12.1: Introduction

Your author likes to refer to impression evidence as evidence that is the most ignored or overlooked evidence in the crime scene for the simple fact that it takes a trained eye to observe, and preserve, impression evidence before it is destroyed. Regrettably, the average patrol officer, firefighter, or emergency medical technician either lacks sufficient training, or has other priorities, to appreciate the importance of impression evidence to solving the crime. Impression evidence is the evidence in the scene that can link a suspect, or the tool the suspect used, with the crime scene. Impression evidence can be found in the form of footwear impressions, tire tread impressions, tool marks, ballistics, bite marks, as well as foot, hand, and fingerprint impressions.

As mentioned in previous chapters, impression evidence is designated either as having class characteristics or individual characteristics. Class characteristics are common to anything that is manufactured using mass-production. One pair of tennis shoes made by the same manufacturer using molds and production techniques that produce identical tennis shoes with the same material, patterns, size, and shape, will have characteristics that are common with the next pair of tennis shoes that come off the assembly line. Once the tennis shoes are purchased and worn, irregular imperfections resulting from wear, use or misuse, and damage will provide characteristics that are unique or individual to that pair of shoes. When a hard object is impressed or forced into a softer surface, the resulting impression may demonstrate the class and individual characteristics of the hard object. Shoes and vehicle tire tread impressions will provide size and pattern information as a class characteristic, as well as the wear, damage, or imperfections and defects caused by normal or unusual usage. Molded or plastic fingerprint impressions left in a soft material such as a candle, mud, or other soft medium like clay, will provide a mirrored image of the ridges, furrows, and minutiae that is unique to the individual who deposited them. Tools such as chisels, screwdrivers, saw blades, bolt cutters, knife blades, and hammers will leave the same class and individual characteristics when forced against softer materials in the form of compressed, scraped, or striated evidence. These marks can be matched to the tool which made them. Although the DNA collected from a bite mark may be of more use to identifying the individual who did the biting, some bitemarks left on skin or food items can be matched to available dental records by a skilled forensic odontologist. Most modern firearms are manufactured using steel alloys for those portions of the weapon that are designed to withstand the extreme pressures resulting from the discharge of a cased cartridge using modern propellants. Brass and aluminum cartridge casings are softer than the metal breech, breech face, or cylinder of the firearm. When discharged, the casing expands within the breech, inner cylinder wall, and against the breech face. Small imperfections in the metal surfaces of these pressure chambers will be imprinted into the sides and back (headstamp) of the casing. As the casing is extracted from the breech or cylinder, the same imperfections will drag or scratch across the casings. The firing pin is also much harder than the soft primer cap on centerfire cartridges or along the edge of rimfire cartridges, thus leaving a distinct impression of the firing pin as well as the mark of the side of the firing pin as it is retracting into its housing. This impressed or striated information that has been transferred to the shell casing can be compared to casings fired from a firearm to determine if there is a common link. Bullets fired through rifled barrels will similarly have individual characteristics that can be matched to one firearm.

Now that we have established a basic understanding of impression evidence, let us take a deeper look at each discipline excluding, of course, skin-print impressions that have already been addressed in previous chapters.