Impression Evidence: The Right Materials for the Job
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.
One of the most popular products for these castings is Mikrosil, which is durable and provides good detail. Mikrosil comes in brown, white, grey, and black, with each color used for different purposes. Brown is used for processing tool marks; the experts at the lab prefer it for the contrast it provides. Grey works well when you need good contrast from a light colored surface and shadows created by side lighting. White and black allow you to create casts of developed latent prints. If you used white powder to develop the print, use black Mikrosil, and vice versa. White Mikrosil works well for fingerprints on difficult areas, such as textures or multi-contoured surfaces.
To prepare the Mikrosil, squeeze out equal lengths of catalyst and hardener, mix them together using a tongue depressor, and then spread the mixture over the impression. At room temperature, Mikrosil sets in less than ten minutes. If it’s very cold, it may take a couple of hours.
Another useful product is AccuTrans. This polyvinylsiloxane (PVS) comes in white, brown, and clear, and works well on tool marks, fingerprints, shell casings, gun barrels, and Styrofoam. AccuTrans is used in a dispenser gun with a cartridge. The mixing tip combines the catalyst and the harder when you squeeze the trigger, so the product is properly mixed when it comes out. The clear version offers an advantage when lifting prints done with black powder: once you lift the prints, they can be added directly into AFIS without being reversed. AccuTrans sets up in only two minutes in 90° weather and 32 minutes at 14°.
As time goes on, more products and techniques are sure to come our way. Stay current with advances in the field, and then follow through. Try out the materials and techniques before you get to the scene. Figure out what works best for you and your department, and put those items in your crime scene kit. Then the next time you get called to a scene, you’ll be ready to get the job done.
Dick Warrington is in research and development and a crime scene consultant and training instructor for the Lynn Peavey Company. firstname.lastname@example.org