How is a latent fingerprint revealed through powder dusting?

A latent print is made of a thin deposit of oily and sweat residue left by the papillary ridges. When a fine powder (graphite, aluminum, flour, cocoa) is applied to a smooth surface, the particles adhere preferentially to this oily deposit, revealing the pattern of the ridges. The contrast between the powder and the background then makes the print visible.

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Fingerprints: Revealed by Powder

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To identify the culprit behind a poisoning by suspicious soup, Maomao asks for cotton, flour, and a brush. She dusts the flour onto a cotton pad, taps it gently against the polished silver bowl, blows off the excess with the brush, and obtains four distinct sets of prints, distinguished by their size and position. She concludes without hesitation on the culprit’s identity. In a minute’s work, she independently reinvents forensic powder dusting, a technique that would not enter official judicial procedure in the West until the following century.

The subject in depth

The inner surface of the fingers and palms is covered with papillary ridges, microscopic skin features whose arrangement in arches, loops, and whorls is unique to each individual and stable from birth to death ^[Wikipedia]. These ridges are pierced with sweat pores: every contact with a smooth surface deposits an invisible film of sweat (water, chlorides, amino acids) and sebum (fatty acids, triglycerides). This deposit is what’s called a latent print, invisible to the naked eye on a clean surface, but physically present.

Papillary ridges of a finger and diagram of sebaceous residue revealed through powder dusting — Fine powder clings to the ridges’ oily deposit and reveals the pattern

The principle behind dusting is simple: a fine-grained powder, applied gently, adheres preferentially to the oily areas of the deposit rather than to the bare surface. The color contrast between the powder and the background then reveals the precise geography of the ridges. Modern forensic labs use specialized powders: graphite (dark gray on a light background), aluminum (silvery), lycopodium (yellow on a dark background), or even fluorescent powders observed under ultraviolet light. Wheat flour, rich in starch and protein, can play a similar role on very smooth, polished surfaces like silverware, adhering to the lipid residue through physico-chemical interactions close to those governing professional powders. The resulting contrast is markedly lower, but the principle still holds.

Modern methods go well beyond dusting. Ninhydrin reacts with the amino acids of the sweat deposit to form a purple compound (the Ruhemann reaction), which reveals prints on porous paper. Cyanoacrylate fuming (heated superglue) produces a white polymer that binds specifically to the amino acids and chlorides of a print and can treat complex surfaces. Each of these methods targets a different chemical fraction of the deposit: lipids for dusting, amino acids for ninhydrin and cyanoacrylate.

The uniqueness of fingerprints is a property established empirically over millions of comparisons: no two individuals, nor even two fingers of the same individual, have ever been found to share the same ridge pattern ^[Wikipedia]. This uniqueness, combined with the permanence of the pattern, underpins the entire forensic value of the discipline.

In real life

The history of dactyloscopy is far older than 19th-century Western criminalistics. In China, under the Tang dynasty (7th century), thumbprints on official contracts already served as signatures. The jurist Song Ci, author of the earliest known treatise on forensic medicine, the “Xi Yuan Ji Lu” (“Washing Away of Wrongs,” 1247), describes the use of finger marks in judicial investigations ^[Wikipedia]. In medieval India, palm marks appeared on official deeds. These practices point to an ancient intuition about the uniqueness of prints, without formalizing its scientific demonstration.

The scientific approach begins in 1823, when Czech botanist Jan Evangelista Purkinje describes and classifies nine types of papillary patterns in his thesis. In 1880, Scottish physician Henry Faulds publishes a letter in The Lancet proposing the use of fingerprints to identify criminals, and succeeds in clearing a suspect with them. Francis Galton establishes the first statistical basis for the uniqueness and permanence of fingerprints in 1892, and Inspector Edward Henry develops the classification system that bears his name, adopted by Scotland Yard in 1901 and then by most of the world’s police forces ^[Wikipedia]. The first criminal conviction based on fingerprints dates to 1902, in England.

Today, AFIS (Automated Fingerprint Identification System) databases hold hundreds of millions of digitized prints and allow automated comparisons within seconds. Law enforcement mobile terminals, biometric border control, and smartphone unlocking all rely on the same physico-chemical property discovered empirically more than a thousand years ago.

The fact-checker's eye

Maomao’s demonstration is physico-chemically sound. Flour can indeed adhere to the lipid deposits of latent prints on a surface as smooth and non-porous as polished silverware. The principle is identical to professional dusting: only the performance differs, since specialized powders give far better contrast than flour. Maomao’s technique, a cotton pad followed by dusting off with a brush, even resembles a brush-powder protocol, which reinforces the scene’s coherence.

The nuance lies in how clean the result is. Getting prints as legible as those described in the series with ordinary flour is probably idealized: in real conditions, the contrast would be weak, the ridges less crisp, and qualitative identification (size, relative position) difficult to carry out from simple visual observation. The scene illustrates the forensic principle perfectly, but amplifies its resolution for the sake of the story.

The series’ fictional setting, an imperial court inspired by the Song or Ming dynasties, is otherwise consistent with real Chinese practices of using fingerprints in contracts and investigations. Maomao doesn’t reinvent the use of the fingerprint as a signature, but rather powder-based revealing on a non-porous surface, which is not documented before the 19th century in the West.

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Published on May 30, 2026

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