Friday, February 22, 2008

Prints on Metal

I received an e-mail from a person who asked for some possible solutions to a recurring problem. When examining metal objects like a pistol magazine for latent fingerprints, they could see a faint fingerprint on the surface that did not have enough contrast to photograph without additional processing. The problem is that none of the standard processing techniques improved the visibility of the fingerprint. Powder doesn’t stick, Cyanoacrylate doesn’t adhere, and other chemical techniques discolor the entire surface, obliterating the fingerprint.

Before presenting a solution, let’s discuss the problem. Powders and Cyanoacrylate require some form of moisture to produce results. Since powder and Cyanoacrylate are not sticking, then there must not be enough moisture left in the print. So why is the print visible? The answer to that lies in the type of surface. This type of problem rarely occurs on surfaces other than metal. The reason is that the faint fingerprint you can see on the metal is actually oxidation. Human oil and fingerprint residue contains salts and acids that produce a chemical reaction on the surface of the metal. This reaction consumes the natural moisture that powders and other techniques require to be successful but produces a slight discoloration that is barely visible.

Now that we understand the problem, we are better able to define a solution. In my experience, the method that has produced the best results has been side lighting. By holding a light at an oblique angle to the item, you can improve the contrast of the fingerprint. This can sometimes be done with a flashlight but an alternate light source usually produces better results. Since the wavelength can be controlled within a few nanometers, you can experiment with different wavelengths until you find the one that produces the most contrast. Add to that the addition of a filter through the use of goggles or a lens attachment and you can improve the contrast even more.

Sometimes, the glare from the background can produce undesirable effects. To reduce this problem, use a Styrofoam coffee cup with the end cut off to diffuse the light. This technique is covered in a previous post about the use of Amido Black.

Once you find the best angle and combination of light and filters, you will need to photograph the fingerprint. Place a scale on the same plane as the fingerprint and set up a camera on a tripod perpendicular to the surface of the print. Turning off any ambient light in the room will help tremendously.

After a while, you will be able to recognize this type of print before you ever try to apply powder or Cyanoacrylate. In any case, all evidence should be examined visually and with an alternate light source (when available) prior to the application of anything.