Crime Science: Methods of Forensic Detection

6. Impression Analysis

IMPRESSION ANALYSIS Just as the markings left by firearms on bullets and shell casings are distinctive and the impressions of the bare hands and soles of the feet are unique, so may other mechanical markings and other impressions be individualized for forensic purposes. The familiar principles apply. Remember that no two things are precisely identical and that forensic comparisons involve matching features of the questioned or unidentified specimen with known standards. In this chapter we look at fabric prints, shoe and tire impressions, and tool marks. FABRIC PRINTS Fabric prints are sometimes encountered by the fingerprint expert in the form of glove impressions. With use, gloves become contaminated with substances such as dirt, grease, oils, and even sweat that enable them to leave their imprints, either latent or visible. They also may leave plastic impressions in materials like putty. Such impressions are developed, photographed, and collected just as if they were fingerprints because they may be their equivalents. Cloth gloves may have snags, tears, or holes that make them distinctive, along with imperfections in the weave pattern or other identifiable characteristics. Leather gloves may have wrinkle or crease formations where they do not fit properly or they may exhibit surface cracks, tears, or other imperfections or characteristics by which they may be individualized.1 In an English case, some unusual prints were found at the scene of a theft, and investigation soon led to a suspect who 6 148 IMPRESSION ANALYSIS 149 had in his possession a pair of pigskin gloves. The fingertips of the gloves were compared with the latent impressions recovered from the crime scene and showed sufficient similarities for a match.2 Developing glove prints requires special care because they are typically less strong than fingerprints and may be destroyed by use of too much powder. Therefore, they should first be searched for with the aid of a flashlight, laser, or other light source and then developed with sparing use of powder. Rather than lifting the print, the object on which it is found should, if possible, be taken to the lab for direct comparison with a suspect's gloves. According to one authoritative text: Comparison prints from the gloves of a suspect are best made on glass, which is generally the most convenient even when the original prints are on furniture. In certain special cases, however, it may be necessary to form a print on the same kind of material as that which was the scene of the crime. Where possible, such material should always be enclosed when a print and gloves from a suspect are sent for examination. Comparison prints should be made in a manner similar to the original ones. Thus if it is possible, in view of the placing of the original, to decide, for example, how the hand of the suspect gripped when making the grip, this information should be communicated to the expert so that the same grip can be used for the comparison print. Consideration must also be given to the degree of pressure that may have been used in forming the original print, and statements forming a guide forjudging the pressure should also be submitted. It is of great importance that neither too great nor too small a pressure should be used in making the comparison prints since their appearance is greatly affected by pressure.3 Clear prints may be difficult to make, but results may be improved by breathing slightly on the surface of the glove's finger. Caution should be used in applying powder, leather polish, or the like in order to avoid destroying subtle details.4 Apart from glove prints, other types of fabric impressions may be found at the scene of a crime under certain circumstances. One of these is the hit-and-run accident in which fabric impressions from the victim's clothes (along with actual fibers, blood, hair, etc.) may be left in the dust on the assault vehicle's bumper, grille, fender, or other area. A hydraulic lift should be used so that the undercarriage may also be examined.5 Whether latent, visible, or plastic, fabric prints should be treated in the manner recommended for glove prints. In photographing them, the camera should be at right angles to the surface to avoid distortions. Photographing should be done both with and without a scale, as discussed in 150 CRIME SCIENCE chapter 2 in the section on documentation. If such prints are...