MadSci Network: Genetics

Re: science project for study of fingerprint forensics

Date: Thu Oct 11 05:20:10 2007
Posted By: Dale L. Laux, Staff, Serology/DNA, Ohio Bureau of Criminal Identification
Area of science: Genetics
ID: 1191503796.Ge

Hello Callie,

CSI makes forensic science seem magical at times, doesn't it.  
Unfortunately, in the real world, the magic runs out and we are forced to 
use real science, good techniques and hard work.  

Fingerprints, or more properly, latent prints, are not always left behind 
at a crime scene.  The deposition of latent prints depends on conditions 
such as temperature, humidity, and type of surface touched.  The use of 
gloves will naturally deter the deposition of a latent print.

We have found that latent lifts are deposited well on clean, hard, dry, 
smooth surfaces such as glass, metal, smooth hard plastic, and paper.  
Latents are not deposited well on textured surfaces such as molded plastic 
gun grips, molded plastic dashboards and automobile trim, wet surfaces, or 
fabric clothing.

Different processes are used to develop and enhance latent prints.  Your 
mentor will be able to show you super glue fuming, dusting with 
black/white powders, use of fluorescent dyes and alternate light source 

A nice experiment is to take your finger and rub your forehead, then touch 
a smooth surface such as a piece of glass or a mirror.  You will leave 
behind a nice, detailed latent print.  The latent consists of mostly water 
and salts plus some oils.  Wash your hands well with soap and water, dry 
them, then repeat the process.  The latent, if present, won't be nearly as 
evident.  You've removed the oils from your skin and lessened your chance 
of leaning behind a latent.

Another experiment that you can do will test the affect of age on a 
latent.  An examiner can never determine when a latent was deposited.  
However, as latents age, they dry up, and become harder to see.  To test 
this, place several latents on a clean plastic or glass surface in a row.  
Process the first latent (far left) one day, the second one week later, 
the third one week after that, and so on.  

Sometimes, we find latents in blood.  This is very good evidence because 
it places that individual at the scene when blood was shed.  However, 
being a liquid, blood sometimes smears.  A nice experiment that you can do 
is to test how long after blood is shed will you leave behind a latent of 
value (with ridge detail).  To do this, you will have to prick your finger 
with a sterile needle (have your Mom or Dad help you :-). It just needs to 
be a little stick.  Start a second timer and begin to touch a clean piece 
of glass or plastic trying to leave behind a latent.  The first ones will 
appear "runny" and smeared.  But as the blood dries on your finger, it 
gets sticky, and you will begin to leave behing latents with fantastic 
detail.  Eventually, the blood completely dries up, and you no longer will 
leave behind a latent at all.  So you can show how limited the time period 
is in which you can leave behind a bloody latent with ridge detail.  It's 
really just a few minutes.

Another experiment that you can easily do is to put on a pair of latex 
surgeon's gloves (the thin ones doctors and nurses use).  Criminals 
sometimes where gloves when they commit a crime.  Try and leave behind a 
latent on a piece of glass.  Initially, you won't be able to.  But 
continue to wear the gloves and exercise, run in place, etc.  Remember, 
criminals are sweating and in a hurry.  As time goes on, you will notice 
that you can begin to leave behind latents through the gloves, especially 
if you press hard.  The oils and water seep through pores in the gloves 
and leave behind trace latents.

I hope this helps.  Sounds like you will have an interesting and fun 
project.  Good luck.

Best regards,

Dale L. Laux
Forensic Scientist

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