Promising early results from research undertaken by the University of Abertay Dundee and the Scottish Police Services Authority could lead to fingerprint evidence being obtained from clothing, for use in criminal prosecution. Refining an existing technique that's been used to successfully recover print detail from smooth objects such as glass and plastic, forensic scientists have managed to create a kind of photo negative of fingerprint impressions on fabric. It's a bit hit and miss at the moment, but even when clear ridge detail isn't retrieved, the technique could still prove useful to investigators looking for other evidence.
The researchers used a method known as vacuum metal deposition that's already been used to recover print detail on smooth surfaces like carrier bags, plastics and glass since the 1970s, but has not previously been applied to fingerprint detection on fabrics.
The fabric is placed in a vacuum chamber. Gold is heated and evaporated and spread in a fine layer over the fabric. Heated zinc is then applied, which attaches to the gold layer where the fabric has no fingerprints, leaving the original fabric to show through where contact has been made.
"One way of explaining it is like a photographic negative, where colors show up as their opposites," said Abertay University's forensic sciences teacher and researcher Joanna Fraser. "Here the fingerprint ridges show through as clear fabric, but where there are no ridges we see the distinctive gray color of the metal. Previously it had proved difficult to reveal a clear fingerprint on fabric, but we've shown that this is now possible. This is great, but the challenge is to develop this further and confirm its effectiveness."
The success rate for recovery is still quite low, with only around 20 percent of the public said to consistently leave good ridge detail or indicate target areas for DNA collection due to the presence of sweat. Folks who have drier skin prove to be poor donors, but the technique could still lead investigators to target areas of clothing for DNA procurement and may reveal other useful facts, such as the shape of a hand or an indication of whether a victim was pushed or grabbed.
Paul Deacon, fingerprint unit manager at the Scottish Police Services Authority, said that "an impression of a palm print on the back of someone's shirt might indicate they were pushed off a balcony, rather than jumping."
"The research is still in its early stages but we are starting to see results," he continued. "We have shown that fabrics with a high thread count are best for revealing a print and have recovered identifiable fingerprints on a number of fabrics including silk, nylon and polyester."
The research paper entitled Visualisation of fingermarks and grab impressions on fabrics. Part 1: Gold/zinc vacuum metal deposition has now been published in Forensic Science International.