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Visualization of Latent Fingerprint Corrosion of Brass

NCJ Number
Journal of Forensic Sciences Volume: 54 Issue: 5 Dated: September 2009 Pages: 1034-1041
John W. Bond, D.Phil.
Date Published
September 2009
This paper reports on how a measurement of the spatial distribution of the variation (deltaV) in applied potential (V) over the surface of planar brass disks, which are subject to corrosion by latent fingerprint deposits, can be used to visualize fingerprint ridge detail.
This is an extension of the research team's finding that leaving fingerprint deposits on brass in air at room temperature for several days caused sufficient corrosion of the metal to enable the fingerprint to be visualized even after the residue of the fingerprint deposit had been removed by cleaning the metal in warm water, to which a few drops of commercial detergent had been added. Subsequent research showed corroded parts of the brass to have a potential lower than that applied to the bulk, typically up to 12 V for an applied potential of 1400 V. Further research discovered that the junction between the bulk and corroded brass could exhibit the characteristics of a rectifying metal semiconductor contact with the corroded brass exhibiting the properties of a p-type semiconductor, with this semiconductor most likely being copper or copper oxide. The current research measured the spatial distribution of deltaV for brass disks that exhibited a range of visible corrosion and related this to the measured work functions for both the bulk and corroded brass. A model is proposed for the corrosion of brass by fingerprint deposits that can account for the formation of a Schottky barrier and the experimental results obtained. In this model, both electrodes (anode and cathode) are formed on the metal surface beneath the fingerprint deposit, and the electrical circuit is completed by electron flow through the metal. Such an electrochemical cell obeys mixed-potential theory and consists of both oxidation and reduction half-cell reactions with no net accumulation of charge, i.e., the rate of oxidation equals the rate of reduction. 10 figures and 31 references