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A technique for determining whether a fingerprint appeared on a piece of paper before or after text was printed on it could help resolve fraud cases. The approach uses standard forensic tools and so would be simple to put to use.

In forensics, the 鈥渨hen鈥� is often as important as the 鈥渨ho鈥�. In a case, for example, there could be a situation where someone is suspected of tampering with a will or contract by printing on top of it. Determining when someone鈥檚 fingerprint was made on the document could bolster any other evidence of tampering, says at Foster + Freeman, a forensic instrument company based in the UK.

King and his colleagues have now created a technique that goes some way to achieving this. It uses a thin layer of gelatin to lift the print, which reacts differently if the fingerprint was left on the paper before or after printing.

To use the technique, a technician places the gelatin over a fingerprint that overlaps with some printed text. They then peel off the gelatin and place it inside a vacuum-sealed glass box filled with a vapour of a chemical called disulphur dinitride. This vapour binds to the microscopic fingerprint ridges imprinted on the gelatin鈥檚 surface so that after a few minutes a blue-coloured fingerprint is revealed.

The team tested this process for a fingerprint that had text printed over it and another that was made on top of printed text. In the former case, the gelatin touches the text instead of the fingerprint first so the pattern developed at the end of processing was noticeably different 鈥� the team could tell which came first, the fingerprint or the text.

鈥淭his approach could be a much simpler and cheaper alternative to what has been proposed previously,鈥� says at the University of Surrey in the UK. Past experiments involved specialised instruments that may not be common in forensics labs, she says.

King says that crime scene examiners in the UK routinely carry gelatin lifters and that the rest of his team鈥檚 process would not require extensive further training. Additionally, the new technique can be combined with other forensic techniques as gelatin lifting does not destroy the fingerprint, he says.

So far, the researchers have only tested combinations of fingerprints and laser jet inks, but they want to expand the method to other types of printing. They also want to extend it writing with ballpoint pens, relevant for signature fraud, and stamps for possibly forged passports.