Bang Bang, He Shot Me Down Bang Bang, I Hit the GroundBang Bang, That Awful SoundBang Bang, My Baby Shot Me Down

Technically, ballistics is the science of how projectiles travel in flight. Forensic Ballistics is a misnomer since it doesn't concern itself with a projectile's flight but rather about analyzing the firearms used in crimes. Perhaps, ballistic fingerprinting is a slightly better term. Ballistics is actually a subset of the forensic science of Toolmark Analysis. Guns are a tool and ballistics experts study the marks left on bullets by the guns. I'll explain this as we go.

Bullets are made up of three components: primer, propellant (held in a cartridge) and the actual bullet.

The primer is a shock sensitive explosive – it will detonate if struck or if a spark occurs. The primer is kept in a tiny cup in the cartridge's head. The gun's hammer has a pin on it which directly strikes the primer. The detonation goes through a small hole in the cup and ignites the propellant. The explosion in the confined area forces the bullet out the barrel. By Newton's Third Law when the bullet explodes out the barrel, the cartridge must recoil backward. A piece of metal in the gun, called the breech, stops the cartridge from flying into the shooter's face.

Pause for a moment and realize that the hammer striking the cartridge will undoubtedly leave a mark and that you could most likely tell the gun manufacturer from this mark alone. Additionally, The breech will leave a mark on the head of the cartridge. Finally, an extractor will grab the cartridge and eject it also leaving another set of traceable markings. Back to the bullet:

The bullet is now cruising through the barrel. Back when guns were first invented, the projectiles would be very unstable and tumble in the air. This made them extremely imprecise. Rifling changed all this. The barrel of the gun is given a set of helix grooves. The heat from the gun powder causes the bullet to expand slightly. It expands into these grooves and then gets locked into the tracks. When the bullet comes out of the barrel it is spinning! Get it?

Why spinning? Because the spinning adds stability (and reduces air friction apparently). Consider a spinning top. It is very hard to knock off course compared to non-spinning top. We already discussed this when we talked about boomerangs.

But clearly, the grooves have dug into the bullet. By comparing the marks on a bullet found at a crime scene to a bullet fired from a confiscated gun, an expert can determine from these marks whether or not the bullet was fired from the suspected gun.

The data from ballistic forensics is not just class evidence but can also constitute individual evidence. Every time the bullet goes through the barrel it changes the rifling grooves, just as they change the bullet. The scratches and imperfections cause each ballistic fingerprint to be different.

There are a couple of things to be aware of:
Just because two bullets were fired from the same gun does not guarantee that the markings will match. Rust build up can change the stria or repeated firings of the gun will cause an evolution of the stria – imperfections in the bullet alter the groves in the barrel or the barrel of the gun could be replaced or purposely altered.

PS It seems fitting to add that if you want to read about Batman's relationship with guns rather than forensic ballistics you should check out the definitive post over at Silver Age Comics.

Batman's Fingerprints

Pattern evidence (or here) is the oldest branch of forensic science, although it's use as individual evidence. Individual evidence is any object which can be said to come from a one particular source. There is no chance (no whatsoever, wink) that it came from any other source. The pattern evidence covered in Jay Siegel's beginner's guide are:
Finger Prints,
Firearms,
Handwriting,
Accelerant Residue
Blood Splatter

Other kinds of individual evidence include paint chips, fractured fragments of glass and of course DNA. Individual evidence should be compared to class evidence. Class evidence can not be associated with any one particular person or source. Class evidence is still useful of course. Hair and fibers are class evidence. So is ink and dye or certain physical characteristics of drugs. Show prints or tire tracks might be class evidence. Class evidence is tied to groups not to individuals.

Finger prints, forensic ballistics (a term the author has a problem with) and handwriting are all supposed to be individual evidence. They are all supposed to be able to identify whether or not a potential source has produced the piece of evidence.

Despite the fact that forensics has been using certain types of pattern evidence for over 100 years, it can justifiably be criticized. Pattern evidence can grouped into the so called “experience-based forensic sciences”. Since there can be no absolutely quantitative analysis of these, forensic “scientists” who work with these types of evidence rely on experience and judgment. That's hardly what I would refer to as hard science.

Finger Prints
Finger prints are pretty cool. You have them on your fingers, palms, toes and souls. We've all stopped to look at them in our life but have you ever wondered what they might be for? They add traction to your grip so that you can hold on to things better. The ridges appear during the eighth week of a pregnancy and remain unchanged (other than growing in size, unless of course they are artificially altered which by the way just makes identification that much easier, duh) throughout a persons entire life. The root of the ridges is in the middle layer (called the dermal papillae) between your dermis and epidermis.

Finger prints are not determined by your genes (cough... not completely determined by your genes). Identical twins have easily distinguishable prints. People argue that it is impossible for two people to have the exact same set of finger prints (although how they would have legitimate statistics on this is beyond me). In any case, finger prints are usually an acceptable form of individual evidence.

When the surface of your skin comes in contact with a surface (glass, plastic but even paper and fabric) biological (like perspiration, skin cells or proteins) and alien (like ink or oil) material is left behind forming a latent print. If the print is visible to the naked eye and needs no amplification, it is called a patent print, such as if it were left in paint or blood. If the print is actually impressed into some matrix (like wax) then it's called a plastic print.

When they are latent (i,e, invisible) they must be found. Smooth surfaces can be dusted or fumed with superglue. Magnetic powders can be used for surfaces with fine texture. Dusting isn't the only way. Chemical fuming with iodine creates temporary reddish images which must be photographed. Silver nitrate can be applied to latent prints as a aqueous solution but therefore can't be used on surfaces that can't get wet. Ninhydrin can be sprayed onto latent prints. It reacts with amino acids to forms a a colored compound. Ninhydrin even works on porous surfaces (i.e. paper!). Laser luminescence can also be used.

So they can find them but what do forensic scientists see? Fingerprints are made up of various topographical elements.

1) The print will have a sort of pattern. There are four major patterns: either the ridges loop, arch, tent or whorl. The four general types are typically broken down into eight patterns which can be seen in the figure.

2) Besides the general pattern there are little details in the ridges, called “minutiae”. Ridges can do all kinds of things:
Bifurcations - when ridges split (trifurcations occur too)
Ending Ridge – self explanatory
Dot – a tiny round island
Short Ridge – a small isolated segment
Enclosure – when a ridge bifurcates and then reconstitutes

3) And looking at even more detail (under a microscope) gland pores, the shape of edges or little scars can help to compare prints

As a final note finger prints can be used to identify some sorts of drug use by individuals.

This was a little more extended than I would have like. Tomorrow I'll cover handwriting and ballistics.

Batman The World's Greatest...Forensic Scientist?

I picked up an introductory book on forensics this weekend and thought I would recapitulate and review it for you. Batman is undoubtedly well versed in forensic science. I have read an online discussion or two on what kind of scientist Batman is.

Like a few people state: He's a forensic scientist. And it's true! Forensic scientists are multidisciplinary just like Batman. And really anybody who applies the techniques and methods of science to solving crimes is a forensic scientist.

Here at Building Batman, I'm introducing myself to everything that someone would need to know to be Batman. And so, on with forensics. Because as everybody (who's read the last paragraph) knows Batman is the world's greatest forensic scientist!

The book I picked up is generically titled 'Forensic Science' and it's written by Jay Siegel who is the “Chair of the Department of Chemistry and Biology, and Director of the Forensics and Investigative Sciences Program, at Indiana University-Purdue University”. I read “ Chemistry and Biology” as a single department and “Forensics and Investigative Sciences Program” as a supplement to a diploma of some kind. Otherwise, this guy must not sleep.

In any case, he seems to have recognized the public interest in forensics due to fiction like the CSI franchise. As such, he doesn't really explain anything in the book but rather just introduces concepts and basic ideas. His intent is only to inform the general public on the topics and limitations of forensic science. He discusses the “CSI-Effect”, the exaggeration of forensic science's ability to magically and quickly collect and analyze all the evidence needed to convict bad guys. Furthermore, the CSI-Effect means that juries expect prosecutors to present incriminating forensic evidence. They tend to discount evidence that doesn't have the Authority of Science (notice the capital letters, please) backing it up.

But not only does forensic science have limitations but so do forensic scientists. Observer Bias (<---that's a great link) can creep in and corrupt the scientific method. Extra information (that may not even be true) can (and presumably does) influence the way that a forensic scientist interprets their results causing the scientist to get in the way of the science. Perhaps, forensic scientists should only be given the evidence and asked to apply scientific techniques to discover what they can. But if you think about that, it's not a very practical way of doing business. How should they know what to search for? or what tests should they apply? Perhaps the some piece of withheld information could be the key in solving a crime. Who decides what is “domain irrelevant”? It's a big question. Even wording in reports can cause subtle bias. There are other kinds of bias. There is Conformational Bias, where the results of one forensic scientist influence another's toward agreement. Or even a pro-prosecutor bias. The author certainly seems to think that regulatory bodies are lacking and that certification and supervision are needed.

And bias is definitely not the only human error that can hinder forensic science. Somehow, the evidence has to get from the scene of the crime to the lab. Only rarely do forensic investigators accompany detectives and police investigators to crime scenes. All evidence must be searched out and collected without damage, which can be tough if say the evidence is a single fiber or buried under the debris from and explosion or liquid blood that will putrefy. The author makes a very tight analogy. Crime scene investigations are immediate archaeological digs. Investigators sift through the evidence and clues to reconstruct a past event.

Forensic also should be limited by the courts. Not all evidence is relevant or competent and so not all evidence should be allowed to be admitted.
- Evidence must be material meaning it must have something to do with the case at hand.
- Evidence must be probative meaning it must prove something and not just be a fact.
- Evidence must competent meaning that it must abide by a set of rules – a spouse may not have to testify against their partner or a religious confession might not meet competence constraints.

Forensic scientists have expert witness status when they testify which simply means that they have knowledge or skills to draw conclusions from a set of facts that your average juror could not. This means that not only must they be good scientists but they must also be good teachers. Despite all this, we should remember that just because a scientist has arrived at some conclusion does not mean that it is de facto truth. People tend to believe scientists and so if they testify in a court case jurors may accept their words without question.

That's the background information. Over the next couple of days, I will summarize the material covered in “Forensic Science” by Jay Siegel. We'll talk about

Pattern Evidence
Evaluating PMI
Serology
DNA
Drugs
Fires and Explosions
Trace Evidence