A great deal of the forensic scientist’s time is spent peering into a microscope. Our fictional heroes aren’t interested in that tedium, and neither are the readers of their adventures. But understanding what the results of that microscopic inspection reveals, and what it means to an investigation, that’s important. And as with any other aspect of this, a hero or villain getting it wrong through incompetence or malice adds to the conflict in a book, even if we don’t want to see it in real life.
I think all of us can grasp the concept of a tool leaving a mark when it is used to, say, pry open a window or a door. But logically, this isn’t going to be terribly helpful in identifying the specific tool that was used – after all, hundreds if not thousands of crowbars have been made in various configuration. However, through the use of a comparison microscope that allows the side-by-side comparison of the marks and the tool, individualization can be made in some – not all – cases.
I’m not sure I covered the two classes of evidence before, so this is a good place to touch on it. Classification is the sorting of physical evidence that will match a class of something. Cotton fibers, that mark was made by a crowbar, those footwear impressions were made by Doc Marten boots. Classification is useful in narrowing the field down, but what every investigator wants and hopes for is individualization. This cotton fiber with cochineal dye belongs to that shirt. This crowbar with the nick in the pry surface made that mark. These boots with the cut on the sole made that impression. Such evidence can tie a suspect or his tools to a scene.
Tools can acquire unique characteristics through wear and tear. I’ve chipped and dented many by dropping them, personally. Tools that have come in contact with paint can carry away traces of that paint, which can be classified or individualized. There is, and this is pretty cool albeit very nerdy of me to call it that, a paint database. The RC Mounted Police maintain PDQ, the pain data query, which is used worldwide to determine where that paint came from, what it was used on, and so forth. Even if you can’t individualize something, enough classifications can resolve into the pointing finger of probability.
Side note for our heroic investigator (or the curling-mustachioed Snidely (sorry, channeling Dudley here)) is that you would never, I don’t care what you’ve seen on TV, attempt to match the tool with the mark on scene. You can alter and destroy evidence that way. The correct procedure is to make a cast of the toolmark with a special kind of silicone called Mikrosil. Later, at the lab, a stereomicroscope will be used to compare the mark with the tool.
Firearm evidence is a huge field, but I’ll cover the basics here, of what marks a gun leaves on bullets and casings. When a bullet is fired, and forced through a rifled barrel, minute striations are left on the soft lead of the bullet by the rifling marks in the barrel, called lands and grooves. In theory, a second test-firing through that same barrel can produce a bullet which will have matching striations, and they can be matched under a comparison microscope. Those of you who are familiar with firearms will immediately see the problems inherent with this. Not all firearms will have rifling. Shotguns, for instance, cannot be matched in this way. Alteration of, or removal of, a barrel can also alter the test firing results. My instructor a while back, an actively serving detective sergeant, would get entertainingly frothy in class about the moron on TV who routinely picked up suspect weapons by inserting a pen into the barrel. Granted, this will not alter the inner map of the barrel much, but it would not take much for the defense to introduce doubts in the jury’s mind.
Sometimes the other end of the action, the casing, can be more helpful than the projectile. While a revolver leaves no casings behind for our intrepid investigator, a semi-automatic ejects them automatically (heh). A savvy criminal might police his brass, if he has time. But the marks left on casings by the ejector, the firing pin, and even fingerprints from loading, can be possible individualized evidence in building a case. It helps if the investigator knows what he is looking for. A while back in a non-investigative CJS class, I had to read and write an analysis of a case. In it, the evidence was listed, and one thing they found in and around the house of the shooter in question were numerous (something like 40 without looking at the case file) unfired casings. That, to me, was hinky. I would have been very unhappy with taking that evidence anywhere listed as it was (the professor of that class got very impatient with my hang-up on the evidence, which I found funny). Do you know why it struck me wrong?
I will finish up the bulk of the series next week with toxicology (and a foray into serology). You can find the first part here, the Crime Scene here, Evidence and Investigation here, and Blood Spatter and Ballistics here. I’ll wind up the series in two weeks with a linkful post, and a summary with Q&A session in the comments.