Forensic Information: How Experts Collect Evidence

Forensics technician can capture clues of detailed prints in snow using molten sulphur. Information on tricks, techniques and practices of such an expert.

It is 3:00 am on a cold winter's night as the Forensic Technician reaches for his bedside telephone. It is the Staff Sergeant on duty calling, and he's got to get out of bed and go to work. There's been a homicide and he's needed at the scene. With a groan he hauls himself out of bed, tells his wife what's going on, and stumbles off to find his clothes. An hour goes by while he gets dressed in his warmest clothes, grabs a coffee at the local coffee shop to warm up, goes to the station to get his equipment, and heads out to a dark country road to see what's up. Upon his arrival, he checks in with the investigating detectives, who show him the scene. There's a body, obviously dragged there from a car, and there are both good quality tire prints and footprints in the packy snow. The detective asks him if he can get some decent photos of the footprints and tire prints. His response is, "I'll do better than that; I'll get castings of them!" The detective gives him a puzzled look, as he's not aware that this is possible. After making sure that none of the other officers are going to wander into any of the prints, he sets to work.

Forty-five minutes later, after having documented and photographed the entire scene, including the tire and foot prints, he's ready to set up for one of the most bizarre techniques in forensic evidence collection; the casting of snow prints. Out comes a box from the trunk of his car, labeled "Snow Prints". Inside is a collection of items, including a small camp stove, a couple of beat up old aluminum pots, a wooden spoon, and a big bag of sulphur.

He fires up the camp stove over next to one of the foot prints, and puts enough sulphur chunks in the pot to cover the footprint. He then puts the pot on the stove and turns up the heat, slowly melting the sulphur in the pot. Meanwhile the detective watches him incredulously, wondering what the heck he's doing. While the sulphur is melting, he carefully gouges a small chute out of the end of each print. Now he goes back to the stove, stirring the sulphur slowly in the harsh glare of the cruiser headlights, which illuminate the scene. The sulphur is taking a while to melt, so he accepts the offer of another coffee from an officer sent to get some to warm up the near frozen officers at the scene. Coffee done, he turns back to the pot, and notes that all the sulphur crystals have melted, and the liquid sulphur is now hot.

The detective is still puzzled. "How the heck are you going to cast in snow with molten sulphur?" "Just watch!" is his reply. He moves to the second step of the process, which is to place the pot in the snow, far enough away from the footprint to avoid melting it. He puts the second pot-load of sulphur on the stove so it can start melting. Now comes the slow part as he stirs the sulphur continuously, monitoring the surface of it with his flashlight. The continuous stirring is critical, as the entire pot of sulphur has to be at the same temperature for the technique to work. Suddenly, he sees crystals forming on the surface of the molten sulphur as he stirs, which means that the liquid is just at the temperature where it wants to become a solid again. With a quick movement he snatches the pot out of the snow pours it as fast as possible down the chute into the waiting footprint. "You just melted it!" states the detective. With a grin he states "Don't be so sure!"

The technique gets repeated with the other prints and the tire marks, and then comes time to collect the castings. Each casting sits in the middle of a patch of melted snow, with a grumbling detective standing over them. But to his amazement, when the castings are picked up by the Forensic Tech and turned over, they are highly detailed copies of the original snow foot print. "How can you do that without melting the print?" asks the detective incredulously. The Technician explains that the sulphur has to be at exactly the right temperature or it won't work. When the sulphur is ready to turn into a solid again, even a small drop in temperature will cause this to happen instantly, and when the molten sulphur hits the cold snow, it solidifies so fast, the snow doesn't even have a chance to melt. This forms a layer of hardened sulphur which has already permanently captured the detail of the print, and then after a few minutes the rest of the liquid sulphur behind it hardens up and it can be removed. The quality of the resulting castings is only limited by the quality of the original print. The castings can then be photographed, molded, and used to do comparisons with suspect shoes and tires, hopefully resulting in an exact match. Just another example of how basic science is of huge value to investigators and the courts in the apprehension of criminals.

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