TOOL MARKS

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Fundamentals &

Investigations, Chapter 16

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WHAT ARE TOOL MARKS?

• 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

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WHAT ARE TOOL MARKS?

Types of Impressions

Indentation Marks

• Result when a tool is pressed against a softer surface

• Tools usually leave distinctive marks

• The hardness of a tool influences the resulting marks left in the softer object

• May indicate the size of the tool used in a crime

Forensic Science:

Fundamentals &

Investigations, Chapter 16

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WHAT ARE TOOL MARKS?

Types of Impressions

Abrasion Marks

• An object’s surface can be ground or

worn away by a tool

• The harder object causes abrasions on

the softer surface

• Indentation and abrasion marks

sometimes occur at the same time

Forensic Science:

Fundamentals &

Investigations, Chapter 16

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WHAT ARE TOOL MARKS?

Types of Impressions

Cutting Marks

• Edged instruments can penetrate a

softer object and separate it into parts

• Cut marks are produced along the

edge as a surface is cut

Forensic Science:

Fundamentals &

Investigations, Chapter 16

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WHAT ARE TOOL MARKS?

Examples of Cut Marks on Bones

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Investigations, Chapter 16

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Q: HOW DO WE DOCUMENT TOOL MARK EVIDENCE?

• Collect tool mark evidence when possible

• Photograph the evidence with a measuring device to show scale

• Measure the size of the impression

Forensic Science:

Fundamentals &

Investigations, Chapter 16

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Q: HOW DO WE DOCUMENT TOOL MARK EVIDENCE?

• Castings preserve tool mark impressions

• silicone or rubber-based casting materials

• Cast impressions retain the unique indentation marks made by a specific tool

Forensic Science:

Fundamentals &

Investigations, Chapter 16

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Q: HOW DO WE DOCUMENT TOOL MARK EVIDENCE?

• Take pictures and dust for fingerprints before applying casting material

• Use magnetic dusting powder and silicone material

• The size of the impression should be measured and recorded.

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Fundamentals &

Investigations, Chapter 16

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Q: HOW DO WE COLLECT AND PRESERVE A SAMPLE?

• Correctly label evidence

• Wrap small objects with clean paper and place

them in small containers or plastic bags

• Pack large objects in cartons or boxes

• Record—who, where, when, and why

• Maintain the chain of custody

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Investigations, Chapter 16

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Q: HOW DO WE ANALYZE TOOL MARK EVIDENCE?

• 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

• Forensic comparison microscopes examine tool

mark characteristics that match a suspect tool

TIRE IMPRESSIONS

Q: What are Tire Impressions?

• Tire Impressions are the tread design and dimensions of a tire.

• Tire Tracks are the related dimensions of the 4+ wheels.

Q: What are Tire Impressions?

•Different Types of Tires:

• Original Equipment (OE) Tires

sold on new cars

thousands of cars have same brand and size

• Replacement Tires:

purchased to replace old, worn tires

usually different than OE tires

Q: What are Tire Impressions?

•Design Elements:

• Grooves: around a tire

• Slots: across a tire

• Sipe: small grooves

• Tread Wear Indicator: raised bar under design; shows after wear

Q: What are Tire Impressions?

•Noise Treatment:

• Tire designs are

repeated around the tire

to result in noise

• Design repeated at different pitch lengths • = tires produces different

noises

Q: What Evidence Can We Gather From Tire Tracks?

• Track width: measured from right

center to left center

• Track width of front tires and rear

tires are not always

the same.

Q: What Evidence Can We Gather From Tire Tracks?

• Wheelbase: • Measured from front center to back center

Q: What Evidence Can We Gather From Tire Tracks?

Turning Diameter: Diameter of a circle made when the wheels are fully turned.

Q: How Can We Recover Tire Evidence?

Photos taken along with log of location.

Since tire tracks are variable along length (pitch), entire track should be photographed (examination quality) with overlapping photos.

Ruler included

Q: How Can We Recover Tire Evidence?

Casts can be made of 3-D impressions Long sections (3 feet) should be cast. Use dental stone

Q: How Can We Analyze Tire Evidence?

• Class Characteristics can be used to eliminate a suspected tire or suggest a tire could have made the track.

• Class Characteristics: Size

Tread design

Some wear marks

Q: How Can We Analyze Tire Evidence?

• To analyze class characteristics:

• Known tires collected.

• Inked or 3D impressions can be made for comparison.

Q: How Can We Analyze Tire Evidence?

Similar to shoes, class characteristics can be used to identify tire brands or manufacturers

Need individual characteristics to narrow down to the specific tire

Wear patterns, cuts, gouges, cracks, temporary alterations (things stuck in treads)

Remember, individual characteristics can change over time.