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It is late, and a break and enter artist parks his car in a dark laneway. He carries his duffel bag with him and slips over the back fence of a town house. He knows no one is home because he phoned first, and no one answered. Now he pulls out a crowbar, jams it in between the door and the doorframe, and with a solid push, he's inside. He cleans out the jewelry box, grabs some valuable electronics, and he's gone. He's an experienced B&E man, and he has used gloves to avoid leaving prints.
Now it's the next day. The distraught homeowner is talking to the investigating police officer about the break in. The young officer looks at the pry mark on the door and, remembering what he was told in a recent training session, asks the homeowner not to have the doorframe replaced yet. He calls for a Forensic Identification Officer to attend the scene. The "Ident" man comes and checks for fingerprints, with negative results. He checks the door, and agrees that it's a nice clean pry mark. First he gets his camera out of the car and photographs the doorframe with the tool mark "in situ". Then he borrows a saw from the homeowner, cuts a one-foot long chunk out of the doorframe, and takes it back to the station. Labeled, dated, and initialed, it goes on file in the Police Department's Forensic Identification Section. On his way to lunch, he lets the detectives in the Break and Enter Section know that he has a good pry mark from the latest, in what has become a string of similar B&E's. The B&E detectives are glad, because this has been a tough string of robberies to break. They post a notice in the "parade room" (where the patrol officers are inspected prior to their shift) stating that they should carefully check for prying tools during any arrests or routine pullovers of suspicious vehicles.
Several weeks go by, and several more B&E's have occurred. No one has seen anything, and the B&E Detectives are getting frustrated. They ask the patrols to step up their routine stops of vehicles at night, hoping for a break. After each stop, the patrol drivers submit "contact cards", listing who was in the car, so the detectives can look for familiar names. Then, a break. During a routine stop, an officer notices a duffel bag in the back seat of the car. The car was seen cruising around the neighborhood and a suspicious citizen called it in. The man is very nervous, and the officer decides to check him for outstanding wants and warrants. Sure enough, a "fail to appear for court" warrant is outstanding, and the man is arrested. The car is searched, and the duffel bag found to contain a number of tools, obviously burglary tools. They are turned over to the Forensic Identification Section the next day.
The Forensic Officer who examines the tools is also a qualified tool mark expert. This means he has training that allows him to examine a tool, examine a mark made by a tool, and determine whether or not the mark was made by that particular tool. In court, he can then give an expert opinion on this subject. But how can you possibly tell that a particular crowbar or hammer made a particular mark?
The secret is in how closely you look at an object. All objects have two types of characteristics; "Class" and "Individual". A Class Characteristic of a crowbar might be that it is "two feet long". There are many two-foot long crowbars, so that doesn't prove anything. But if you look at the end of the crowbar under high magnification, you will see that it has scratches, nicks, and gouges from being used, dropped, and abused. It is these small bits of damage that are referred to as "individual" characteristics, because they are individual to that particular crowbar. A new crowbar or hammer will not have as many individual characteristics as an old one, but nonetheless, they will be there. There will be marks from handling, being thrown in bins, grinded on various equipment, etc., and it is those things that the tool mark examiner catalogues.
Now comes the hard part; making the characteristics of the tool mark and the tool visible to the naked eye. Photography is the key here. The tool itself is the easier of the two to photograph. Simply set it up in the photo studio, put on a powerful macro (close-up) lens, and then apply "oblique lighting" to the end of the tool. Oblique Lighting is light that is hitting the tool on a very shallow angle, which creates dark shadows on even the smallest surface features. Skimming just above the surface of the object, the light catches even the most minor scratch or gouge. Observe the shadows thrown by small objects late in the day. They are often much longer than the object is high. If the tool is dark in color, it can be very difficult to see the shadows properly, so sometimes the tool will be coated with the smoke from burning magnesium. This deposits a light gray surface on the tool, without filling in fine detail. Carefully setting up his camera and focusing on the fine scratches and nicks in the end of the crowbar, the Ident man takes a number of pictures. Now comes the more difficult job; photographing the tool mark. Since it is a recessed area in the piece of wood from the doorframe, it's not possible to apply oblique lighting to it. So the tool mark examiner takes a different approach. He pulls out a kit, which includes a special silicone molding compound in a tube. Known as "Room Temperature Vulcanizing Rubber", this compound, when mixed with a catalyst, will solidify within 20 minutes or so. He mixes it up and uses a small artist's trowel to fill the tool mark with it. Twenty minutes later, he carefully peels the rubber casting from the tool impression and examines it. It is a perfect copy of the tool mark, with every characteristic shown in exquisite detail. Silicone rubber is so sensitive that even a pencil line will be reproduced. Now he coats this rubber casting with magnesium smoke and photographs it just like he did with the crowbar.
After making some 8 X 10 enlargements of the photos of both the tool and the impression, he is ready to do a comparison. By cataloguing the accidental characteristics on the crowbar, and then comparing them to the tool mark, he is able to form the opinion that the mark in the doorframe was made by that actual crowbar. There are a few additional marks on the crowbar, which don't match any marks on the impression in the door. This sometimes happens when a tool continues to be used after it has left a mark. Nonetheless, the characteristics he can see are very distinctive, and there is no doubt in his mind that he has a positive match. This is bad news for the bad guy, and great news for the B&E detectives, who proceed to interview the suspect and drop the news of the tool mark match in his lap like a bomb. Faced with the cold hard facts, he decides to make a confession, knowing that judges are easier on people who plead guilt than on people who force costly time consuming trials when they are obviously guilty.
Whether it is a mark left by a hammer, a crowbar, or any other hard object, often a qualified tool mark examiner can tell you for sure whether a particular item left a particular mark. It's just another tool in the arsenal of the Forensic Identification Officer.
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