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
Prepare dental impressions and match them with bite marks

2. Introduction Impressions fall into three basic categories:
People, vehicles, and objects leave evidence of their presence at an accident or crime scene.
Impressions fall into three basic categories:
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
*Depending on how it is made, impression evidence may be either
class evidence (type of shoe) or individual evidence (wear pattern)

3. Shoe Impressions Shoeprint size indicates the foot size
The depth of a foot or shoe impression indicates a person’s weight
The type of shoe can tell something of the person’s job or personality
When a shoe impression is found at a crime scene, the crime scene investigator will search
Databases contain the names of specific manufactures and tread designs

4. Shoe Wear Patterns Factors that personalize a person’s footwear:
Walks on toes or heels
Body weight
Walks straight ahead or toes inward or outward
Shape of the foot
Surface on which the person usually walks
Unique holes, cuts or debris

5. 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;
The number of people
Movements (one room or several)
Entrance or exits.

6. Collection of Shoe Impression Evidence
Why would the following steps be important?
Take photos as soon as possible
Take multiple photos of the impression from at least two different orientations
Place an identifying label and a ruler in position with the impression for the photo
Use oblique lighting when possible

7. Lifting Latent Impressions
Different methods to make latent prints visible:
Luminol makes bloody footprints visible for photography
Dusting the latent print reveals an impression for lifting or photography
Electrostatic lifting and gel lifting (image below) techniques can capture hidden impressions

8. Electrostatic Dusting and Lifting
Electrostatic dusting reveals dust left with each step and creates an impression
Electrostatic charges can lift impressions from
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

9. Casting
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.

10. Comparing Shoe Size and Height

11. Tire Treads and Impressions
Motor vehicles can leave patent, latent, or plastic patterns
Tire treads—ridges and grooves channel water away and provide traction
Patent tread patterns—impressions made after tire runs through a fluid material
Latent tread patterns—impressions from tire oils used to keep tires soft and pliable
Plastic tread patterns—three dimensional impressions left in soft surfaces

12. Tire Treads and Impressions
Tread patterns can indicate the type of vehicle that left the mark
Link a suspect or victim to a crime scene
Reveal events that took place at the scene

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

14. Recording Tread Impressions
Count ridges and grooves across the tire width
Note unique characteristics—wear or pebbles embedded in the grooves
Create a print of the suspect’s tire impressions through one revolution
Compare impressions from the crime scene and suspect’s tire
Identifying tread patterns may not be enough to link a suspect with a crime scene

15. 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

16. Identifying a Vehicle Turning diameter
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.
Databases can be checked to find the vehicle with these specifications
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.

17. Accident Reconstruction
Drivers may not recall the exact series of events before, during, and after an accident
People, vehicles, and objects, however, can leave evidence of their actions at the scene of an accident
Debris patterns and tire marks can be clues to speed, direction, and vehicle identification

18. Accident Reconstruction
There are three basic types of tire marks:
Skid marks—clues to the distance traveled after brakes are applied and the vehicle’s speed
Yaw marks—shows a sideways skid
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)

19. Dental Impressions
Occasionally a perpetrator will leave behind a bite mark—considered individual evidence
Note differences in the size of teeth and jaws, position, fillings, crowns, caps, breakage, and crowding

20. Tooth Anatomy Protects the living dentin, hardest
substance in the human body
- Calcium and Phosphate, similar to bone
27 baby teeth
32 permanent adult teeth including
wisdom teeth (usually 17 years old)
8 incisors (front) 4 upper, 4 lower
4 canines (beside the incisors) 2 upper,
2 lower
8 premolars 4 upper, 4 lower
12 molars 6 upper, 6 lower

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

22. Dental Patterns in Forensics
The investigation consists of recognizing, docu-menting, collecting, and analyzing evidence
What are the two basic ways dental patterns can be used in forensic investigations?
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?

23. 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.
Differences in dental patterns can connect a person to a crime scene.
Documentation (including early photos) is extremely important in an investigation.
Forensic Science: Fundamentals & Investigations, Chapter 15