The Handy Forensic Science Answer Book by Patricia Barnes-Svarney

Author:Patricia Barnes-Svarney
Language: eng
Format: epub
Publisher: Visible Ink Press
Published: 2018-03-20T16:00:00+00:00

IN THE CRIME LAB: TOOLMARKS, FIREARMS, AND BALLISTICS

DEFINING TOOLMARKS

What are toolmarks (also seen as tool marks)?

Toolmarks are striations or impressions made by the forceful contact between two objects that differ in hardness. For example, in terms of tools and toolmarks, a hard crowbar will mark the softer wood of a door that is pried open. In terms of firearms, the barrel of a firearm is hard and leaves minute impressions on the softer bullet as it passes through the muzzle after firing.

What are two common classes of toolmark characteristics?

Experts often divide toolmark characteristics into two classes. The first class of characteristics are those that are common to a group of objects, such as a hammer with its distinctive shape and typical size. The other class is the individual characteristics, or those that are unique to a certain object, such as gouges or scratches from use or the manufacturer. These tools can also be identified based on such things as paint on a handle, which may match paint at the crime scene.

What are two general ways of looking at toolmarks in forensics?

Forensic experts look at toolmarks in two general ways. The first way concerns the imperfections, patterns, and marks on physical tools used at a crime scene, such as from a burglary, theft, or vandalism. The tools’ unique marks are formed naturally during the manufacturing process of the tool and can change as the owner uses the piece, further making the marks unique to that tool. In a situation such as the forcing open of a window, forensic experts can analyze the unique marks made by a tool to possibly determine what type of tool was used, and if the tool is found at a scene, they can match the markings of the tool itself to the toolmarks found, such as in a doorframe. The second way of looking at toolmarks is in association with firearms. Such marks as striations are usually found on a bullet or casing made as the projectile travels from the breech through the barrel of a firearm. For example, a toolmark forms when a weapon’s firing pin hits the softer primer material of a cartridge. This forms a pin impression on the cartridge. (For more about toolmarks and firearms, see this chapter.)

How are toolmarks in tools analyzed?

Toolmarks from tools are usually found as negative impressions (almost like stamping by the tool) or as an abrasion mark (as it slides by where the tool was used). Some of these patterns, striations, and other marks are visible to the naked eye, while others are only seen under a microscope, with most patterns and marks made by the tool’s cutting surface.

What are some of the causes of a tool’s toolmark impressions?

Toolmark impressions are made in several ways. For example, static marks are made when a tool is pressed into a softer material, such as a crowbar used to open a window that has a wooden sill. Cutting marks are made when pressure is applied on both sides of an object, such as cutting a screen with scissors.

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