A trace of evidence

New material aids investigators in lifting trace evidence


     It's long been known trace evidence, which contends that every contact no matter how slight will leave a trace, can help solve a case. This minute evidence is normally left by objects or substances coming into contact with one another, leaving a small sample on the contact surfaces. Today's investigators rely on many types of trace evidence, but some of the most commonly and successfully used are fingerprints and toolmarks.

     To successfully use this evidence, an efficient means of collection must be used. A new casting material from Ultronics Inc. incorporates ease of use, eliminates mixing and applies simply. Researchers set out to examine whether this casting material is a reliable, sensible product for forensic investigators by testing its performance on a variety of surfaces.

What is AccuTrans?

     Casting products traditionally are mixed from a tube then applied to a surface, and variables affecting their use typically include temperature, odor and hardening times. AccuTrans Auto Mix Dispensing System, from the Cuyahoga Falls, Ohio, company, utlilizes a new material — polyvinylsiloxane — which assists the investigator because it does not require mixing and is easy to use. In addition, AccuTrans is available in a transparent material, alleviating photographic reversals.

     This casting silicone is applied with an extruder gun. The material is flexible and does not distort the image. Once the impression is dry and lifted, it cannot be smeared or smudged, making the lift permanent. The impression is 1:1 and can be placed under a scanner or camera and searched in the Automated Fingerprint Identification System (AFIS) in either the local, state or federal fingerprint data banks.

     It is available in white, brown and a transparent material. The transparent version allows for instant print comparison without reversing the image. The transparent silicone enables an investigator to place the lift on any color background, allowing photographic image capture. In fact, it works well for all photographic purposes.

     The main material in AccuTrans, polyvinylsiloxane, can be used on curved surfaces, and flat, horizontal or vertical planes. When used on vertical planes, only a small amount of silicone is needed. The material will smooth itself as it runs down over the area. This casting silicone also can be used on smooth or rough surfaces, human skin and blood evidence. The silicone can even be used to make an impression inside a gun barrel by using a light release agent before application.

     AccuTrans has a boiling point of less than 150 degrees Fahrenheit (F), a relative density of 1.2g/l, and is insoluble in water and soluble in methyl ethyl ketone.

     The compound doesn't irritate the skin, and wipes up easily. However, as a general precaution, it is recommended that users do not eat, drink or smoke when handling, wear gloves and avoid contact with eyes and skin.

Using AccuTrans

     It should be noted that before treatment with silicone, forensic investigators should dust rough surfaces with magnetic fingerprint powders to bring out the fingerprint ridges. (See Figure 1 below.) Magnetic powders come in a variety of colors, which is beneficial for surface contrast and photographic purposes. Magnetic powders contain ferromagnetic particles, and are applied using a magnetic applicator or wand. Excess powder is easily removed by moving the applicator back over the print. However, magnetic powders should not be used on metallic surfaces.

     At the beginning of the application, users must squeeze out a small amount of AccuTrans to properly blend the contents in the mixing tube. Doing this prevents large air bubbles in the casting. If small air bubbles are present in the cast, they do not interfere with ridge detail and usually occur in the beginning of the application, or if the user stops and starts or removes the extruder's tip. The extruder gun allows the material, and not the tip of the gun, to come in contact with the fingerprint, thus preserving the print's integrity.

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