Forensic Science: Fundamentals & Investigations, Chapter 15 1 Chapter 15 Cast and Impressions By the end of this chapter you will be able to: distinguish

Forensic Science: Fundamentals & Investigations, Chapter 15 1

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

distinguish between patent, latent, and plastic impressions

describe how to make foot, shoe, and tire impressions

use track width and wheel base information to identify vehicles

All Rights Reserved South-Western / Cengage Learning © 2009


Introduction

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

This evidence is often in the form of an impression.


3 Types of Impressions

Patent – visible, two-dimensional Latent – invisible, but can be visualized

through special dusting and electrostatic techniques

Plastic – three-dimensional prints left in soft materials (snow, mud, soil)


Patent impressions

A dust shoe print on the fabric of a chair.

A fingerprint on wallpaper.


Latent impression


Plastic impression

Shoe print in mud.


Individual or Class Evidence?

Impressions can be either class or individual evidence– Class – tread pattern, brand, size– Individual – split on shoe sole or unusual wear


Shoe Impressions

The size of a shoeprint can tell the size of foot of the person.

The depth of a foot or shoe impression can tell something of the person’s weight.

The type of shoe can tell something of the person’s job or personality.

CSIs use databases that contain the names of specific manufactures and tread patterns to identify different types of shoes.


Shoe Wear Patterns

Each person wears down his/her shoes differently, so shoe wear patterns can provide clues about a person:– Body weight. – The way a person walks (on toes or heels,

toes pointed in or out, etc). – The surface on which the person usually

walks. – Debris that became embedded in the tread or

unique holes and cuts in the tread.


Shoe wear patterns (cont.)

– Gait – length of strides can indicate if person was walking or running

– Limp/Injury – can be indicated if one foot is angled differently or if one foot makes a deeper impression than the other

– Trail of prints – can assist in recreating events

– Different gaits are also caused if a person is carrying a heavy weight


Collection of Shoe Impression Evidence

Photographing Impressions:

1. Take photos before impressions are touched or altered.

2. Fill view finder with impression.3. Lens should be perpendicular to impression

in order to reduce distortion.4. Take multiple photos from at least two

different angles.


Photographing Impressions (cont).

5. Place an identifying label and a ruler in position with the impression.

6. Use oblique lighting when possible (sunlight causes shadows).

7. Use flash if needed.

8. If impression is faint, spray with color-contrasting paint.


Lifting Latent Impressions

– Latent impressions are left when a shoe or bare foot walks on a smooth surface.Bare foot – leaves thin layer of body oilShoe – leaves thin film of substances from

the plastic in the sole or dirt– CSIs have to know where to look to find

latent impressions


Lifting Latent Impressions (cont).

Different methods to make latent prints visible include: Luminol makes bloody footprints visible. Dusting (similar to dusting a fingerprint) reveals

an impression. Electrostatic lifting and gel lifting (impression

shown above) techniques


Luminol

Shoe print visualized by Luminol


Electrostatic Lifting

A dry shoe deposits a small amount of dust with each step.

Apply an electrostatic charge to lifting film and place it over the latent print; the film picks up and holds the dust. (Like static cling)

View the film with a special light source, and the impression becomes visible.

Electrostatic charges can lift impressions from paper, carpeting, wood surfaces, linoleum, asphalt, and concrete.


Gel Lifting

Gel lifter = a layer of thick gel between paper backing and plastic cover sheet.

The gel conforms to uneven surfaces.Peel off the plastic cover sheet and press

firmly over print; then lift off and replace cover sheet.

The gel lifter is then photographed to reveal the print.


Gel lifter

Gel lifter

Shoe print taken from gel lifter


Casting Plastic Impressions

If imprint is made in sand or dirt, a Plaster of Paris cast is made.

Apply a light film of hairspray to prevent the impression from collapsing under the weight of the plaster, then pour in the plaster.

Wait for the Plaster of Paris to harden.


Plaster of Paris cast


Foot Length and Shoe Size

The shoe model must be identified first, then an estimate of shoe size can be obtained.

A person’s height is generally related to his/her foot size, but it is impossible to predict exact height from shoe size.

Considered class evidence.


Tire Treads and Impressions

Tire evidence:– Can indicate type of vehicle that left the mark– Link a suspect or victim to a crime scene– Can reveal the events that took place.

Examples:– Speed a car traveled when it left the road– Direction car traveled when fleeing a crime

scene– Determine who is at fault at the scene of a fatal

accident.


Tire Treads and Impressions

Tires can leave patent, latent, or plastic tread patterns. – Patent – if vehicle has tracked through a fluid

such as oil, tar, or blood– Latent – left on asphalt by manufacturer’s oils

used to keep tires soft and pliable– Plastic – left on off-road surfaces such as mud,

lawns, sand, or snow


Anatomy of a Tire

To channel water away and provide traction, a tire’s tread is divided into ridges (elevated) and grooves (indentations).

Every model of tire is unique.

A single tread can usually indicate the general type of vehicle.


Recording Tread Impressions

Ridges and grooves of discovered tire impressions are counted across the entire width.

Unique characteristics such as wear or pebbles embedded in the grooves are noted.

The impression from a crime scene may be matched to a suspect’s vehicle.

A print of a suspect’s tire impressions is taken– Ink is painted onto the suspect vehicle’s tire, and the vehicle

is driven over paper or cardboard. The print is then compared to the impression from the

crime scene.


Identifying a Vehicle

Identifying tread patterns may not be enough to link a suspect with a crime scene. Other things can help with this: – Front and rear track widths. – Wheelbase measurement. – Turning diameter.

Databases can be checked to find the vehicle with these specifications.


Accident Reconstruction

Drivers may not recall the exact series of events, or the story may not be consistent with the evidence.

Debris patterns and tire marks can help reconstruct the events of an accident.

The ultimate goal of accident reconstruction is to determine who was at fault.


Accident Reconstruction

Tire marks are helpful in:– Hit and run accidents– Head-on collisions– Quick getaways from the scene of a crime

There are three basic types of tire marks:– Skid marks– Yaw marks– Tire scrub


Skid Marks

Formed when someone brakes suddenly and locks wheels.

Provides evidence of the distance at which brakes were applied.

Calculation of velocity (speed) can be made from skid marks.


Yaw Marks

Produced when a vehicle travels in a curved path faster than the vehicle can handle and skids sideways.

Tires and road surface melt from extreme temps.

Audible squeal and often smoke occurs.


Tire Scrubs

Produced by damaged or overloaded tires during or immediately after impact.

Usually curved, irregular in width.

May have striations that look like stripes.

Determine area of impact.


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

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

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

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

Documents

Forensic Science: Fundamentals & Investigations, Chapter 15 1 Chapter 15 Cast and Impressions By the end of this chapter you will be able to: distinguish