Toolmark Impression Evidence STEM

Impression Evidence Any mark made in a softer surface by a harder implement or “tool” – tracks in mud – pry mark in a door frame – fingerprint in wax – bite mark – markings found on bullets & cartridge cases

Imprints Tire tracks & shoe prints most common kinds of imprint evidence found in a crime lab Mass production of tire treads & shoe soles produces only class evidence – tread pattern – trademark logos Individualizing potential comes from random characteristics acquired by use & wear

Toolmarks Any implement can be a tool for the purpose of leaving a mark – crowbars – tire irons – screwdrivers – metal pipes – heavy flashlights – fingernails

Types of Impressions Impressions made by striking, pressing or imprinting – leave a single point impression impressions made by cutting, sawing, dragging or prying – leave characteristic striations indicative of movement across a surface

Definition: Firearm & Toolmark Identification An empirical comparative analysis that can determine if a mark was produced by a particular tool.

Definition: Class Characteristics General and/or measurable features of a specimen which indicate a restricted group source. They result from design factors, and are therefore determined prior to manufacture.

Definition: Subclass Characteristics Features that may be produced during manufacture that are consistent among some items fabricated by the same tool. These are not determined prior to manufacture and are more restrictive than class characteristics.

Example of Subclass

Definition: Individual Characteristics Distinctive marks or features produced by the random imperfections or irregularities of tool surfaces. These characteristics can be used to individually associate a tool to a toolmark.

How are individual characteristics produced? These random imperfections or irregularities can be produced by: Manufacture Wear from Use Wear from Abuse

Striation Patterns Pattern of striations produced by the edges of a tool can be traced back to microscopic irregularities in the sharp edges or surfaces intended for cutting

Striation Patterns Mass machine manufacturing of tools causes them to be very similar when they leave the factory – the greater the wear patterns the higher the potential for individualization

Striation Patterns Tool marks are often compared with another mark made by the suspect tool rather than the tool itself – suspect tools are never fitted into the tool mark – generation of the reference mark becomes a potential source of error different marks may be produced depending on the angle & amount of pressure used the substrate used may alter the appearance of the tool mark

Striation Patterns A reference mark made by a tool dragged at various angles

Tool Mark Examination Magnification is usually needed to reliably compare the minutiae of marks & striations Comparison Microscope – Allows two samples side by side in the same microscopic field