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For the past several years, I’ve taught a class on developing and lifting prints off unusual surfaces. This class is very popular because it shows Crime Scene Officers that the only “surfaces” where you can’t get prints are air and water; everything else can be processed with the right products and the right technique. In this issue, I’ll provide an overview of advanced ways to deal with unusual surfaces.
Some of the most challenging surfaces to work with are multi-textured or multi-contoured. If you find evidence on one of these surfaces, you need to be prepared. Otherwise, you may have to settle for simply photographing the evidence instead of casting impressions and lifting latent prints. Let’s take a look at a the products and techniques that you can use on these kinds of surfaces.
Tool marks, bite marks, other kinds of impression marks, and latent fingerprints are often found on multi-textured or multi-contoured surfaces. The AccuTrans system, which uses polyvinylsiloxane (PVS), is ideal for this type of evidence because it is easy to use, provides great detail, and sets relatively quickly. You’re also likely to get better results because the system mixes the chemicals for you. AccuTrans works like a caulking gun. Simply insert a double cartridge, add the special mixing tube, then squeeze the trigger. The mixing tip combines the catalyst and the hardener, so the PVS is properly mixed when it comes out. Apply the PVS directly to the area you want to cast, then use the tip to stir the compound to remove any air bubbles. If applying over a latent print, be careful not to allow the tip to touch the print directly, or you could ruin the print. The AccuTrans PVS comes in different colors. White is used primarily for dusted fingerprints, while brown is for tool and bite marks and gun barrels. The clear version offers an advantage for lifting prints with black powder because the lifted prints can be added directly into AFIS without needing to be reversed.
Lifting prints from multi-curved surfaces such as lightbulbs or doorknobs can be especially difficult. Polyethylene tape works well on these surfaces because it stretches and conforms to the shape of these surfaces without distorting the latent print. It is also easy to use. Cut off the desired length of tape. Press it directly on the print with your thumb. Smooth the tape out from the center.
Trying to lift latent prints from textured surfaces can seem impossible. Lifting the powdered print with standard lifting tape won’t work well. DIFF-Lift Tape, which is 20 times stronger than regular tape, gives much better results. DIFF-Lift molds itself to textured surfaces to get all of the details of the powdered print. It can be used to lift prints from vinyl car dashes, pebble-grained leather, textured plastic, metal, paper, and wood. I’ve even used it to lift prints from a paper towel. DIFF-Lift is used just like standard lifting tape. Place the tape over the print. Use your index finger to press firmly on the dusted latent print only. You should be able to feel the texture of the surface through the tape. Lift the print with the tape and place it on a contrasting backing card. Note that the DIFF-Lift needs to be cut with scissors or a sharp knife. After cutting, fold the end over on the roll, so it’s ready for the next use.
Prints found on the adhesive side of things like duct tape, masking tape, and mailing labels, can be developed and lifted, but it’s often quite difficult to do so. The key is to use a specially designed product, called an adhesive side powder, for the job. The adhesive side powder is mixed with equal parts water and dispersing agent until it reaches a consistency similar to thin paint. The mixture is then applied to the adhesive side of the tape or label using an all-purpose brush. After the solution sets for 10–15 seconds, it’s rinsed with water. If it sets too long, it will fill in the background.
When you find dust fingerprints and footprints, collect them with an electrostatic dust lifter such as the PathFinder. The PathFinder is a wireless device that can lift prints from most surfaces, including wooden window ledges, carpet, tile, and upholstery. To use the lifter, place the dark side of a piece of mylar film on top of the print. Apply a charge to the shiny side of the film. The charge will cause the dust print to adhere with a static charge to the film. Use a foam paint brush to lightly smooth out any air bubbles. Remove the lift film. You should have lifted your print. Be sure to photograph your lifting with side lighting. Because of the static charge, the lift will attract dust and other items to it. Protect it by placing it in a shallow box (such as a clean pizza box).
Most people think you can’t get prints from a wet surface, but you can if you use Small Particle Reagent (SPR). SPR is like liquid fingerprinting powder and can be used on non-porous surfaces. Spray SPR onto wet evidence then rinse with water. The reagent adheres to the latent print. Most people think you then have to air dry the print before you can lift it. Again, not true! Simply place the lifting tape over the latent print. Push in at the center and work out to remove excess water. I use a squeegee, but you can also use a business card or something similar.
Like anything else, learning these techniques takes time and practice. But once you gain these techniques, you’ll be prepared to handle some of the most complicated crime scenes.
Dick Warrington is in research and development and a crime scene consultant and training instructor for the Lynn Peavey Company. firstname.lastname@example.org