Fingerprints and footwear marks
Fingerprinting methods
Over the years we have published information on a range of techniques to develop fingerprints on exhibits retrieved from scenes of crime. This information helps police detect more fingerprints more successfully.
Fingerprints and DNA evidence
HOSDB and the Forensic Science Service, working together, have shown that DNA can be extracted from exhibits previously treated with fingerprint development techniques. This is a vital step forward, enabling more evidence to be gathered from the same article. By first indicating where finger contact had been made, police forces can target their DNA swabbing to greater effect.
Superglue
The use of superglue - the same glue in everyday use - was pioneered over 20 years ago when we discovered that relative humidity was the critical factor for fingerprint development on certain surfaces. This resulted in the development of the ‘Sandridge superglue cabinet’. This provided a sealed, humidity-controlled environment enabling the superglue to be heated and react with the water and other components in the fingerprints to form a visible, white polymer layer.
Since then, other self-scrubbing superglue cabinets and a system for use at crime scenes have been developed and the process is used successfully all over the world.
Powders
A variety of fingerprint powders have been used successfully for over 100 years and fingerprints powdered at crime scenes still account for over 50% of fingerprint identifications in the UK. This demonstrates the continued importance of this cheap and easy technique. Recently HOSDB has published guidance on the most effective powdering techniques for a variety of surfaces.
Fluorescence examination
Another key method for making fingerprints visible is by using fluorescence. Fluorescence is the absorption of light at one wavelength (colour) and its re-emission at a longer wavelength (colour). Fluorescence plays an important role in the perceived colour of many objects: the unnatural brightness of "day-glo" paints is due to fluorescence. Phosphorescence is similar but may persist long after the stimulating radiation is extinguished. This is used on many watches and clocks.
A high-intensity light source is required to induce fluorescence from fingerprints. There are many types; lasers, filtered arc-lamps and LEDs are among the most commonly used. We intend to produce a standardised way to compare how effective the different light sources are for the detection of chemically treated fingerprints. This effectiveness measurement will help police forces in their purchasing decisions.
Most recently, a green laser, often referred to as a 'YAG Laser', also used in the repair of retinal damage in the eye, was found to be more effective at visualising fingerprints not previously doped with a fluorescent substance. Police forces throughout the UK are now using this equipment to great effect.
Alternative Vacuum Metal Deposition (VMD) techniques
HOSDB optimised the VMD technique in the mid-1970s and the process has long been recognised as the most sensitive technique for developing fingerprints on non-porous articles such as plastic bags. One major advantage of the technique is that it can detect fingerprints on articles that have been previously immersed in water.
The recent increase in the use of recycled material in plastic bags has meant that the original process, deposition of a thin layer of gold followed by zinc, has not been as effective as expected. HOSDB has carried out research into alternative VMD processes that are can develop fingerprints on a variety of problematic surfaces. Deposition of silver as a single metal has been found to be effective in these situations and the technique is now in operational use, with many additional fingerprints detected.
Fire scenes
Traditionally, fire scenes have presented a challenge to scene examiners trying to recover fingerprint evidence. Our recent work with the fire investigation training organisation Gardiner Associates has demonstrated that fingerprints are capable of surviving temperatures up to 600°C and has identified the best processes for soot removal and subsequent chemical treatment. Since we published this in a recent HOSDB Newsletter, police laboratories have reported successful recovery of fingerprints from articles previously thought too badly affected by fire to be treated.
Blood reagents
HOSDB has recently introduced two new reagents and modified another for enhancing fingerprints in blood. Police forces are now able to enhance fingerprints in blood on the wide range of surfaces they may encounter. Two, which are different colours, are used for porous surfaces like paper and wood and the third is a fluorescent reagent for non-porous surfaces such as metal and plastic.
