Revisiting the Thermal Development of Latent Fingerprints on Porous Surfaces: New Aspects and Refinements*

Abstract

Although the ability to develop latent fingerprints on paper using heat alone has been noted previously, it has been considered impractical for casework and inferior to other techniques. Here a new refinement of the technique is demonstrated for the high quality development of latent fingerprints on porous surfaces such as paper. Fingerprints deposited on various papers were developed by exposing them to hot air with a temperature in the vicinity of 300 degrees C, for periods of c. 10-20 sec. Several different heating methods were tested. The novel observation was made that after shorter heating times, fluorescent prints could be observed. These became visible after longer heating times, as noted by earlier workers, but with greatly improved contrast compared with their results. Prints from various donors (and aged prints) were developed with excellent ridge contrast. Direct heating methods (such as with a hot plate or press) produced inferior results. The refined technique, which is simple, safe and inexpensive compared with conventional methods, has great potential for use in forensic laboratories.

Thermal development of latent fingermarks on porous surfaces—Further observations and refinements

“…[9] This technique was reproduced in the current work with similar results. Fingermarks were developed over a temperature range of 240 to 315 °C.…”

“…[9] We also observed that the latter (visible) stage could yield improved contrast when observed under ultraviolet rather than visible illumination. At first, it seemed remarkable that stage (i) had not previously been reported, and that stage (ii) had not previously been optimized to give results that could compete with those yielded by chemical reagents.…”

“…[9] Fingermarks developed to fluorescent (i) and visible (ii) stages were achieved with both hot air and the direct contact heating methods described above. As noted in previous work, [9] the use of UV illumination on thermally developed (visible stage) fingermarks greatly improves the contrast between the fingermark ridges and the paper background.…”

“…[9] Fingermarks developed to fluorescent (i) and visible (ii) stages were achieved with both hot air and the direct contact heating methods described above. As noted in previous work, [9] the use of UV illumination on thermally developed (visible stage) fingermarks greatly improves the contrast between the fingermark ridges and the paper background. This is because the charring of the paper under the fingermark ridges removes or blocks the natural UV-stimulated fluorescence of the paper, which persists in the background, provided the heating time has not been execessive.…”

“…This is borne out by the observation that clean paper, when heated, goes through the same stages of fluorescence followed by visible browning, but more slowly than in the presence of fingermark residues. [9] However, to understand and perhaps better control the thermal development of fingermarks on paper, it may be instructive to consider the thermal degradation of cellulose (a major component of paper), which is known to result in chemiluminescence, [1][2][3][4][5] as well as whether similar thermal development phenomena can be observed with other cellulose-based materials.…”