Toolmarks (World of Forensic Science)
A toolmark is defined as the impression left by the contact of a tool (or a similar object) onto a surface. When the tool or object contacts the surface with sufficient force to create an indentation, the pattern of the tool is permanently reproduced onto that surface. Toolmarks examination is an important discipline of criminalistics. Its goal is to establish a link between a toolmark and the tool that created it. Such links are crucial in forensic sciences, as tools are often used in criminal activities, particularly in burglaries, and can help to identify a criminal. For example, when a burglar uses a pry bar to force entry into a house, the marks left by the tool on the door frame are direct evidence of the presence of that tool for that particular use at the crime scene. If the tool is found with, or near, a suspect, it permits the establishment of a link between the suspect and the crime scene. Thus, the recognition and collection of toolmarks at the crime scene and their examination at the laboratory are paramount.
Toolmarks bear two kinds of characteristics: class and individual. The class characteristics of a toolmark include the type of impression, its general shape, and its general dimensions. Class characteristics typically allow the examiner to determine what type of tool created the impression and how the mark was created. Conversely, they do not permit for the identification of the exact tool that created the impression. This means that if only class characteristics are available on a toolmark, it will not be possible to distinguish which tool, among a series of similar tools, made the impression. Individual characteristics, also called accidental characteristics, are the striations and small particularities exhibited by the tool that are individual to one unique tool. They consist of small, commonly microscopic, indentations, ridges, and irregularities present on the tool itself. For example, the tip of a screwdriver is never perfectly flat, but shows small ridges along its edge. These are created by the history of the tool such as its use and misuse, its cleaning, and its maintenance. These characteristics are the only ones that permit a formal identification. If such characteristics are present in the toolmark, it is possible to identify the actual individual tool that created the impression, even among a series of identical tools.
There are two main types of toolmarks that can be distinguished: slipped and molded impressions. The slipped impression occurs as the tool drags or slides across the surface. The resulting toolmark is a series of striations running parallel to each other following the direction of the drag. For example, such impressions are created by slipping a key across the door of a vehicle, by cutting with a knife (not used in a sawing motion) through a given material, or by cutting an electrical wire using a pair of lineman's pliers. The molded impressions are the result of the contact of a tool onto a surface with no lateral motion (no drag nor slip). The resulting toolmarks are a three-dimensional mold of the part of the tool that contacted the surface. Examples of such impressions are the leverage of a door from its frame with a pry bar, or the serial number stamped onto a fire-arm's barrel. Some toolmarks are made of a combination of molded and slipped impressions.
Toolmark examination is a term that includes a wide variety of impressions that are not necessarily directly related to tools but that are created via the same fashion and are, therefore, examined with the same techniques. A clear example is the impression left by a firearm's barrel onto a bullet or by the firearm onto the cartridge. These are a specialized category of toolmarks. Other examples include the impressions left by human teeth or even the impressions left by shoes or tires. Very often, the toolmark examiner is the person responsible for examining and rendering expert opinions on such impression's identifications.
The examination of toolmarks is conducted in different phases. First, the toolmark is observed, measured, and described. Second, a photograph perpendicular to the toolmark, is taken. This provides a permanent record of the class and some individual characteristics of the toolmark. Then, if the support onto which the toolmark is located cannot be collected as evidence, a cast of the toolmark is made. This cast is usually made with polymeric dental paste. When a tool is discovered and its class characteristics match the ones exhibited by the toolmark, the comparison process is started. Usually, the tool is observed and photographed. Then, comparison tool-marks are made with the tool on a soft material so that extra marks are not created on the tool. A comparison microscope is used to perform the comparison process. The incriminated toolmark is placed on the left side of the microscope and the comparison mark on the right side. If a match exists between the individual characteristics, the common origin between the incriminated toolmark and the tool is established.
SEE ALSO Casting; Impression evidence; Microscope, comparison.