Heat and let dye: Fixing blood-contaminated fingermarks using heat

Blood-contaminated fingermarks are significant evidence for forensic investigators in high-profile cases providing a direct link between the suspect and the crime. Although these marks are often visible, blood enhancement techniques are operationally used to recover maximal ridge detail. The standard protein dye-staining procedure includes a chemical blood-fixing step, which requires an initial, prolonged drying period, for natural coagulation to occur. However, in special cases, when it is crucial to detect forensic traces quickly, there is a need to speed up the enhancement process. This study explored, both theoretically and empirically, the use of heat as an alternative method to the standard chemical fixing. Three consecutive experiments were conducted in which blood-contaminated fingerprints were deposited on different types of surfaces (car parts, glass, and flooring tiles), and heated for different periods, prior to development by amido black solution. The results showed that heat was successful in fixing blood, while the required temperature and heating durations, were inversely proportional. This observation was in correlation with theoretical heat-transfer data, calculated by the Lumped Heat Capacity model, also demonstrating the impact of the thermal time constant of each surface, on the conditions required for the full fixing of blood. The experimental findings led to a design of a portable, and tailor-made heating device, examined for the use in crime scenes, allowing to shorten the necessary fixing process from hours to minutes. For future crime-scene work, this novel approach may be utilized for a rapid blood-fixing, especially in cases when the scene cannot be preserved.