Often when CSOs get called to a crime scene, we are so focused on searching for fingerprints and DNA evidence that we forget to look for other evidence that suspects leave behind. But impression evidence from tire tracks, footprints, tool marks, extruder marks on different casings, etc. can be just as important in making your case. When you do find impression evidence, you need to carefully preserve it so that it can be sent to the lab for analysis and compared to the original item.

Take the time when you first arrive at the scene to check for tire tracks, footprints, and tool marks. A good flashlight often comes in handy, especially for finding footprints. Next, before you do anything else with impression evidence, you need to photograph it. If something goes wrong during the processing and you forgot to take photographs, you’ll be left with nothing. Remember to use the proper labeling, lighting, and scale with the evidence. 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, always insert the scale at the same depth as the track impression.

Now, when we talk about impression material, we think about casting. A number of products are available for casting larger items like tire tracks and footprints. In the past we used Plaster of Paris because it gave good results and was inexpensive, but it had some drawbacks. You had to use frames or it would run all over the place, and besides being messy, it’s not very strong. It had to be about two inches thick and reinforced with sticks or screens. Even with reinforcements, it still might break.

Today, we use dental stone instead of Plaster of Paris. Dental stone is stronger, so the casts can be thinner and don’t need reinforcement. Dental stone is also cost effective and readily available from dental supply companies. What’s more, it’s easy to use, making it ideal for taking out to the field with you. You simply place two pounds of dental stone in a Ziploc bag, thoroughly mix it with water, then pour it onto the impression evidence. And if you do break the cast, you can glue it back together!

In many cases you’ll need to cast smaller items like tool marks, firing pin marks, breech face marks on weapons and cartridge casings, and latent finger prints. For this evidence, you need sharp, well-defined impressions which can be examined under a microscope and compared against the actual tool. Dental stone has the wrong consistency for this type of evidence.