Isolating fingerprints from clothing is now possible thanks to a method refined by forensic experts at Abertay Dundee University and the Scottish Police Services Authority (SPSA).
The new method will help to recover fingerprint ridge detail and impressions from fabrics — something that has so far proved difficult.
The team used a technique known as vacuum metal deposition (VMD), where small amounts of gold and zinc vapour attach to parts of a surface not covered by fingerprint residue — leaving something akin to photographic negative.
VMD has been around for some time and is already used to detect fingerprint marks on smooth surfaces such as carrier bags, plastics and glass.
Using a trial-and-error approach, the team investigated how different VMD conditions affected fingerprint retrieval in test swatches of fabric with varying composition and weave.
‘It’s kind of like a reverse pressure cooker. It’s under vacuum but there’s a glass window in it so you can look at the fingerprints developing as the process proceeds. If you go too far, it ends up completely covered and so, consequently, you can’t see the fingerprint — there’s a fine dividing line,’ said Abertay’s Prof David Bremner, who worked on the project.
Fabrics with a high thread count — especially silk, nylon and polyester — were best for revealing a print, with cotton and wool proving more of a challenge.
It also depends on individual variation — or, in plain terms, ‘how sweaty a person is’, according to Bremner. Only 20 per cent of the public are classed as ‘good donors’ for leaving fingerprints.
While the success rate is low for recovering a full fingerprint from items of clothing, the researchers have had great success in revealing contact points and hand prints from a number of fabric types.
This can still be enormously useful in criminal cases, such as an impression of a palm print on the back of someone’s shirt might indicate they were pushed off a balcony, rather than jumping.
According to Bremner, however, the real power of the latest advance will be in corroboration with other techniques, especially DNA typing. Obtaining DNA from a large garment with no bodily fluids to guide an investigator can be an exceptionally laborious process. However, with the outline of a fingerprint, even a rough and incomplete one, forensic investigators can hone in and dramatically increase their chances of getting a usable DNA sample.
Bremner added that they have done some preliminary tests of DNA purification after the VMD process and it seems the DNA was not overly damaged.
Director of SPSA Forensic Services Tom Nelson said: ‘Used on their own, fingerprints and impressions recovered on fabrics will not necessarily convict a criminal, but used alongside other evidence they will present a more robust case to the court.’