Fingerprints(pt. 1) – aarfo – Feb11 (7/_)

Dactylography – A Lot of History, A Lot We Don’t Know

Before Fingerprints

Criminal investigators have struggled to link marks, impressions, and traces to individuals since the concept of “criminal investigator” was recognized. Some of these we have already discussed, such as trace elements, spent casing impressions, or bullet striations. However, there are many others in this same broad field – toolmarks, teethmarks, and yes fingerprints.

Before fingerprints were realized to be a wellspring of (potentially) unique information, the police used a complex scheme of body measurements. I don’t have the information in front of me but I think they took something like 27 or 28 different measurements of your body and wrote them down upon arrest. Then, predictable catastrophe, two people had the exact same measurements. Ever seen two people and thought, “Gosh, they could be brothers!” Well you weren’t just seeing things. There are a significant number of people out there who could be your identical twin. You will have no relation to them at all beyond ancient anthropological coincidence. This was the end of that source of ID. This was all at least a century ago now, maybe longer.

Luckily, fingerprints were just making a splash among criminologists (as forensics hadn’t been conceived of as an independent field much less a name). They seemed to be the perfect replacement for the old ID as well as having many advantages. For one, it seemed like you left fingerprints on goddamn near everything! Maybe you could even link a fingerprint found at a crime scene or on a piece of evidence to a suspect! You probably know most of the rest (or at least the end point) of the history by watching Law & Order and CSI.

In this chapter we will learn about the multitude of steps necessary in order to start at a human hand, go to a surface, then going back to a human hand. The print has to be left by a person, ID’d as a print, visualized, recorded, analyzed, and then matched. Each individual step must be scientifically valid, and performed correctly!

What is a Fingerprint?

When people say fingerprints they could mean the ridges on your hand, the latent fingerprints you leave all over everything, or the visualized and lifted fingerprints presented at trial. People tend to use the term interchangeably depending on the context. I’ll do the same because it’s a pain to invent new and awkward words. Everyone is born with fingerprints. They form in the womb and you have them your whole life. As discussed with firearms ID, fingerprints have class characteristics and individual characteristics. Class characteristics are things like whorls, arches, or loops. Look at your fingers. You’ll probably see at least two of these types of class characteristics even without trying to know which is which. Each of your fingers will fall into one of a few broad categories.

Individual characteristics are minute and can barely been seen with the naked eye. They can be deltas, breaks, oases, bifurcations, divergences, and the like. Anything lines can do, generally the ridges on your fingerprints can do. The crucial claim is that these characteristics, when taken in the aggregate, are unique to each person.

Leaving a Fingerprint

When you touch something, the oils in your hands leave traces behind on that thing. The oils are deposited according to the pattern of the ridges on your fingers. This leaves a unique outline of residue behind on a lot of surfaces. Leaving fingerprints depends on many factors. Are your hands covered in some substance or are they generally clean? If you leave a black oily perfect print behind, you save everyone a lot of hard work. What kind of surface are you leaving a print on? Hard non-porous surfaces generally result in easiest and most traditional fingerprints. Finished wood, glass, metal, stone, and similar surfaces result in fingerprints that even your mom can see every time she yells at you for putting your hands all over her sliding glass doors. Porous surfaces like fabric, cardboard, unfinished wood, and paper also retain fingerprints. Generally, these are invisible to the naked eye. Forensics experts have, on occasion and under ideal circumstances, been able to detect fingerprints left on skin and flesh. The lesson boils down to this: if you touch a thing, you may have left a fingerprint even if you can’t see it.

Visualizing a Fingerprint

This area contains simple processes with no complex science behind them, simple processes with a shitload of complex science behind them, and complex processes of both types. Most fingerprints aren’t visible to the naked eye. If they are visible, they are on surfaces with complex patterns or poor contrast.

As shown on every cop drama from the 1950’s onwards you can gently spread powder on a surface and it will stick to the oils left behind by your fingers. The type of powder use generally depends on the color of the surface you are spreading it on. Light surface, dark powder. Colored surface, highly contrasting colored powder. There are even powders that fluoresce under UV light and powders that can be applied and cleaned up magnetically. Typically, the investigator spreads the powder with a special superfine brush and let me tell you, it is a pain in the ass and hard as shit. Unless you have a lot of practice, it is damn near impossible not to put too much powder on the print. Even if you put the right amount of powder on the brush, you have to spread it properly. Oh and I hope you are using the right kinds of powder because there’s no do-overs! Fuck up any of these steps and the latent print is ruined forever. Nice job, bozo. A real credit to the force you are. This is a “simple with simple science” technique.

Another technique of visualization is chemical. This is the “simple with complex science” technique. Suffice it to say if you have ever been a research chemist you will be accidentally familiar with one or more of these techniques because you will have spilled the chemical and realized your fingerprints are all over your goddamn samples. I’m not going to go over what chemicals they are because it’s overly complex and boring but the technique is idiot proof. Pick up spray bottle with chemical, spray surface, look at perfect print become visualized in seconds. There is almost no way to fuck it up. Sometimes there are other chemicals on the surface or in the object that will react with the visualizing chemical. This is bad news as you’ve probably destroyed the print. It happens and there’s not much you can do about it if you are following directions on which chemical to use on which type of surface/manufacturer/whatever. Sometimes shit is unpredictable. Occasionally you will have to apply a second chemical treatment to visualize the print. For example if you have a print on black electrical tape you have to spray one chemical then submerge the tape in what is effectively a crystal dye. The dye sticks to the chemical, the chemical sticks to the print, booya.

Finally, there are some more sci-fi techniques to visualize prints. Pointing a laser at them can make prints fluoresce. It looks pretty cool you get a neat gun and you wear special goggles. As a bonus, you can zap it around the room at a distance of up to NINE GODDAMN FEET, which really makes you feel like you are doing Star Trek forensics. Just push that button, scan that crime scene. After you find the prints, you can visualize them traditionally if possible.

There are varieties of chemical techniques to make a surface fluoresce more or less, so you can just zap it with UV light and not ever really touch the fingerprint. The fingerprint will be left glowing more or less brightly than the surface around it. It also lets you do other stuff to the print that a traditional visualizing technique would make impossible. E.g. DNA.

Super Glue fuming falls into this category as well. You take the object you want to print, stick it in a box with a cap full of super glue and heat up the cap. The super glue evaporates and sticks to fingerprints pretty precisely. It’s pretty cool to watch and it’s functionally impossible to do wrong. The only drawback is that the prints are always white so you have to do some more visualizing if it’s a light surface.

There are also some applications of most traditional analytical chemical spectroscopy – IR, UV, LASER – that are out there and work but they are pretty uncommon.

HUMAN FLESH is generally not a viable surface for fingerprinting. Human skin contains all the same surface chemicals as the skin on your fingers so good luck differentiating. In addition, human skin is good at dealing with deposits of these oils so they are dispersed/absorbed rapidly. Seems like two or three hours is the maximum time a print will last if it is deposited at all. Finally, the skin area has to be smooth and clean. Ladies (and Gentlemen), if you want that crack team of FBI investigators to catch your killer: shave! The inner thigh is apparently one of the best surfaces for fingerprints. Of course, if you leave a perfect neon pink impression because you grabbed a big bowl of open highlighters before you choked someone then there will be a fingerprint.

Recording a Fingerprint

So, now you’ve got this bright orange fingerprint outline on the twenty foot tall, 30-ton granite pillar. You sure as shit aren’t taking that bad boy with you. Also the sexy debutante who owns the mansion doesn’t think orange goes well with her elephant skin entry rug and is chomping at the bit to have her servants wipe it off. So what do you do? You record that visualized fingerprint somewhere permanently!

The most traditional way is just to use clear tape. You put the tape over the print, rip it off, and put the tape on a piece of white cardboard. Now the visualized print is safely in your evidence bag.

The other way is just to take a high-resolution photograph of the visualized print. With the massive popularity of digital photography, this has become by far the simplest method. Problem is digital photographs introduce all kinds of visual errors and noise compared to traditional photography. Even if you take a traditional photograph, the investigators will probably just scan it and destroy the original, reintroducing the problem of digitizing the picture. It’s also a bit of a rules of evidence SNAFU. In any event, it’s still is better to lift the print if it’s a curved or otherwise awkwardly shaped surface.

Fingerprint Analysis

We’ve somewhat addressed this already. Because I don’t have pictures to show what I’m talking about, I’ll keep this pretty brief. The investigator goes over the fingerprint and IDs class characteristics and individual minutiae – line breaks, deltas, oases, bifurcations, etc. If done properly it is a long, tedious, and exhaustive analysis of the print.


The Fingerprint Expert, a human being with eyes, ears, hands and a brain, goes over both sets of fingerprints – the exemplar and the evidence. Then the expert, a human being, starts drawing comparisons between the two. Finally, the expert, a human being, after making what should be dozens upon dozens of matching points, concludes that the prints match. A single fingerprint should have over 100 points of individual distinction so dozens ain’t that much to ask.

Computer Matching

There is no such thing as a computer match. There never was. There isn’t now. Computers don’t match fingerprints. CSI is a lie. Get over it and get it out of your head. There is NO SUCH THING AS A COMPUTER MATCH. Human beings have invented a series of classification categories that boil a fingerprint down into a series of alphanumeric codes. Then a human examiner will take an exemplar print, and apply the classification method. Then the classification chosen will be entered into the computer and associated with the identity of the giver of the exemplar print. Then the exemplar print is stuffed into a file cabinet at the police station or FBI headquarters more likely.

When someone does a computer search, what they are doing is asking the computer to give a set amount of potential matches as determined by the classification system. The expert will choose the stringency of the classification as well as how many possible matches the computer should spit back to the expert. I don’t know what classification system they use now; it was in flux as of 2005.

In short, there never has been a computer match ever. All the computer does is narrow the 198 MILLION entries in the FBI database down to a manageable number which is set by the examiner. This is somewhere between 10 and 100. A human expert then looks at the potentials and tries to make a match to the evidence print.


I’ve already written four full pages on this so I’m going to split this post here and discuss the science or lack thereof, the controversy and the admissibility of fingerprint evidence next time. Until then, lemme give you a preview – Fingerprints are fully admissible everywhere in the country and no one has ever kept them out successfully.


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