Ion Chromatography Links Chloride from Sweat to Fingerprints

To better understand why heating seems to make metal-based fingerprint marks show up more clearly, especially when there's more sweat involved, researchers used a special microscope with a heating stage to directly watch how fingerprint ridges (from 15 volunteers) changed as they were heated. This let them confirm that the marks really do become more visible on their own during heating. Building on these observations, they then compared the amount of chloride, a component found in sweat, between fingerprints with more sweat and those with less using ion chromatography, testing two representative volunteers. A paper based on this work was published in the journal Science & Justice.¹

Why Should Fingerprint Investigators Focus on Non-Combustible Surfaces at Fire Scenes?

Fingerprints are one of the most common types of evidence found at crime scenes, and the usual methods for making them visible rely on physical or chemical techniques that depend on natural secretions left behind by the finger, like sweat and oils. But in fire scenes, where temperatures are extremely high, most of these secretions break down and burn off, making it much harder to find and identify fingerprints. Depending on the material they are on, fingerprint surfaces at fire scenes can generally be split into two types: those that can burn (combustible) and those that cannot (non-combustible). Surfaces that can burn tend to be destroyed in a fire, taking any fingerprints with them, while surfaces that resist burning are much more likely to preserve fingerprints, since they can withstand the heat. Because of this, when investigators are searching for fingerprints at fire scenes, they should focus their efforts on non-combustible surfaces, since these give the best chance of recovering usable evidence.^1,2

What Has Prior Research Found About Visualizing Fingerprints on Heated Metal Surfaces?

Metal surfaces are a good example of non-combustible material, since metals typically have high melting points and can hold their shape even under intense heat, which helps keep fingerprint ridge patterns from getting distorted or destroyed. Metal is also everywhere at crime scenes, showing up in things like weapons, appliances, and door or window handles. Because of this, many forensic researchers have specifically studied how to make fingerprints visible on metal surfaces after they've been exposed to fire.¹

In one earlier study,³ researchers heated four different metals (brass, copper, aluminum, and tin) at increasing temperatures, in steps of 50 °C, ranging from 50 °C up to 500 °C, to figure out the best temperature for making fingerprints visible on each metal. They found the ideal temperatures were 200 °C for brass, 350 °C for copper, just 50 °C for aluminum, and 300 °C for tin. Another study⁴ looked at brass, stainless steel, mild steel, and copper, testing each at three different temperatures. The researchers in this case thought that keeping metal at a high temperature for longer might increase oxidation and make fingerprints show up better. However, their results showed that heating the metal longer did not improve fingerprint visibility. A third study⁵ successfully revealed fingerprints on brass and iron surfaces heated up to 600 °C using a specialized detection tool called a scanning Kelvin probe. These researchers then compared how well fingerprints showed up on brass, aluminum, and stainless steel at different temperatures, and found that results varied a lot depending on the type of metal. Brass still showed usable fingerprints at 600 °C, aluminum performed poorly once temperatures went above 280 °C, and stainless steel only produced usable fingerprints when heated to 600 °C or higher. That said, some fine fingerprint details were lost when temperatures reached as high as 900 °C.⁶

Since bullets are briefly exposed to very high temperatures when fired, several studies have examined whether fingerprints left on bullet casings can still be identified afterward. In one such study, researchers placed 1,540 fingerprints onto 223 brass bullet casings, most of which were then fired after varying amounts of time had passed. After processing the casings using a specific method,⁷ 121 of the fingerprints were still clear enough to be identified.

In a separate real-world case, Brazilian police used a technique involving cyanoacrylate smoke (sometimes known as "superglue fuming") to reveal fingerprints on five cartridge casings collected from a crime scene. Using an automated fingerprint identification system, they were then able to identify the suspect.⁸

How Does Temperature Affect Fingerprint Visibility on Metal Surfaces?

To get a closer look at what was happening on a microscopic level, the researchers examined the fingerprint marks after heating using energy dispersive X-ray analysis (SEM/EDAX) that let them see fine surface details and identify chemical elements present. Based on this, they found that how well fingerprints show up after heating follows a pattern with three distinct stages depending on the temperature. Below 500 °C, higher temperatures made the fingerprints harder to see, because the organic material from sweat and skin oils started breaking down, and chloride (a sweat component) wasn't yet accelerating the corrosion process much. Between 500 °C and 800 °C, things changed: chloride began significantly speeding up corrosion on the metal surface, which made the fingerprints show up more clearly. Above 900 °C, chloride was still speeding up corrosion, but a different effect took over: protective layers of certain minerals formed on the surface and blocked oxygen from reaching the metal underneath. This covered up the corrosion patterns that had been created by the chloride, making the fingerprints harder to see again. On top of this, fingerprints with more sweat (and therefore more chloride) consistently showed up more clearly at every temperature tested, since more chloride meant more of this corrosion-accelerating effect.¹

“The results,” write the authors of the paper,¹ “enhance understanding of high-temperature fingermark behavior and provide both empirical data and theoretical insights relevant to forensic investigations in fire scenes.”

References

1. Wang, Y.; Zang, Z.; Liu, Y. et al. Impact of Sweat on Visualization of Latent Fingermarks Deposited on 304 Stainless Steel During Heating Based on in-situ Observation. Sci Justice 2026, 66 (3), 101426. DOI: 10.1016/j.scijus.2026.101426
2. O'Hagan, A.; Banham, R. B. A Review of Fingerprint Recovery Within an Arson Crime Scene. Forensic Res. Criminol. Int. J. 2018, 6 (5), 315-325. 10.15406/frcij.2018.06.00223
3. Peel, A.; Bond, J. W. Effect of Temperature on the Visualization by Digital Color Mapping of Latent Fingerprint Deposits on Metal. J. Forensic Sci.2014, 59 (2), 490-493. DOI: 10.1111/1556-4029.12344
4. Wightman, G.; Emery, F.; Austin, C. et al. The Interaction of Fingermark Deposits on Metal Surfaces and Potential Ways for Visualization. Forensic Sci. Int. 2015, 249, 241-254. DOI: 10.1016/j.forsciint.2015.01.035
5. Williams, G.; McMurray, H. N.;  Worsley, D. A. Latent Fingerprint Detection Using a Scanning Kelvin Mmicroprobe. J. Forensic Sci. 2001, 46 (5), 1085-1092.
6. Lam, R.; Hockey, D.; Williamson, J. et al. Latent Fingermark Development on Fired and Unfired Brass Ammunition Under Controlled and Blind Conditions. Forensic Sci. Int.2022, 337, 111369. DOI: 10.1016/j.forsciint.2022.111369
7. Girelli, C. M. A.; Segatto, B. R. Identification of a Suspect in a Murder Case Through Recovery of Fingermarks from a Fired Cartridge Case. J. Forensic Sci. 2019, 64 (5), 1520-1522. DOI: 10.1111/1556-4029.14045