Suspects often leave important evidence throughout crime scenes: tire tracks, footprints, tool marks, extruder marks on different casings, etc. Casting can preserve this impression evidence for comparison work and analysis at the lab. Since this evidence can be crucial for your court case, you need to know the proper way to handle it. While a thorough discussion would require a separate article for each type of evidence and product, I can help get you started with an overview of the products available to you.
Let me begin by reminding you that before you do anything with impression evidence, you need to photograph it. This is a key step that you can’t afford to miss. If something goes wrong during the processing and you forgot to take photographs, then you’ll be left with nothing. So get in the habit of taking your photographs first, and remember to use the proper labeling, lighting, and scale with the evidence. For example, if you’re working with tracks, you need side lighting, which creates shadows that provide 3-D depth and allows you to see the details of the evidence. Also, you should always insert the scale at the same depth as the track impression—otherwise, if you place the scale on the surface, you’ll throw off its accuracy.
A number of products are available for casting larger items like tire and shoe tracks. Today, the primary product used for casting is dental stone. In the past, Plaster of Paris was widely used because it gives good results and is inexpensive, but it does have some drawbacks. Besides being messy, it is not very strong, so it has to be about two inches thick and reinforced with screens or sticks. But even with reinforcements, it still might break.
Dental stone, on the other hand, provides excellent results and is durable, convenient, and cost effective. Because it is so strong, it only needs to be a half inch thick and does not require any reinforcement. It is also very easy to use. You can buy dental stone in a pouch that has a water bladder right there—when you’re ready to make the cast, you just break the bladder and shake to mix.
Dental stone is also versatile. It is the only product that works on concrete: you just apply powder to the track on the concrete, and then apply dental stone to make the cast. It can also be used in hot or cold weather, and with tracks in dirt, mud, sand, and snow. In some cases you do need to take an extra step to protect the impression before you begin casting with dental stone. For loose or sandy soil, you first need to spray the impression with hairspray to hold it in place. For snow or slush, you must apply Snow Print Wax™ first. These layers build up the impression and keep it from collapsing once you pour in the dental stone.
You can also cast prints left in snow with liquid sulfur. Sulfur comes in pellets that melt at a low temperature, so you first melt the pellets in a pot over a heat source like a camp stove. Once the sulfur is melted, you 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.