There’s a distinction to be made right out of the gate regarding fingerprint techniques. There’s possible, and then there’s possible. Given unlimited time, money and equipment, and a stress free, protected situation, you could preserve prints on just about anything. But patrol police in the field don’t have the luxury of this.
There are two situations in which fingerprint development occurs: field investigation and lab analysis.
Most fingerprinting is done in the field. This is most often done in the wake of a burglary or vehicle break-in, but could occur in a number of crimes. When I arrive on scene, I’ll review the scene and identify likely areas of contact.
What does that mean? Well, unless the crime in question is a Class A felony (which I’ll discuss more thoroughly in a second), I’m not going to scrutinize your entire residence or car and dust everywhere for prints.
I’m going to try to retrace the suspect’s path through the residence, or what he disturbed in the vehicle, and focus my efforts there.
I’ll identify the surfaces I’m most likely to develop prints on. For all practical purposes in the field, these surfaces will be
– dry, or
– non- or semi-porous.
Think furniture with a glossy finish, clear glass, lightswitch panels, and the like. Curved surfaces probably won’t give a full print; textured surfaces will be missing most of the print data from the recessed areas; lift tape won’t stick to wet surfaces; porous surfaces can absorb fingerprint oil and present some of the same challenges textured surfaces do.
I’m probably not even going to try to develop prints on a surface type other than these, because it will honestly be a waste of time. Even if I were to find evidence of a print, there won’t be any way (for me in the field) to lift the actual print for analysis.
There are multiple ways of developing a print for analysis. In the field, I only carry a simple print kit:
– carbon powder,
– a simple dusting brush, and
– lifting tape.
I dip the brush in the carbon, tap off the excess, and roll the brush between my thumb and forefinger to spin the brush on the surface and spread the dusting powder.
Most of the excretions from someone’s skin consists of water, but a small percentage consists of lipids and amino acids – what most people term call “oil.”
This is what a fingerprint actually consists of, and is what the carbon sticks to upon its application. It’s also why it’s exceedingly difficult to lift prints from absorbent or woven materials like paper, cardboard, and fabric.
The sad truth is that, in the field, the severity of the crime is going to dictate whether I dust for prints and how much time I invest in the process if I do. I’m sorry, but I’m not going to dust for an infraction or a minor misdemeanor like second degree trespassing, with no extenuating circumstances or suspect information.
I realize you have been wronged, but my time as an officer is a departmental resource – every minute I spend chasing ghosts is a minute I’m not elsewhere.
In the case of a major crime (armed robbery, rape, homicide, etc.), though, the metrics change drastically.But, you’re not going to get me as an investigator in this situation.
You’re at least going to get a specialist, sometimes called a Major Crimes Investigator (MCI) These are usually veteran officers with specialized training in forensics and investigations.
As you know from TV, in the case of a homicide, there’s often a unit of detectives on call to investigate these. They bring much more forensics firepower to bear, as you might expect.
In this situation, anything able to be moved or manipulated would probably be bagged, tagged, and taken to the forensics lab for analysis.
Anything fixed-in-place would be developed, photographed, and lifted as well as possible, though detectives have also been known to make non-portable things portable – taking drywall sections and portions of furniture out with reciprocating saws and other awesomeness. (In Privileged Killers, samples of flooring from the Unicorn Killer’s apartment were sawed and removed).
So then class, what have we learned about ease of lifting?
- Easy: Glass, glossy tile, porcelain, lacquered furniture, smooth metal.
- Involved: Paper, painted surfaces, drywall, cardboard, leather, most dashboards.
- Difficult: Organic surfaces (tree leaves, fruit peels, feathers).
- Formidable: Fabrics, human skin, and rough or textured surfaces (think checkered handgun grips).
Class Dismissed !
An aside to readers before you accept all this as gospel and include it in your next crime novel. Realize that a rule of thumb taught many officers is that you do not know what prints you can lift until you’ve tried. Processing involves many techniques other than dust and brush!
There is a wide range of chemicals used to visualize latent or hidden prints on various porous surfaces, or to make visible prints deposited by, say, blood transfer. One of the best tools around for screening prints is UV light direct, viewed through a low light viewing system maximized for UV spectrum.
It is extremely challenging to get prints from the following
– oily, rusty, or extremely dirty surfaces.
– high traffic surfaces with multiple overlapping prints, and
– prints smeared by movement.
However, we’ll be posting a blog soon about the amazing techniques and heroics involved in some contemporary lifting procedures.
* Much of this “lecture” is based on info taken from police manuals and 2011-2017 Quora answers, esp those given by patrol officer Justin Freeman.