Fingerprinting

Recently a question came up in one of my Facebook groups.

Do forensic techs still use black fingerprint powder? How do they life the prints? Do they cleanup the scene afterwards?

The answer wasn’t as simple as it sounds. But let’s start with how fingerprints are left. When you touch an object, the oils from your fingers are left on the object. This oil residue remains in the shape of your fingerprint. Fingerprints are different on everyone (almost, more on that in a moment). Even identical twins have different fingerprints. And, fingerprints start developing in the womb. Some criminals have tried to burn off their prints, but modern techniques still allow forensics to lift the print. Now, about that almost. There is a very rare genetic condition called Adermatoglyphia that causes the person to not develop fingerprints. It is so rare that only four families worldwide are known to have it.

Next we have to understand different types of prints. There are latent, which are invisible to the eye. This is what you see on TV and what I’m discussing here. Patent prints are in a substance like blood. And plastic prints are in a three-dimensional object such as putty. Law enforcement has even lifted plastic prints from a block of cheese.

The last thing to consider is the surface where the print has been left. For latent prints, the surface needs to be non-porous. That means it has to be hard like metal, wood, glass, or Formica.

But what about the powders that the poster asked about? Powders are used to find latent prints. In general, black fingerprint powder is used about 99% of the time, but they’ll use the powder color that gives the greatest contrast. So, if trying to get prints off of a light-colored object, a dark colored powder is used. If the object is dark-colored, a light colored powder is used. The doesn’t necessarily mean black and white. Powders come in several different colors including grey, pink, blue, green, silver-black, and others. Also, powders are made with different materials, some are magnetic, which are used in dry, desert climates or when the possible print has dried out. Powder can also be fluorescent, making it show better in the dark.

The tech will apply the powder to the area that is being fingerprinted. You don’t want to use too much as powder is expensive. The 2 oz. jar above costs $11.35. Also, if too much powder is used, then more has to be removed. The brush is used to help distribute and then remove excess powder. Ideally, only the fingerprint and a bit of residue remains. Then you use lifting tape. This is clear tape made especially for this. There are different types of tape for different surfaces. The tape is more like clear package sealing tape than cellophane tape (Scotch tape.) The tape is placed directly over the fingerprint. Technically, you are lifting the powder, which adheres to the oils left by the print.

The tape is then pressed onto a Lift Card. This is a 3″x5″ card. The back is blank and shiny. This is where the print is placed. The front has spaces for information such as case number, where the print was lifted from, date and time, officer or technician information, etc. This video shows the process.

The print can then be run through databases for a match. If you have been arrested, your prints will be on file. Computers are used to identify possible matches. A human must determine if the prints taken from the scene match any of the possible matches returned by the computer.

In older shows, you see the suspect has her fingers rolled onto an ink pad then onto a card. She then wipes the ink off her fingers with a paper towel. This card is not the same one for the print taken at the crime scene. Today, you mostly see the suspect fingerprinted digitally, so no more messy ink pads.

One important note for you authors. When a fingerprint has a circular shape, the correct term is whorl, not swirl.

So, who cleans up? It isn’t forensics. It isn’t the police. If your house or office is a crime scene, you clean up. Or you hire someone to do it.

And there you have it. If you want hands-on training for lifting fingerprints, I highly recommend The Writer’s Police Academy. I’ve taken fingerprint classes there taught by forensics experts, many of whom teach law enforcement the same techniques.

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