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Footwear and tire track evidence can be essential to your case, but it’s often overlooked. In some cases, CSIs identify the evidence but assume they can’t do anything with it. Weather extremes and difficult surfaces can make casting very challenging, but that doesn’t mean it’s impossible. In this two part series, I’ll first review the basics of casting footwear and tire track evidence. Then in the next issue, I’ll provide an overview of techniques you can use in more difficult situations.
Obviously, before you can do any casting, you need to find the evidence. When you arrive at a crime scene, stop and observe before you jump in. Think about how the suspect might have moved through the scene. Outdoors, look for points of entry and exit. Check the dirt near windows and doors for prints. Determine routes a suspect might have walked between the crime scene and a vehicle, then check for evidence. Don’t neglect to check for evidence indoors. The suspect may have walked in with dirt on their footwear, or the floor may be dusty or wet. Again, pay careful attention to entry and exit points. Look for other obvious places where a suspect would have walked. If you’re dealing with a garage, check for tire tracks on concrete. Stop, take a flashlight, and look for prints.
In some cases, footwear prints and tire marks might be your only evidence. If you rush into a scene, you might miss this evidence, or even worse, accidentally ruin it. Make a point of training other officers and first responders in techniques for locating footprint and tire track evidence. There’s nothing more frustrating than arriving at a scene and identifying dust footprints, only to discover that they’re ruined because someone walked on them.
Before casting, thoroughly photograph the evidence. That way, you’ve protected your evidence in case anything happens during processing. When photographing, remember to use a scale for close-ups and to shoot at a 90º angle. Also remember that the photos need to establish location and detail, so take overall, medium, and close-up shots. For example, if you find footprints under a flower box at the side of a house, take overall shots that show the footprint next to the house; medium range shots that show the window box and footprint; and close-up shots that show details of the footprint. Don’t be afraid to take a lot of photos; with digital cameras, you can easily take as many shots as you need.
In most cases, dental stone is the best material for casting footprints. In the past, we used Plaster of Paris, but dental stone is superior. It’s easy to use and strong; it’s also readily available through dental supply stores. Since dental stone is sold in large, 25 lb. containers, your best bet is to store smaller amounts in gallon-sized Ziploc plastic bags. When you’re at a crime scene, mix the stone with water in a separate container or right in the Ziploc bag. Add approximately 6–10 ounces of water for every two pounds of dental stone. Mix thoroughly. The consistency should be similar to pancake batter.
A casting frame can be used to place around the area to cast, but is not always necessary. The casting frame will keep the casting material in a specific area. Once you have the dental stone mixed, pour it slowly and carefully. Start from a point outside of the impression and let it flow into the impression and cover it. The cast should be about 1 inch thick. Before the cast sets, use something sharp to etch identifying information such as date, case number, item number, and officer’s initials, onto the surface. Allow the cast to dry completely before attempting to lift it. In moderate temperatures with low moisture, the cast should dry in about 30 minutes.
Extreme temperature affects casting. The warmer it is, the faster it sets. In extreme heat, the cast will set quickly—a matter of minutes. Make the solution a little thinner than you normally would when dealing with extreme heat. The thinner solution will allow you to get the details you need in the cast. Extreme cold requires a different procedure. Place approximately 2 cups of dental stone in a Ziploc bag. Add a tablespoon of potassium sulfate to the dental stone and mix the dry powders. The potassium sulfate accelerates the curing process. Once you pour the cast, allow it to thoroughly harden or freeze before lifting, usually about 1 hour. After lifting the cast, turn it upside down and let it thaw indoors overnight.
Once you have a solid foundation in casting, you’ll be ready to tackle more challenging situations. In the next issue, I’ll discuss casting prints in sand, water, and snow; I’ll also show you how to deal with prints found on tricky surfaces such as inclines and concrete.
Dick Warrington is in research and development and a crime scene consultant and training instructor for the Lynn Peavey Company. firstname.lastname@example.org