  Mystery
This week: Fingerprints III Edited by: The Milkman  
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| In this last chapter of Fingerprinting I will show you and your characters the tools of collecting fingerprints. |
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In the first part of, what is now my fingerprint trilogy, I gave you knowledge of the history of fingerprints. The second part was devoted to fingerprint classification. This, the final chapter, will be used to help you find, lift and collect those identifying prints.
Since I'm sure many of us have not seen an actual crime scene, except for television, I will begin by telling you how Hollywood actually gets it right. Crime Scene Investigators, police officers and those who play them only choose to fingerprint those areas in direct crime scene. Some exceptions are if the victim is killed in the bedroom with a steak knife and a kitchen drawer where steak knifes are stored, an investigator will fingerprint the drawer. And remember that not all police forces have the budget for CSI's so the police officers will collect prints from crime scenes in a large portion of cases.
A fingerprint comes in three general types.
Patent Prints occur when a substance such as blood, ink, paint, dirt or grease on the fingers of the perpetrator of a crime leaves behind a readily visible print.
Plastic Prints have a three-dimensional quality and occur when the perpetrator impresses a print into a soft substance such as wax, putty, caulk, soap, cold butter or even dust.
Latent Prints are invisible and can't be seen without special lighting or processing.
By definition the patent and plastic prints are the easiest to see, record and classify. Most can be photographed, and the photo can be used for matching. The latent prints, the prints most often mentioned by police officers or crime scene investigators, are prints that are accidentally or uuintentionally left at a crime scene or on an object. They are trickier to track down but with some tools, which may be as simple as a flashlight or black powder, as sophisticated as chemical reactions and lasers, or as goofy as Super Glue.
The best places to find latent prints are on hard surfaces such as a murder weapon, tools or objects left behind or potentially touched by a criminal, opened drawers, out-of-place furniture, and entry and exit points. In short, any place the perpetrator may have touched and therefore left a print.
Hollywood uses a flashlight held at an angle with or without the help of a magnifying glass as the simple approach to find these latent prints. This technique along with lasers and ultraviolet lights are also used by the real crime scene investigators.
Once these prints are discovered the investigator has to rely on his tools to bring those print patterns to the light of day. This is where a investigator will use the various fingerprint powders. There are several colors of powder. The crimologist will use the color that gives him the greatest degree of contrast with the background surface. They also use some magnetic and flourescent powders.
After the the powdering process is complete, the print is either photographed or lifted. Liftingis done by gently laying the sticky surface of a strip of transparent tape over a print. As the tape is peeled off, the print pattern sticks to the tape, which is then placed on a card for later examination and matching. Because smears can render a print unusable, lifting a print takes a very steady hand.
How do you retrieve fingerprints from more porous surfaces? The answer is chemicals such as cyanoacrylate vapor, iodine fuming, ninhydrin, and silver nitrate.
Super Glue is the marketable name for cyanaoacrylate. When this glue is heated or mixed with sodium hydroxide it realeases vapors that bind to amino acids that are present in print residues, thus forming a white latent print. Once the print is exposed it is photographed as is or is treated with a flourescent dye that binds to the print, which will make the print glow under a laser or ultraviolet light.
Iodine fuming is done when solid crystals of iodine are heated in a fuming chamber. The iodine fumes combine with oils in the latent print to produce a brownish print. This kind of print fades quickly, so it must be photographed right away or fixed by spraying it with a solution of starch in water, which preserves the print for several weeks or months.
Ninhydrin is a liquid that is used as a dip or a spray at the crime scene. The reaction between the Ninhydrin and the oils of the print make take several hours but increases the temperature of the object to 80 to 100 degress speeds up the process. This print will appear as a purple-blue print.
Silver nitrate is a component of black-and-white- photographic film. When investigators expose a latent print to silver nitrate, the chloride in salt molecules present in the print residue reacts with the silver nitrate and forms silver chloride. this colorless compound develops, or becomes visible, when it's exposed to ultraviolet light, revealing a black or reddish-brown print.
I hope this extended, but brief, look into the world of fingerprinting has lead you and your characters down a road of discovery.
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|  | Malice Intended (ASR)
Who is the slapdash, foolish criminal behind a crime scene in a hotel?
#445131 by Joy  |
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Bikerider 
Hi,
I enjoyed reading this issue of the newsletter. In my 20+ years in law enforcement I made hundreds of cases based on fingerprint matches. But until now I had never known any of the history of fingerprinting. Very interesting. I'm looking forward to the next issue.
Joy
Very informative. Thanks, Milkman.
These are just a couple of responses I've recieved for this Fingerprinting articles. Thank you and keep reading. |
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Mystery: March 23, 2011 Issue [#4293]
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