five independent texts, dated between the fifth and tenth centuries CE. The manuscripts came from five collections across Europe: the Austrian National Library in Vienna, the Institute of Papyrology of Sorbonne University, the Michaelides collection at Cambridge University Library, Leiden University Library, and the State and University Library in Hamburg. Crucially, the team employed an entirely non-invasive methodology, meaning none of these fragile artifacts was damaged or sampled during analysis—a critical consideration when dealing with materials that have survived more than a thousand years.

The technical backbone of the investigation rested on three complementary techniques. First, two-colour digital microscopy using a Dino-Lite USB microscope operating at magnifications of forty to fifty times allowed researchers to image ink strokes under both visible light and short-wave near-infrared illumination at 940 nanometers. This provided an initial diagnostic signature: carbon inks retain their absorptivity and remain opaque under near-infrared light, while iron-gall inks lose opacity, and pure plant inks become completely transparent. Second, infrared reflectography was performed with an OPUS Apollo imaging system equipped with a cooled indium gallium arsenide sensor sensitive in the 900 to 1700 nanometer range. Using a long wave-pass filter restricting detection to 1510–1700 nanometers, the researchers could definitively distinguish ink types: pure iron-gall ink becomes fully transparent at around 1400 nanometers, whereas any ink containing carbon—including mixed formulations—continues to absorb radiation and remains legible. Images were processed with a custom MATLAB script and normalized against a white Labsphere Spectralon diffuse reflectance target reflecting ninety-nine percent of light across the sensor’s range. Third, X-ray fluorescence spectroscopy provided elemental fingerprints of the inks. The team used a Bruker CRONO spectrometer with a 10-watt rhodium X-ray tube and a 50-square-millimeter silicon drift detector, as well as a Bruker M6 Jetstream micro-XRF instrument with polycapillary optics and spot sizes as small as 30 micrometers, operating at voltages between 35 and 50 kilovolts. Rather than relying on single-point measurements, the researchers favored mapping acquisitions, which account for the notoriously heterogeneous nature of papyrus as a writing support. The resulting elemental distribution maps were rendered as heat maps in which red indicated the highest signal intensity and dark blue the lowest.

The results for the papyrus protocols were astonishingly uniform. All twenty-four Arabic and Arabic-Greek bilingual protocols, spanning the seventh through tenth centuries CE, were written with iron-gall ink containing iron, accompanied in most cases by potassium—attributable to the tannins and gum arabic used in ink manufacture—and in roughly half the samples by manganese, an impurity associated with iron. Notably, copper and zinc sulphates, which typically appear as satellite impurities when commercial iron sulphate salts are used, were absent from every Arabic protocol analyzed. Sulphur was detected in only eleven of the twenty-four fragments. This chemical profile implies either the deliberate use of an unusually pure iron sulphate source—perhaps the green or yellow vitriol referenced in Arabic recipes—or the preparation of ink from metallic iron, such as nails or filings, reacting with a vitriolic solution. The researchers connect this observation to the Arabic ink-making tradition: the Persian physician and alchemist Muḥammad ibn Zakariyyāʾ al-Rāzī, writing his Kitāb al-asrār (Book of Secrets) around 900 CE, identified six vitriolic substances and described purification methods, and later Arabic treatises transmit recipes combining iron nails with vitriol—chemistry that would naturally yield an iron-gall ink containing only iron, sulphur, and traces of manganese.

To place the Arabic protocols in historical context, the team also analyzed seven Byzantine protocols dating to the fifth and sixth centuries. Here the results revealed a temporal split with profound implications. The early Byzantine protocols, dated between the early fifth and mid-sixth centuries, were written with carbon-based ink—a conclusion confirmed for three fragments by supplementary Raman spectroscopy using a 532-nanometer laser when the infrared reflectography camera was unavailable. The later Byzantine protocols, from the mid-sixth to late seventh centuries, however, employed either a plant ink or an iron-poor iron-gall ink, with XRF mapping detecting iron, potassium, and traces of copper. This means the Arabic protocols, though their textual formulas derive from the earlier Byzantine tradition, followed the ink practice of the later Byzantine era. When the Arab conquest transferred ownership of Egypt’s papyrus manufacturing facilities to the new rulers, the incoming administration adopted not merely the bureaucratic system of certifying rolls with protocols but the very ink chemistry associated with that function.

Papyrus protocols occupy a unique position among documentary texts, and understanding why illuminates the significance of the finding. A protocol is the text written on the prōtokollon, the outermost sheet of a papyrus roll, certifying its production in a state-controlled facility. Introduced in the Byzantine period around the fifth century CE and mandated as proof of authenticity for notarial documents by Justinian’s decree of 537 CE, the protocol evolved from a few lines in the center of the first sheet to text covering nearly the whole surface, including nearly illegible perpendicular writing known as Schraffenschrift. The bilingual protocols transitioned to purely Arabic text around 720 CE, likely as part of an empire-wide adoption of Arabic as the official administrative language, and Qur’anic verses became common elements in the formula by the second half of the eighth century. Because protocols were necessarily tied to a single writing support and produced in a localized industry—predominantly in the Nile Delta—they form a coherent corpus united by function and context of production, making them ideal subjects for correlating material choice with documentary purpose.

The contrast with other document categories was equally telling. Nine official letters from the dossier of Qurra ibn Sharīk, governor of Egypt from 709 to 714 CE, written in both Greek and Arabic and dispatched from the chancellery at al-Fusṭāṭ to Basilios, pagarch of Aphrodito, were all written with carbon-based inks. Interestingly, when metallic impurities appeared, they correlated with language: copper was detected mostly on the Arabic papyri, while iron appeared mostly with the Greek texts. Combined with the known palaeographic evidence that the Arabic letters were written with a broad-cut reed pen and the Greek with a narrow-cut pen, this suggests that distinct inks, inkwells, and likely scribes were used for each language within the same chancellery—a hypothesis consistent with the staged production of these bilingual documents. Thirteen further administrative and legal documents, including contracts of sale, deeds of donation, and receipts, along with twelve private and religious texts, were also written with carbon-based inks, irrespective of their degree of formality. The single exception was a private letter, P.Hamb. Arab. 6, dated 927–928 CE, written with a carbon–iron-gall mixed ink, detectable because the ink lost opacity under infrared reflectography while XRF revealed calcium, titanium, manganese, and iron.

The researchers propose several overlapping explanations for this functional division. One factor may be accessibility and cost: producing high-quality soot for carbon ink is labor-intensive and requires specialized facilities, yet lower grades could be mass-produced and distributed, whereas iron-gall inks are easy to make in small batches once raw materials—iron sources and tannins, often imported oak gall nuts—are obtained. A second, arguably more compelling explanation lies in the physical chemistry of the inks themselves. Iron-gall ink penetrates the writing support through the formation of an iron(III)-polyphenol complex, whereas carbon ink largely sits on the surface, making iron-gall text considerably harder to erase. For protocols serving as certificates of authenticity under a state monopoly—where Justinian’s decree required the protocol-bearing sheet to be attached to legal documents—permanence against tampering was paramount. Arabic erasure recipes compiled centuries later confirm that removing iron-gall writing is feasible only under mildly acidic conditions. In contrast, the documentary letters of a governor, though official, had limited temporal validity and did not demand the same permanence, making convenient, erasable carbon ink the practical choice.

Perhaps the most striking demonstration of the principle came from a direct comparison: two bilingual protocols, P.Vindob. G39766 and P.Vindob. G39767, were produced during the very same governorship of Qurra ibn Sharīk as the carbon-ink chancellery letters, yet the protocols were written in iron-gall ink while the letters used carbon ink. The choice of ink, the authors conclude, was not determined by the person in power but by documentary traditions that predated the Islamic conquest of Egypt. The study thereby provides material evidence of how pre-Islamic scribal conventions shaped the development of Arabic administrative culture, showing that the chemical signature of an ink can serve as a witness to institutional continuity across the Byzantine-Islamic transition. The authors caution that their corpus, while broad, remains small relative to the thousands of extant protocols—the analyzed Arabic protocols represent roughly five percent of catalogued examples from the seventh to tenth centuries—and that qualitative XRF data carry inherent limitations, including the difficulty of detecting low-atomic-weight elements like sulphur. Even so, the work demonstrates how non-invasive archaeometry can decode the silent, deliberate choices of scribes whose recipes were never written down, transforming faint brown strokes on yellowed papyrus into a readable record of technological tradition, bureaucratic logic, and cultural inheritance.