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Forensic Science: Fundamentals & Investigations, Chapter 16 1
Chapter 16 Tool Marks By the end of this chapter you will be able to:
o Describe the three major types of tool mark impressions
o Summarize the steps of a tool mark examination and analysis
o Explain how technology is helping in criminal investigations
o Match tool marks with the instrument that produced them
All Rights Reserved South-Western / Cengage Learning © 2012, 2009
Forensic Science: Fundamentals & Investigations, Chapter 16 2
Introduction
o Tool mark—any impression, abrasion, or cut made when contact occurs between a tool and an object
• An example of physical evidence • Even mass-produced tools have minor
differences • The impressions can link the tool to a crime
scene and potentially to the owner
Forensic Science: Fundamentals & Investigations, Chapter 16 3
Tools and Crime Scenes
o Tools: • increase our ability to handle manual tasks, but • can also be used in crimes
o How can a tool used in a crime lead investigators to the criminal?
o Why is ownership of a tool used in a crime circumstantial evidence?
Forensic Science: Fundamentals & Investigations, Chapter 16 4
Tool Mark Impressions
Indentation Marks o Result when a tool is pressed against a softer
surface o Tools usually leave distinctive marks o The hardness of a tool influences the
resulting marks left in the softer object o May indicate the size of the tool used in a
crime
Forensic Science: Fundamentals & Investigations, Chapter 16 5
Tool Mark Impressions
Abrasion Marks o An object’s surface can be ground or worn
away by a tool o The harder object causes abrasions on the
softer surface o Indentation and abrasion marks sometimes
occur at the same time
Forensic Science: Fundamentals & Investigations, Chapter 16 6
Tool Mark Impressions
Cutting Marks o Edged instruments can penetrate a softer
object and separate it into parts o Cut marks are produced along the edge as a
surface is cut o Different types of saws and blades leave
distinct cutting marks
Forensic Science: Fundamentals & Investigations, Chapter 16 7
Tool Mark Impressions
Examples of Cut Marks on Bones
Forensic Science: Fundamentals & Investigations, Chapter 16 8
Tool Mark Examination
o Some experts specialize in tool mark investigations
o Evidence can include: • tool marks at the scene • the tool if left behind
9
Documenting the Evidence
o Collect tool mark evidence when possible
• Cut tool mark out of door or door jamb
o Photograph the evidence with a measuring device to show scale
• Best way to preserve evidence
• Oblique lighting highlights details with shadow
• Magnesium smoking also highlights detail
o Measure the size of the impression
Forensic Science: Fundamentals & Investigations, Chapter 16 10
Documenting the Evidence
o Castings preserve tool mark impressions
• silicone or rubber-based casting materials
o Cast impressions retain the unique indentation marks made by a specific tool
o Takes 10-30 minutes to dry and solidify.
Forensic Science: Fundamentals & Investigations, Chapter 16 11
Documenting the Evidence
o Dust for fingerprints before applying casting material • Use magnetic dusting powder and silicone material
o The size of the impression should be measured and recorded.
Forensic Science: Fundamentals & Investigations, Chapter 16 12
Collecting and Preserving a Sample
o Correctly label evidence o Wrap small objects with clean paper and
place them in small containers or plastic bags
o Pack large objects in cartons or boxes o Record—who, where, when, and why o Maintain the chain of custody
Forensic Science: Fundamentals & Investigations, Chapter 16 13
Analyzing Tool Mark Evidence
o Laboratory tool mark analysis identifies: • major characteristics defining the type of tool used
in a crime • unique characteristics that might distinguish
between the same kinds of tools o Forensic comparison microscopes examine
tool mark characteristics that match a suspect tool
Forensic Science: Fundamentals & Investigations, Chapter 16 14
New Technology in Tool Mark Identification
Courts now require more than visual pattern comparisons
o Tool mark databases (with images acquired by forensic comparison microscopes)
o Algorithms to statistically analyze tool mark patterns
o Scanning tools measure the depth or height of tool marks to create a 3D image.
Forensic Science: Fundamentals & Investigations, Chapter 16 15
Tool Mark Evidence in the Courtroom
o The tool mark witness prepares a written report to present to a jury.
o When available, provide: • Original evidence • Castings • Magnified images of tool mark comparisons
o Such evidence may link a series of crimes
Forensic Science: Fundamentals & Investigations, Chapter 16 16
. . . . . . . . . . . . . . . . . Summary
o Tools have major and minor surface differences o Tool marks are indentations, abrasions, or cut
marks o Impressions made by tools could link them to a
crime scene and ultimately to the owner o Tool mark evidence should be photographed,
documented, and collected or cast