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TOOL MARK COMPARISONS

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p567-568

2026-09-05

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TOOL MARK COMPARISONS

The potential sources for tool marks are many, and some are surprising. Metal tools are made by a variety of methods, but most are finished in a way that leaves microscopic striations on their working surfaces. A tool may leave class characteristics that may help to identify what kind of item it is: An ice pick will leave different markings than would a flat-head screwdriver, for example (see Figure 1). The types of markings left will depend not only on the type of tool, but also how it was used, the angle of contact, the force of contact, and what was contacted, among other factors. It is important for the tool mark examiner to have a foundational knowledge about how various tools are made and machined. Bullets travel down the barrel of a firearm in nearly the same way each time; the same is not true of a tool mark. The forensic tool mark examiner must pay close attention to the potential orientations of the questioned tool; otherwise, the test marks may not be comparable with the questioned marks (see Figure 2). Like bullets, however, the questioned tool cannot be compared directly to the mark it may have left. Crime scene personnel or the tool mark examiner may take casts of the tool mark(s) for comparison purposes. These casts are lightweight, easy to handle, and easy to store; fine-grained polymer materials are sold by most forensic science supply companies. Dental stone is also a favored medium for making tool mark casts.

FIGURE 1 The machining of metals leaves microscopic striations that can be transferred to a softer surface, such as a screwdriver prying open a wooden window casing. The class characteristics can also be useful in identifying (but not individualizing) the tool used.

FIGURE 2 Examples of tool mark striations. (a) A pair of bolt cutters was used to cut a lock shackle. The comparison microphotograph shows the evidence lock shackle on the left side and the test cut produced by the bolt cutters in a piece of lead on the right. (b) An unusual example of a tool mark comparison is shown on the left where a knife edge from a suspect’s knife was compared and matched to a piece of rib cartilage from a victim. On the left is a silicone rubber cast of the cut in the victim’s rib cartilage compared with a cast of a test cut produced with the suspect’s knife using a plastic material (Dip-Pak).