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In the last issue, I reviewed the basics of casting footwear and tire track evidence. I discussed ways to identify and photograph evidence and the materials and procedures used for casting. As I mentioned in that article, footwear and tire track evidence can be essential to your case, but it’s often overlooked. In some cases, officers identify the evidence but figure they can’t do anything with it. This is especially true when you have extreme weather or difficult surfaces. Those situations present more of a challenge, but you can still get the job done. In this issue, I’ll give you some techniques for casting beyond the basics.
Sand or Loose Soil
Prints left in sand or loose soil can be cast with dental stone. However, sand or soil will stick to untreated dental stone when you pour it, ruining the impression. To avoid this problem, you need to create a barrier that will protect the detail. You can purchase a silicone release agent for this purpose; hairspray also works. Remember to photograph all evidence with appropriate lighting, scales, and labels before you begin working. That way, you’ll have a record of it if the worst happens.
Photograph the print. Spray the silicone release agent, or hairspray, over the print. Don’t get too close or you’ll disturb the print. Once the spray is set, place a frame around the perimeter of the print. Go about a half inch larger than the footprint. Carefully pour the dental stone over the print and let it set.
Imagine finding footprints in mud on an embankment. How will you get the casting material to stay on the incline? The trick is to build up the impression so the back and the front are at the same height. I’ve used combinations of rock, mud, and dirt to build retaining walls at the lower end of the impression. Follow the usual process for using dental stone. Remember to spray the impression first to create a barrier. Also note that the impression may take longer to dry because it will be thicker in places.
Casting wet tire tracks and footprints off of concrete can also be difficult, but it can be done. Keep in mind the properties of the surface. Prints will seep into concrete and become distorted; if the prints are in the sun, they’ll eventually evaporate. To avoid losing evidence, act quickly but carefully.
After you photograph the prints, dust them with magnetic powder. The powder will develop the print and preserve it at the same time. The process is like fingerprinting — the powder makes the print darker. The water in the print is then like a contaminant. Once you apply the powder and develop the print, photograph the developed print, then use dental stone to cast it.
Have you ever found a print at the edge of a pond or some other body of water but assumed you couldn’t cast it? Well, that’s not the case. You can even cast a print under a couple of inches of water. After you photograph the print and the area, place a casting frame around the print. The frame will help keep the powder from washing out. Fill a flour sifter with dental stone. Hold the sifter over the frame and allow the stone to filter into the print. Once you get a good layer of stone in the frame, it will set up This process takes time, but be patient, and you’ll get good results.
Snow prints are fragile, of course, but you can get good impressions. The first option involves using dental stone, but you’ll need to make a few adjustments to get it to work in these conditions. Again, photograph the impressions. Then, spray the impression with gray primer if the snow is frozen, or with Snow Print Wax if the snow is soft or slushy. The Snow Print Wax builds up the impression to keep it from collapsing once you pour in the dental stone. Prepare the dental stone by placing approximately 2 cups of dental stone and 1 tablespoon of potassium sulfate in a Ziploc bag and mix together. The potassium sulfate will accelerate the curing process.
Place the bag with the dental stone mixture on the snow and allow it to cool to the temperature of the snow. Cool the mixing water by adding snow until a slight amount of slush is present. Add the cold water to the cold dental stone and mix to a consistency of thick pancake batter. Try not to get ice crystals in the mix. Pour the dental stone mixture into the impression, being careful not to damage the impression. Allow the impression to thoroughly harden or freeze before lifting (about 1 hour). After lifting the cast, thaw it indoors overnight, keeping the cast upside down the whole time.
Another option for snow prints is liquid sulfur. Sulfur comes in pellets that melt at a low temperature. Melt the pellets in a pot over a heat source like a camp stove. Once the sulfur is melted, spray the print with grey primer and then carefully pour in the sulfur. The cast sets in a minute. Sulfur gives great results, but it’s less convenient to use than dental stone since you need a heat source and extra container.
Sometimes you arrive at a crime scene and find ideal conditions — 70? and sunny, with perfect prints left on a nice flat surface. Most of the time, though, you’re going to have to deal with situations that are far from optimal. If you think it through ahead of time, you’ll be ready for those challenges.
Dick Warrington is in research and development and a crime scene consultant and training instructor for the Lynn Peavey Company. email@example.com