The principle: black is not a colour
Many historical inks, especially dark ones, appear identical to the naked eye, but black is almost always a combination—an equilibrium of chromatic components reacting differently to light, oxidation, paper, and ageing.
RGB triplets make this combination measurable. Each pixel in a digital photograph is described by three numerical values: R (red), G (green), and B (blue), each ranging from 0 to 255. If two strokes appear identical but show different RGB distributions, the difference is not an opinion but a datum. This approach is known as Chromatic Triplet Analysis (CTA).
From Osborn to digital methods: same principle, better tools
At the beginning of the twentieth century, Albert S. Osborn proposed analysing ink colour through microscopy and filters: enlarging the text, isolating specific chromatic bands, and describing the ink tint.
Today optical filters are no longer necessary because digital photography is already decomposed into RGB. Chromatic Triplet Analysis extends that principle with computational tools, allowing the RGB triplets of every pixel to be read and compared without altering the document, yielding objective, measurable, and replicable results.
Acquisition conditions
CTA works only if the photograph is reliable. Change the lighting or white balance and you change the numbers; at that point you are measuring the lamp, not the ink.
Practical requirements:
1) Comparable paper. Ideally the same paper type within the corpus, since absorption and reflectance affect the values.
2) Uniform illumination. No shadows, no hot spots, no variation across the page.
3) Constant white balance. It must be maintained identically across images.
4) High resolution. More detail means less statistical noise in the measurements.
If these conditions are not met, the results become fragile—not false, but difficult to defend.
Procedure. Isolate the stroke, measure the triplets, define the ranges
The operational workflow of Chromatic Triplet Analysis, in essential form, is as follows:
1) Image acquisition/reading. Work is conducted on HD photographs under controlled conditions.
2) Cleaning. The software removes the background (paper) and noise (speckling, grain, dirt) that would interfere with chromatic reading.
3) Pixel-by-pixel reading. For each stroke pixel, the RGB triplets are recorded.
4) Threshold detection. Identify the point at which a component (e.g., R) begins to appear consistently and where it reaches observable saturation.
5) Definition of the characteristic range. For example, R between 40 and 70 may describe a specific ink tint for that sample.
6) Highlighting. All pixels within the range are coloured with an arbitrary colour (e.g., pure red) to make the ink distribution immediately visible.
This highlighting is not retouching; it is an objective map.
Comparison, percentages, statistical stability
Once an RGB range is selected, CTA enables comparisons between different sections of the same page, between pages of the same corpus, and between different graphic elements.
The comparison can also be expressed in percentage terms: how many stroke pixels fall within the selected range and how this value changes across areas. If the difference is stable and the standard deviation is very low, the indication is statistically robust.
Typical applications in musical documents
In musical manuscripts, Chromatic Triplet Analysis is particularly useful for distinguishing later additions, overwritings, different working phases, and chromatic coherence anomalies within a corpus.
In many cases, minimal but systematic differences in RGB triplets reveal interventions invisible to the naked eye.
Why it is a forensic method: non-destructive and presentable
The main advantage of CTA is its non-destructive nature. No reagents or sampling from the document are required.
In return, it provides numerical results that are replicable and verifiable, and easily presentable in forensic or expert contexts.
What RGB does NOT promise
Chromatic Triplet Analysis is not an autonomous time machine. Once iron gall ink has fully matured, colour alone cannot provide reliable absolute dating.
The method is primarily comparative and works best when relating parts of the same document or homogeneous corpora.
Scientific reference (JFDE)
For the complete methodological description and peer-reviewed context, see the article published in the Journal of Forensic Document Examination (JFDE), the official publication of the Association of Forensic Document Examiners (AFDE).
Article link:
https://jfde.org/index.php/jfde/article/view/11417
JFDE publishes research papers, case studies, and technical articles in the field of document examination. Opinions expressed remain the responsibility of the authors and do not represent the AFDE or the editors.
Conclusion: from colour to evidence
Chromatic Triplet Analysis (CTA) transforms a visual impression into a measurable verification. In the study of historical musical documents, this reduces interpretative arbitrariness and strengthens the material basis of analysis.
When colour is properly measured, ink ceases to be a visual detail and becomes structural data.
Writing is a process, not an image.
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