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Chapter 15 Cast and Impressions By the end of this chapter you will be able to:

o Distinguish between patent, latent, and plastic impressions

o Describe how to make foot, shoe, and tire impressions

o Use track width and wheel base information to identify vehicles

o Prepare dental impressions and match them with bite marks

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Introduction Introduction

1. Patent impressions are two-dimensional, ex. Blood print

2. Latent impressions are hidden to the eye

3. Plastic impressions are three-dimensional, ex. Mud, snow, soil

People, vehicles, and objects leave evidence of their presence at an accident or crime scene.

Impressions fall into three basic categories:

*Depending on how it is made, impression evidence may be either class evidence (type of shoe) or individual evidence (wear pattern)

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Shoe Impressions Shoe Impressions

o Shoeprint size indicates the foot sizeo The depth of a foot or shoe impression

indicates a person’s weighto The type of shoe can tell something of the

person’s job or personalityo When a shoe impression is found at a crime

scene, the crime scene investigator will search

o Databases contain the names of specific manufactures and tread designs

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Shoe Wear Patterns Shoe Wear Patterns

Factors that personalize a person’s footwear: o Walks on toes or heelso Body weighto Walks straight ahead or toes inward or outwardo Shape of the footo Surface on which the person usually walkso Unique holes, cuts or debris

Gait and Track

Numerous prints together tell an investigator about the person’s gait, or walking habits.

Information that can be obtained from shoe impressions include;

1. The number of people

2. Movements (one room or several)

3. Entrance or exits.

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Collection of Collection of Shoe Impression Evidence Shoe Impression Evidence

Why would the following steps be important?

1. Take photos as soon as possible 2. Take multiple photos of the impression from

at least two different orientations3. Place an identifying label and a ruler in

position with the impression for the photo 4. Use oblique lighting when possible

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Lifting Latent Impressions Lifting Latent Impressions

Different methods to make latent prints visible:o Luminol makes bloody footprints visible for

photographyo Dusting the latent print reveals an impression for

lifting or photography o Electrostatic lifting and gel lifting (image below)

techniques can capture hidden impressions

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Electrostatic Dusting Electrostatic Dusting and Lifting and Lifting

o Electrostatic dusting reveals dust left with each step and creates an impression

o Electrostatic charges can lift impressions from

o Gel lifters also recover latent impressions by dusting and then placing the gel between paper backing and a plastic cover sheet

paper carpeting

wood surfaces linoleum

asphalt concrete

Casting

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Three dimensional impression, such as a shoeprint in mud or snow, is called a cast and may be made to preserve physical evidence.

The casting material used for snow is called dental stone, it hardens faster than Plaster of Paris.

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Comparing Shoe Size and HeightComparing Shoe Size and Height

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Tire Treads and Impressions Tire Treads and Impressions

o Tire treads—ridges and grooves channel water away and provide traction

o Patent tread patterns—impressions made after tire runs through a fluid material

o Latent tread patterns—impressions from tire oils used to keep tires soft and pliable

o Plastic tread patterns—three dimensional impressions left in soft surfaces

Motor vehicles can leave patent, latent, or plastic patterns

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Tire Treads and Impressions Tire Treads and Impressions

o Tread patterns can indicate the type of vehicle that left the mark

o Link a suspect or victim to a crime sceneo Reveal events that took place at the scene

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Anatomy of a Tire Anatomy of a Tire A tire’s tread surface is divided into ridges (elevated regions) and grooves (indentations)

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Recording Tread Impressions Recording Tread Impressions

o Count ridges and grooves across the tire widtho Note unique characteristics—wear or pebbles

embedded in the grooves o Create a print of the suspect’s tire impressions

through one revolution o Compare impressions from the crime scene and

suspect’s tire o Identifying tread patterns may not be enough to

link a suspect with a crime scene

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Identifying a Vehicle Identifying a Vehicle

Track widths—From center of tire to center of opposite tire

Wheelbase length—From center of front axle to center of rear axle

Wheelbase Track width

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Identifying a Vehicle Identifying a Vehicle

o Turning diameter

Databases can be checked to find the vehicle with these specifications

A measure of how tight a circle can be driven by a vehicle OR the minimal space required for a car to make a U-turn.

When a vehicle turns a sharp corner, even at moderate speed, a track is created by the additional stress put on the front out tire.

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Accident Accident Reconstruction Reconstruction

o Drivers may not recall the exact series of events before, during, and after an accident

o People, vehicles, and objects, however, can leave evidence of their actions at the scene of an accident

o Debris patterns and tire marks can be clues to speed, direction, and vehicle identification

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Accident Reconstruction Accident Reconstruction

There are three basic types of tire marks: o Skid marks—clues to the distance traveled after

brakes are applied and the vehicle’s speedo Yaw marks—shows a sideways skido Tire scrub—determines the area of impact

How are each of these formed? Through experience and experimentation, investigators can also estimate speeds of vehicles using the “skid-to-stop” formula. (weight of car, texture of surface and length of skid marks determines speed)

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Dental Impressions Dental Impressions

o Occasionally a perpetrator will leave behind a bite mark—considered individual evidence

o Note differences in the size of teeth and jaws, position, fillings, crowns, caps, breakage, and crowding

Tooth Anatomy

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- Calcium and Phosphate, similar to bone

-Protects the living dentin, hardest

substance in the human body

27 baby teeth32 permanent adult teeth including wisdom teeth (usually 17 years old)

8 incisors (front) 4 upper, 4 lower4 canines (beside the incisors) 2 upper, 2 lower8 premolars 4 upper, 4 lower12 molars 6 upper, 6 lower

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Development of TeethDevelopment of Teeth

The approximate age of a child can be estimated by viewing the child’s teeth. Older child may have a mixture!

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Dental Patterns in Forensics Dental Patterns in Forensics

o The investigation consists of recognizing, docu-menting, collecting, and analyzing evidence

What are the two basic ways dental patterns canbe used in forensic investigations?

o There are 76 points of comparison when comparing a suspect’s dental patterns with bite marks left at a crime scene

What is the common method used to reveal whether there is a match?

Forensic Science: Fundamentals & Investigations, Chapter 15 23

. . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . Summary

o Impression evidence—whether patent, latent, or plastic—will be considered class evidence unless it has individualizing features.

o Debris patterns and tire marks can be clues to speed, direction, and vehicle identification.

o Differences in dental patterns can connect a person to a crime scene.

o Documentation (including early photos) is extremely important in an investigation.