Why inks matter
In historical documents, ink is not a secondary detail. It is a material, chemical, and chronological trace. Each ink reacts differently to light, to the paper support, to oxidation, and to time. Two texts that appear identical to the naked eye may prove radically different when analysed with appropriate tools.
For this reason, ink analysis is one of the decisive keys in source criticism. It helps answer simple but uncomfortable questions: was this text written in a single phase? Do all its parts belong to the same hand and the same moment? Or did someone intervene later, correcting, adding, or rewriting?
From Osborn’s microscope to measurable colour
At the beginning of the twentieth century, Albert S. Osborn had already grasped a principle destined to become central in document examination: every ink possesses its own chromatic identity. Even when it appears black, ink is never neutral; it is the result of a specific chemical composition, an oxidation process, and a reaction with the paper support.
Osborn’s method was based on microscopic observation and the use of optical filters to isolate specific colour components. This made it possible to distinguish different inks that appeared indistinguishable to the naked eye. It was a fundamental intuition, shifting attention from handwriting alone to the material substance that renders it visible.
The limitation of that system was not theoretical, but technological. The analysis remained largely qualitative, relying on the examiner’s experience and verbal descriptions of chromatic differences. What was missing was a decisive element: numerical measurement of colour.
With the advent of high-definition digital photography, this limitation is overcome. The image is no longer merely a representation, but a data matrix. Each pixel contains measurable and comparable information that can be analysed without any intervention on the original document.
The conceptual shift is clear. It is no longer a matter of seeing colour, but of reading it as data. Ink ceases to be a perceived quality and becomes a measurable quantity. It is today that Osborn’s intuition finally finds the tools needed to be carried to its most radical consequences.
The RGB triplet method
Every digital image is composed of pixels. Each pixel contains three numerical values: Red (R), Green (G), and Blue (B), each ranging from 0 to 255. These RGB triplets objectively describe the actual colour of the ink as it reacts to light.
The method developed and applied in our studies systematically analyses these triplets, isolating specific chromatic ranges. In this way, it becomes possible to determine whether two strokes of writing share the same chromatic composition or belong to different inks.
The procedure is entirely non-destructive: the document is neither touched, altered, nor sampled. Everything takes place on the digital image, making the method replicable, verifiable, and usable in forensic contexts.
Conditions and criteria of validity
For the analysis to be reliable, certain fundamental conditions must be met: identical paper support, uniform lighting, certified white balance, and high-resolution images. In the case of documents preserved in archives and libraries, these conditions are often ideal.
The method also takes historical chemical factors into account. In the eighteenth century, for example, the most widespread ink was iron gall ink, whose composition varied significantly depending on recipes, sources of tannins, and degrees of oxidation. This very variability makes ink an extraordinary chronological indicator.
The ink used in the eighteenth century
In the eighteenth century, the most common ink in Europe was iron gall ink. It was not a standardised substance, but rather a family of inks obtained by combining iron salts with tannins of plant origin, generally extracted from oak galls, bark, or other tannin-rich materials.
Recipes varied significantly from place to place, from scribe to scribe, and even over time for the same writer. The proportions of ferrous sulphate, tannins, gum arabic, and water influenced both the initial colour of the ink and, above all, its evolution over time.
Freshly applied iron gall ink could appear greyish or brownish. Only through progressive oxidation of the iron, in contact with air, did the stroke darken to black or deep brown. This process is not uniform, as it depends on the ink’s chemical composition, the acidity of the paper, humidity, and exposure to light.
This controlled instability makes iron gall ink a valuable chronological indicator. Inks that appear similar may in fact present significant differences in residual chromatic components, detectable through RGB triplet analysis. Each mixture leaves a specific chromatic trace, even centuries later.
It is important to stress a point often overlooked: in the eighteenth century, it was not possible to write for years with the same ink without perceptible variations. Containers were exhausted, recipes changed, and ingredients were never perfectly identical. The presence of the same ink in documents distant in time therefore constitutes an anomaly that requires explanation.
Digital colour analysis allows these inconsistencies to be detected objectively. When the same chromatic signature reappears in places that should historically be separated by months or years, the problem is not interpretative, but material.
Identifying later interventions
By applying RGB triplet analysis, it is possible to isolate portions of text written with different inks even years apart, sometimes on the same page. Corrections, underlinings, marginal additions, and rewritings emerge clearly when specific chromatic ranges are highlighted.
This makes it possible to reconstruct the phases of a document’s composition and to distinguish what belongs to the original drafting from what was added later. In many cases, it is precisely these later interventions that betray manipulation.
An objective, not interpretative method
The strength of this approach lies in its objectivity. RGB values are numbers, not impressions. Chromatic differences can be statistically measured, compared, and clearly visualised, even in a courtroom setting.
When two portions of text show systematic differences in chromatic components, with identical paper and environmental conditions, the only variable at play is the ink. And different inks mean different moments, different hands, or both.
From musicology to source criticism
Ink analysis is not a technical curiosity, but a central tool in the critical re-reading of sources. Applied to catalogues, musical manuscripts, letters, and administrative documents, it allows entrenched narratives to be dismantled and stratified texts to be reordered.
In this sense, ink becomes a primary source: it does not tell stories, but disproves them when they do not hold up.
A method already applied and verified
The method described here is neither a theoretical proposal nor an experimental exercise. It has already been systematically applied to a concrete case of historical and documentary relevance, demonstrating its effectiveness in identifying different hands, distinct writing phases, and posthumous interventions within a single document.
The combined analysis of RGB triplets, ink chromatic distribution, and calligraphic features was used to study a complex manuscript corpus in which historiographical tradition had long assumed authorial unity. The results instead revealed a material stratification incompatible with a single, continuous drafting.
This work has undergone peer review and has been published in a Scopus-indexed scientific journal, the Journal of Forensic Document Examination, within an issue devoted to writing materials. The publication documents the method in detail, including image acquisition criteria, analytical procedures, and quantitative results.
The validity of the method lies precisely in the transition from historical analysis to forensic examination, from subjective perception to replicable measurement. Ink, read as chromatic data, thus becomes a decisive tool for source criticism and for reconstructing the actual dynamics of document production.
For a complete presentation of the case study and the practical application of the method, see the scientific publication:
Journal of Forensic Document Examination, Vol. 32 (2024)
Briciole di storia
Ink does not lie (but it can betray)
An ancient ink is never truly “still”. Even when the text appears immutable, the material continues to react: it oxidises, fades, and migrates into the paper fibres. It is a slow process, invisible to the naked eye, but constant.
For this reason, two strokes written with the same ink, but at different times, never age in the same way. They change together only if they were born together. When one remains darker, more saturated, or reacts differently to light, it is not a matter of handwriting, but of time.
In authentic documents, ink ages coherently. In manipulated documents, time stratifies. And time, unlike human beings, cannot pretend.
From this point onward, ink definitively ceases to be a detail and becomes a source in its own right.
