Casting in snow
Taking footwear and tire impressions
By Michelle Perin
The young man snuck quietly around the corner of the house, his breath coming out in puffs, warmth fighting to retain its heat in the sub-zero temperatures. Wiping the windowsill off with a gloved hand, he patted his pocket reassuring himself of the jewelry nestled inside before he made his way into the darkness, soft crunching the only indicator he was outside the single-story ranch-style home that was not his. Disappearing into the darkness he inhaled, deeply satisfied he had accomplished his task and grateful for the distance he put between himself and the house. What he didn’t realize was that he had left something behind. Under the window in the snow rested a perfect size '9' impression of his Xtratuf boot.
Footwear and tire tracks are two common pieces of evidence left behind at a crime scene, especially when there’s snow. Although snow is a perfect medium, many complications exist with locating and collecting forensic evidence. “If you live in a place like I’m at you have snow more than half the year,” explains James Wolfe, a current Alaska certified police instructor who retired from the Alaska State Crime Lab-Anchorage. “Up here they will deal with it more than any other impression. If you’re in an area without snow the problem of finding three dimensional impressions is low when you’ve got a lot of concrete. But, when it snows you have them.” Because of the rich opportunity to gather impression evidence when it snows, it’s important to think about the techniques for locating and collecting footwear and tire tracks for when it does.
Value of track evidence
In his law enforcement training, “Documenting and Collecting Snow Impression Evidence,” Wolfe looks at an investigator’s point of view and asks, “Is it really worth my time?” His answer is absolutely. Track evidence provides immediate feedback at the scene to the investigator about movements of suspects, victims, witnesses and vehicles, number of suspects and types of shoe tires and vehicles to search for.
If known, laboratory comparison of shoes and/or tires can provide important associative evidence, including positive identification or exclusion. Footwear and tire tracks have unique characteristics and snow is a great place to gather them. “It’s one of the most challenging surfaces yet it is a great opportunity to have evidence,” explains Lesley Hammer, Forensic Examiner, Hammer Forensics. “When it snows you get a fresh surface. It’s like a blank canvas.” Other surfaces, such as gravel don’t leave the same quantity or quality of impressions. “It’s difficult in snow to not leave an impression,” she says. Regardless, snow comes with its own set of challenges.
“Snow has its own challenges in a bunch of different areas,” explains Wolfe. For example, snow is transient. Even in areas where several feet exist, snow comes and it goes. At any moment, a footwear or tire track impression can melt or be filled with more snow. Another challenge is the weather itself. “It’s pretty inclement,” states Wolfe. “It’s cold and (officers) don’t want to spend a lot of time outside photographing and casting impressions.” Hammer agrees, explaining she’s often collecting impressions in temperatures ranging from 20 to 40 below zero. Another challenge is snow’s consistency and color.
“We talk about snow transforming,” says Wolfe. “There are different types of snow ranging from real fine grain melting snow to coarse grain non-melting snow and this may affect how you photograph and how you cast the impression. Color also complicates a snowy scene. “It can be more difficult to photograph because it’s white, but also it reflects,” says Wolfe. “You’re trying to photograph something that is made up of clear particles. You get small mirrors reflecting back at you.” In winter, a lot of flat white without contrast complicates the location and documentation of snow impressions. Lighting and coating techniques have to be used to bring up the contrast explains Wolfe.
Recognizing and locating
When responding to a crime scene in snow, officers need to be thinking about what trace evidence might be present even before they arrive. Understand you might drive right over the evidence explains Hammer. Investigators should ask themselves, “What impression might be part of the scene versus part of the first responder effort and protect those,” says Hammer. Once on-scene, all the photography and casting needs to be determined quickly because of snow’s transient nature. “They really need to be evaluated right away,” Hammer explains. “(Snow impressions need to be) prioritized in the other evidence.”
Finding the tracks is important not only to place a suspect at the scene, but they can also lead officers to a suspect. “I had a series of burglaries and the same set of shoe impressions kept showing up in the snow,” says Hammer. “I’ve been involved in several cases where not just the impression but the track of the impression assisted. The officer could follow someone right to their home. In one case, the suspect stopped and changed shoes. He must have had another pair in his backpack. Then, he continued with a different shoe impression so that was obvious in the track.” Tire tracks can indicate what direction the vehicle came from and went. Once impressions are found, using an appropriate method of collecting them is important.
Collecting-photography and casting
“Always photograph and cast,” explains Hammer. “Sometimes one of those might turn out better than the other. Have it in your mind that you need to do both.” One nice thing about snow, states Wolfe is you can use paint to identify where the tracks are. “You can use different colors of paint for different trails,” he says. “You just circle the tracks.” Both Hammer and Wolfe recommend an investigator take lots and lots of photographs. One complication officers encounter is the lack of contrast. To mitigate this, shadows can be created. “You can spray paint the shadow of the impression,” says Wolfe. “We coat the entire impression with gray primer and then use the oblique light source to bring up the details of that impression. You’re coating all the crystals so you see the surface of the snow.”
After photographing impressions, a cast can be made. Casting is important due to the difficulty in getting an accurate shot and the inability of photographs to capture three dimensional details. “If there is a cast then that size is the actual size of the impression,” explains Hammer. Casting in snow again has unique complications. “The problem with casting in snow is you have to have a casting material that can set and hold its shape before the snow melts,” says Wolfe. “The second big thing is the snow grain. You need to make sure that your material is not going to seep through like a snow cone.” Three main casting materials, two that have been around a long time and one that’s relatively new, can be used.
One of the two casting methods utilizes gypsum-based products, including dental stone which has been in use the longest. “Dental stone is more common,” explains Wolfe. “It’s cheap and most agencies have it because that’s what you use for casting mud and dirt tracks.” Using an accelerator to speed up the setting, Wolfe explains the process, “Prior to mixing, we cool the whole thing. We try to get the dental stone down below freezing. We add snow to the water. The idea is the mixture is below freezing so it won’t immediately melt the snow.” Because the consistency of snow can sometimes create the problem of dental stone running right through, many investigators use a layer of snow print wax to coat the impression. “That works well on really wet snow where plaster tends to run down really easily,” says Wolfe. “It works better in warmer temperatures.” In Anchorage, where the weather is colder, Wolfe states using gray paint to coat the snow or putting down some dry dental stone, called dry casting, is more effective. He advises to mix the dental stone thick and it will be less likely to seep through.
Another common method used is sulfur cement. “Some people like it and some people really don’t,” explains Wolfe. “It’s a form of sulfur that’s been around 60 or 70 years. It’s hot when it’s a liquid, then you let it cool until right when it’s about to harden and you pour it in.” Although you’re pouring something hotter than water into the snow impression, the sulfur hardens and makes a shell. “The snow melts but it’s hardened and you have the impression,” he says. Some of the drawbacks to this method are it smells like sulfur, there are safety issues because officers and technicians have to wear protective gear, like gloves and a respirator and requires extra equipment. Regardless, Kathi Young, Forensic Technician II, Alaska State Crime Lab-Fairbanks, utilizes this method quite often. “I do it in all kinds of weather down to 50 below,” she states. Although Young also uses a lot of dental stone, when the weather turns nice she likes sulfur cement. “It was a sunny day,” explains Young in reference to a recent investigation. “This was for a gentleman accused of arson. The ice was thin and the sun was out. When I tried to use dental stone, I got mush.” Using sulfur cement can be difficult because of the need for equipment, like a propane stove says Young. “It’s challenging but it gets good results,” she explains. Because of the complications of this, as well as, dental stone, forensic experts are excited about a new kind of method.
“With dental stone, you mix it and pour it and it takes about 5 minutes or so,” explains Wolfe referencing a new gypsum-based product from Kjell & Jens Carlsson. “SnowStone starts to gel in about 45 seconds and does its initial set in about a minute. This is a technique we’ve been looking at up here because it’s quick.” Young just completed an experiment for validation of SnowStone and was impressed. She used it in weather up to 40 below. “I’m excited to use the plaster cast this winter,” she says. “I enjoy it. It’s going to be faster than the sulfur cement. It’s faster and faster is always good when it’s really cold out.”
With three materials to choose from, agencies can decide what works best for them. “All three techniques can give you a decent cast,” explains Wolfe. “Some work better in certain types of snow conditions.” The most important thing for agencies to do is to practice. “It’s really beneficial to practice the techniques,” explains Hammer. “You can read about them and you can learn them but when it comes to a crime scene it is not the best place to learn it. You’ll approach it with more confidence and success if you take the time to practice the photography and especially the casting before you’re using it at the scene.”
Wolfe gives some additional advice on how to take the time do locate and document impressions well. When photographing, make sure the scale is at the right level. If you don’t use a tripod and camera, which is optimal, hold the camera right above the impression. Utilize placards and clear marking after isolating footwear and tire impressions. “In snow, you know there’s got to be some kind of track,” he states. “So take the time to look for those.” Making the time and having the passion to look for and collect snow impressions can make the difference in a case. “In snow, you have a lot of opportunity to get information,” Wolfe concludes. So when snow comes, be ready and when the suspect leaves his or her mark, locate it, collect it and get put the bad guy behind bars.