Biology of Fingerprints
Fingerprints are made of 99% water; the rest is a mixture of acids, lipids, and salts. Fingerprints form during the second trimester of pregnancy and are the same throughout the rest of a person’s life. Fingerprints are different for everyone--including cloned primates and identical twins.
Due to the different body chemicals in prepubescent kids, their fingerprints degrade very quickly: in six hours, the fingerprints of a kid can disappear. (However, capturing the fingerprints using ordinary processing methods can preserve kids’ prints as well as adult ones--a fingerprint captured on a card will be preserved indefinitely.) Kids’ prints get priority processing throughout the system.
Some people leave better fingerprints than others: people who sweat more leave better ones than people with dry skin. In fact, dryness affects everything about fingerprints: fingerprints tend to degrade faster in dry climates than in humid ones.
Processing Fingerprints in the Field
During the class, we were able to try out several methods of fingerprinting:
- Black powder
- Magnetic powder
- Super Glue fuming (the items had already been fumed due to potential health hazards)
- Fluorescent powder
Black powder. Processing fingerprints in the field can be as simple as brushing them with black powder and lifting the prints off the surface with a piece of tape, then attaching the tape to a card (the tape leaves behind tape marks when you pull it off, by the way). Details about the location and orientation of the print including rough sketches of where the print was found, the date and time the print was lifted, and the name of the person lifting the print are all recorded on the back of the card. The black powder method is usually not destructive and doesn’t cause a permanent stain. (Agencies aren’t required to clean up after an investigation, so this can be a concern--people have complained to departments and even sued them for excessive damage.)
Magnetic powder. The powder used can also be magnetic--which means that it can be picked up again with magnets after use.
Super Glue fuming. In Super Glue (cyanoacrylate) fuming, a sealed container (or room) is filled with Super Glue fumes. The Super Glue collects on the oily residues of a fingerprint and forms a polymer. Prints processed with Super Glue are more resistant to smearing. Once fumed, prints can be processed with black powder and lifted that way.
Fluorescent powder. An alternative to black powder is a fluorescent powder that glows under ultraviolet lights. This makes it easier to pick out prints on multicolored surfaces. Other than lifting prints to black cards rather than white ones, the powder is handled just like regular black powder.
Ninhydrin. On porous surfaces like paper, ninhydrin, a chemical that reacts with the amines in skin, can be used. When exposed to the right amines, ninhydrin turns a pinkish-purple color.
Processing Fingerprints in the Lab
Anyone with training can process fingerprints in the field, but someone needs to be certified in order to testify in court about the identity of the person with those fingerprints.
In the lab, fingerprints will be submitted to the Integrated Automated Fingerprint Identification System (IAFIS). Usually about 15-20 candidates are returned. Fingerprints are listed by State ID (SID) number. The computer will not match fingerprints to specific people; only a human operator can make that call. The system often comes back with “matches” that have completely wrong fingerprint-pattern types.
Women are becoming more prominent in forensics, at about a rate of 5:1 in current university programs. Possibly as a result, there is a lot more emphasis on personal safety, but people who are in the forensics field can’t be germaphobes. Male or female, the squeamish tend to leave the field quickly.
Forging a fingerprint is extremely difficult. Generally, casting mediums like plaster aren’t as pliable as skin and flesh, and reveal tell-tale signs of being a fake.
Easier than forging a print are:
- Taking a fingerprint out of context--for example, taking a bottle with a fingerprint on it and moving it into the scene of a crime.
- Fabricating a print--that is, lifting a print, then claiming that you found it somewhere else. When this happens, this is generally due to police involvement, the cops who “never fail” and manage to get around having another person reviewing their evidence. Criminals tend not to think that far ahead.
Fingerprinting Is Fun
Fingerprinting was a hoot. I had a great time with the magnetic powder (brush! it’s on! magnetic swipe! it’s off!) and with the fluorescent powder; after Mr. Adair said that the UV LED flashlight would pick up on any kind of bodily fluids (and that he would never take one into a motel room again, ewww), I tested it by spitting on a finger and watching my spit glow pearly white as it dried.
Another lesson that hit home--and that I think writers will appreciate--was that it’s almost impossible to get fingerprints from textured areas (like the handle of a milk jug). Mr. Adair pointed out that the books and movies where someone’s always finding fingerprints on a gun is silly; the places you’re most likely to touch a gun are the places with the most texture (the grips, the trigger).
In the end, my main impression was that fingerprinting wasn’t as easy as it looked. It was pretty easy to see that someone had touched an item, but it was more difficult to pick out the general shapes of fingerprints, let alone to see them clearly enough to identify. After all, people don’t tend to make sure they have adequate amounts of oils and sweat on their hands, then carefully place their fingers on the smoothest part of an object, holding it carefully so that nothing smears--and all at the right temperature, humidity, and time before being investigated! I’d say that getting a decent set of prints was maybe a one-in-ten chance, at best.
About the Writer: DeAnna Knippling started freelancing in May 2011 and wouldn’t be able to do it without her wonderful family and friends, especially her husband. In fact, she owes a lot to Pikes Peak Writers for helping her be a better writer, especially through the Write Brains, both in the lectures and in meeting lots of other writers.
Her reason for writing is to entertain by celebrating her family’s tradition of dry yet merry wit, and to help ease the suffering of lack of self-confidence, having suffered it many years herself. She also likes to poke around and ask difficult questions, because she hates it when people assume something must be so.