The influence of materials on fingermark enhancement
Even for established forensic fingerprinting techniques, advances in materials and their applications continue to present new challenges, as Stephen Bleay and
Rory Downham explain.
The use of fingerprints for forensic identification purposes has been established for more than 100 years. The ability to pinpoint criminal activity and identity using validated analytical approaches is of great value to forensic scientists. An essential element of fingerprint identification is to have a crime scene mark to compare with a reference set of prints, and a wide range of chemical and physical processes have been proposed both to enhance fingermarks and to enable an image to be captured for comparison.
Early cases relying on fingerprint identification typically involved crime scene marks that were already visible because they were deposited in contaminants such as blood or grease, or left as impressions in substrates, such as wet paint. However, it was recognised that many unseen, or ‘latent’, marks were also present at crime scenes, and methods to enhance these sufficiently to make them visible to the eye were sought. One of the earliest methods identified was powdering, using soft brushes to apply fine powders to the substrate. Fine particulates in the powder selectively adhere to fingerprint residues, and by appropriate powder selection a mark of contrasting colour to the substrate can be developed.
In the early 1900s, more emphasis was placed on identifying development techniques that selectively reacted with some of the chemical constituents known to be present in the fingermark. A range of reagents were used around this time, such as iodine (absorbed by water/squalene to give a brown mark), silver nitrate (reacts with chlorides to give a brown/grey deposit) and osmium tetroxide (reacts with fats to give a dark grey product). Subsequent chemical reagents included ninhydrin, which gives a purple product on reaction with amino acids and has become the most widely used process for developing fingermarks on paper and other porous materials. Stains developed for biological sections including gentian violet and Sudan Black have also been adapted for enhancing fingermarks rich in fatty contaminants.
The development of fingerprint techniques
In the late 1960s and 1970s, interest increased in the physical properties of fingermarks, and in new methods for developing them. Processes originally implemented for other purposes were adapted for the development of fingermarks, including vacuum metal deposition (VMD), originally intended for producing metal coatings on optical components, such as mirrors, and physical developer (PD), a process from the photographic industry. VMD was found to be highly effective in developing marks on the polymer carrier bags, and PD became invaluable for marks on wetted paper items, something not previously possible. Another noteworthy fingermark enhancement process is cyanoacrylate fuming, discovered accidentally and separately in Japan, America and the UK, where fumes of ‘superglue’ were observed to reveal fingermarks by selective polymerisation of ethyl cyanoacrylate on the fingermark ridges. This process has since been understood, controlled and commercialised.
The selection of processes used for fingermark enhancement is strongly influenced by the nature of the substrate the fingermark has been deposited on. For example, some solvent-based processes appropriate for penetrating porous substrates such as paper and reacting with fingermark deposits may wash fingermarks from non-porous surfaces, such as glass. Powders are routinely used on non-porous surfaces but are unable to discriminate between the fingermark and the slightly textured surface of porous materials, such as cardboard. The texture, colour, surface chemistry and surface physics of the substrate all impact on the selection of the processes used in the fingerprint enhancement laboratory.
In 2014, the Centre for Applied Science and Technology released The Home Office Fingermark Visualisation Manual, which placed greater emphasis on identifying the nature of the substrate being treated than previous editions. It provides more information on the types of materials that may be encountered and ways of discriminating between them, and gives comprehensive guidance on the most effective processes and processing sequences to be used for each substrate. However, the continual drive to change the materials used in everyday objects present new, and sometimes-unforeseen challenges to those involved in fingermark enhancement. The progressive change to polyethylene carrier bags with higher contents of recycled material and additives resulted in the vacuum metal deposition process identified as being most effective for this substrate in the 1980s, recently becoming less effective than other processes, such as cyanoacrylate fuming, and guidance was revised as a consequence.
The new note
The introduction of the new polymer £5 banknote from the Bank of England on 13 September 2016 presents another challenge. These notes are made of biaxially oriented polypropylene, with varied surface treatments applied to different regions of the note. They feature transparent windows, printed areas of varying detail (some of which are raised) and foil patches – non-uniform surfaces for fingermark visualisation.
The currently recommended fingermark visualisation processes for paper-based banknotes, which include the solvent-based 1.8–diazafluoren-9-one (DFO) and ninhydrin, both of which react with amino acids, are unsuitable for the new non-porous substrate. Therefore alternative methods and approaches were investigated in advance of the introduction of the new banknotes to establish processes that could be used to recover latent fingermarks.
The work clearly demonstrated the strong influence of the substrate and surface characteristics on the way in which fingermarks were enhanced, with some processes being very effective in developing marks on certain regions of the note, but being ineffective on adjacent regions.
Experimental work on the banknotes before the introduction has enabled guidance to be given to police laboratories on which processes are likely to be effective on the new notes, and this will be repeated on the £10 note prior to its introduction in 2017. The surface characteristics of the notes will change as they become worn, and further work will be required to establish which processes remain effective.
Several areas of fingerprint research are ongoing. Advanced analytical techniques, such as secondary ion mass spectrometry and matrix assisted laser desorption/ionisation are being explored to provide additional contextual information about the fingermark such as what substances the donor has been handling, and whether the mark placed before or after the printed words – these are all yet to be fully answered.
Advances in sensor materials and technology have made it possible to obtain high-resolution images of fingermarks in the infrared and ultraviolet regions of the electromagnetic spectrum, using advanced imaging methods such as multispectral imaging.
Such methods enable ridge detail to be revealed against highly coloured and/or patterned backgrounds, where marks can be difficult to distinguish under conventional white light.
There are still materials that present challenges for fingermark enhancement, leather being a substrate where the combination of a ‘semi-porous’ nature, surface texture and the ability to deform during contact make identifiable marks less likely to survive. It is also recognised that metallic substrates vary significantly in their properties and the interactions that may occur with fingermarks, and processes for enhancing marks on metals increasingly rely on electrochemical approaches. It is likely that in future a suite of enhancement processes will be available that can be tailored for the different types of alloy encountered in casework. Even for well-established forensic techniques, materials and their applications present new challenges and opportunities.
The authors wish to acknowledge the support of the Bank of England in providing banknotes, and Foster and Freeman, West Technology Ltd and Loughborough University, UK, in collaborating in the work.
Rory Downham and Stephen Bleay are Government Scientists at the Home Office Centre for Applied Science and Technology, UK, conducting research into fingermark visualisation and imaging.