Iron gall ink was the writing medium of Europe from the 12th through the 19th centuries, and its recipe sounds almost alchemical. Scribes mixed iron(II) sulfate with tannin-rich extracts from plant galls and organic binders such as gum arabic. The polyphenols in the tannins react with iron to form dark complexes that deepen to a rich brown-black on exposure to oxygen. The chemistry that made the ink so legible, however, also made it corrosively acidic, and the same reactivity has been slowly eating away at parchment and paper for centuries. In some ancient documents, conservators noticed something stranger still: crystals forming within the ink itself, causing the written text to fade rather than simply darken.

Those crystals turned out to be jarosite, a potassium iron sulfate hydroxide mineral that also occurs in sodium and hydronium varieties. Jarosite precipitation requires a very specific combination of conditions: abundant sulfate, ferric iron, and a strongly acidic environment. Previous attempts to reproduce its formation in laboratory-made inks reached no conclusive explanation, though repeated wetting and drying cycles have long been considered the main trigger. The new study offers a more complete picture, showing that the mineral’s appearance in the manuscript reflects interconnected inorganic and organic processes, including iron redox cycling, organic-iron complexation, and a drop in pH below 3.0, with most of the precipitation chemistry being essentially non-biological.

Under the microscope, the jarosite in the faded manuscript appeared in two telling forms. Dendritic precipitates, branching like miniature frost patterns, marked areas where the mineral had crystallized rapidly from supersaturated solutions. Elsewhere, pseudo-rhombohedral crystals displayed clear evidence of dissolution, their edges etched and corroded. This combination of fresh precipitation and active dissolution is the mineralogical signature of a system in flux, where jarosite forms, breaks down, and re-forms as chemical conditions shift within the ink layer. Crucially, both crystal types were closely associated with fungi and bacteria, placing the microorganisms at the heart of the mineral’s life cycle.

Metagenomic analysis of the ink identified acidophilic fungi belonging to the genera Geotrichum and Coniochaeta as key players. These organisms are extremophiles in a quiet sense, thriving in an environment that would dissolve most organic life. The researchers found that these fungi mobilized elements both inside and outside the inked areas of the manuscript, effectively transporting iron, sulfur, and other components across the document’s surface. In other words, the microbes were not merely passive residents of an acidic niche; they were actively redistributing the ink’s chemistry, driving the dissolution of jarosite crystals and enabling their reprecipitation in new locations.

The mechanism the authors propose is a feedback loop between biology and geochemistry. The ink’s inherent acidity, maintained by residual iron sulfate and the oxidation of iron-tannin complexes, selects for acid-tolerant microorganisms. As fungi and bacteria metabolize the organic binders and interact with the inorganic components, they alter local pH and redox conditions, releasing iron from organic complexes and supplying it to solution. When evaporation concentrates the pore solutions during drying cycles, jarosite precipitates. When humidity rises and conditions shift, the same microbes help dissolve the mineral again, translocating its elements. Each cycle leaves the ink paler and the document more fragile.

This finding reframes a long-standing conservation problem. Ink corrosion has traditionally been treated as a purely chemical threat, managed through deacidification, phytate treatments to complex free iron ions, and strict humidity control. The demonstration that living communities of extremophilic microorganisms participate in the mobilization of iron and sulfate means that microbial management may need to become part of the conservation toolkit. It also explains why laboratory reconstructions of jarosite formation in inks have been inconclusive: without the microbial actors, the full sequence of dissolution, translocation, and reprecipitation cannot be reproduced.

The analytical approach itself is a model for modern heritage science. Micro-Raman spectroscopy allowed the team to identify jarosite and distinguish its potassium, sodium, and hydronium varieties directly on the manuscript without sampling. Scanning electron microscopy paired with energy-dispersive X-ray spectroscopy revealed the crystal morphologies and their elemental signatures, while metagenomics uncovered the taxonomic composition of the microbial community without the need for cultivation, which is essential when the organisms in question are adapted to conditions nearly impossible to replicate on a petri dish. Together, these techniques connected the mineralogical evidence to its biological drivers.

Beyond the archive, the study resonates with research far from the library stacks. Jarosite is a hallmark of acid sulfate environments on Earth, from mine tailings to volcanic hot springs, and its detection on Mars has been interpreted as evidence of ancient acidic water. The Italian manuscript demonstrates that jarosite can form in a human-made, organic-rich microenvironment over historical timescales, provided acidity, iron, and sulfate are available and wetting-drying cycles concentrate the solutions. That a 17th-century scribe’s ink became a natural laboratory for the same geochemistry seen in planetary science is a striking reminder of how universal these processes are.

For the curators and conservators who steward Europe’s documentary heritage, the message is both sobering and actionable. The fading of iron gall ink is not a single chemical event but an ongoing ecological and mineralogical process, one in which acid-loving fungi such as Geotrichum and Coniochaeta and their bacterial companions continuously reshape the ink’s mineralogy. Understanding jarosite precipitation, dissolution, and translocation as coupled biological and inorganic phenomena offers a path toward interventions that address the microbial community as well as the chemistry, potentially slowing the disappearance of texts that have survived centuries of war, humidity, and neglect, only to be threatened by organisms small enough to live inside a single stroke of a pen.

Subject of Research: Microbial colonization of acidic historic iron gall ink and the precipitation, dissolution, and translocation of jarosite

Article Title: Extremophilic microorganisms colonization of acidic historic ink with precipitation, dissolution and translocation of jarosite

Article References: Maisto, F., Pin, L., Pavlović, J., Biocca, P., Bicchieri, M., Cuadros, J., Pangallo, D., & Pinzari, F. (2026). Extremophilic microorganisms colonization of acidic historic ink with precipitation, dissolution and translocation of jarosite. npj Heritage Science. https://doi.org/10.1038/s40494-026-02969-8

Image Credits: AI Generated

DOI: 10.1038/s40494-026-02969-8

Keywords: iron gall ink, jarosite, extremophiles, acidophilic fungi, Geotrichum, Coniochaeta, heritage science, manuscript conservation, micro-Raman spectroscopy, metagenomics, SEM-EDS, mineral dissolution