Question: last year I was invited to the Forensic science camp at armidale and we used iron filings and magnetic wands to reveal fingerprints. Is this the most up to date and effective way to collect evidence or is there a better way?

  1. I’m not too familiar with forensic procedures, but I would guess that the experiments you did were not quite the way forensic scientists do it these days. It was probably the best and easiest way to demonstrate the idea to you guys though. What you did might be an old but effective way of revealing fingerprints. The new way is likely to be more high tech but less fun and more automated. But I could be totally wrong!


  2. That sounds like the grease of the fingerprint slightly affecting the way the filings move in response to the magnet.

    Yes, there are much better ways now. Because we can make lots of different kinds of molecules, we can create molecules that will stick a little bit to the grease of the fingerprint, or react with the stuff that’s in a fingerprint, and then use special light to show where the fingerprint is. There are actually a lot of different ways to get a good picture of fingerprints, with new ways being invented all the time. The tough thing is getting a high resolution picture so that there’s no doubt about whose finger did the printing. And the surface the fingerprint is on can vary a lot too – the material can be glass or paper or wood. It’s important that whatever you spray onto the surface doesn’t do anything funny to the surface and only does stuff with the fingerprint.

    Of course nowadays there’s DNA fingerprinting. If you leave your DNA at a crime scene, that DNA can be duplicated and then examined to reveal you were there. The police would need to match it with some DNA taken from you, but if there’s a match then you’re in trouble!

    Interestingly identical twins, who have the same DNA, will have different fingerprints, though!


  3. Generally there are different methods used depending whether the fingerprint is on a solid (non-poreous) surface (eg glass or metal), or on a poreous surface (eg cloth or paper).

    The easiest and therefore most common methods is for coloured powders to stick to the fingerprints on solid (non-poreous) surfaces. Brushes are still commonly used, but that takes skill as too much pressure can literally scrub away the fingerprint. The magnetic powder techniques are currently used by police forces. The advantage is that the magnetic applicator does not scrub away the print. The disadvantage is that it is hard to use on non-horizontal surfaces. These exact choice of powder depends on the color of the background — light-coloured powders on dark surfaces, etc.

    The fancy techniques you see on TV — superglue fuming, special lasers, etc — are sometimes used, but in real police forces, they are used, but rarely used, because they are much more difficult.

    The powders do not work well on poreous surfaces, so usually a liquid is sprayed onto the paper or cloth, or in some circumstances the paper or cloth is dipped in he liquid. The liquid reacts with the fingerprint to form a coloured dye exactly in the same place and detailed pattern of the fingerprint. My collaborators and I have been working on new fingerprint reagents for use on paper or cloth. There are only about 3 reagents in common use, with a very limited selection (3) of colours. Our reagents have the potential of being less hazardeous and having a great choice of colours for use on different-coloured papers.