Research on temperature‐dependent visualization and enhancement mechanism of latent fingermarks deposited on steel container

In arson cases, the accelerants are typically stored in steel containers where fingermarks may be preserved. To explore the effects of heat on latent fingermark visualization deposited on steel surfaces and the corresponding mechanisms, 304 stainless steel and tinplate were selected. Coaxial illumination and cyanoacrylate fuming were employed to visualize fingermarks. An optical microscope (OM) equipped with a hot stage was utilized to characterize the fingermark ridges evolution in situ. Additionally, the scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to analyze fingermark micromorphology and variations in fingermark composition. The results showed that below 500°C, the visibility of fingermarks on both steel surfaces decreased as the temperature increased, and no ridges were visible at 500°C. This was due to the decomposition/evaporation of organic compounds in the fingermarks caused by high temperatures. However, their outlines began to reappear at 600°C, and ridges became visible to the naked eye at 800°C. This was due to the fact that at temperatures exceeding 600°C, oxidative, electrochemical, and salt corrosion between fingermarks and substrates were accelerated, achieving fingermark enhancement. For tinplate, due to its antioxidant properties and the corrosion resistance provided by FeS₂, it exhibited less apparent oxidation and salt corrosion. Comparatively, fingermark visualization on 304 stainless steel was superior to that on tinplate. This study demonstrated that fingermarks could be retained on steel surfaces even after exposure to high temperatures with different mechanisms related to the chemical composition of steel, which provided crucial data for fire investigations.