Module 27 - Impression Recovery

Three considerations for impression evidence

Type of impression

2D verse 3D

Nature of surface - wet/dry, dirt, dust, snow

impression evidence

Includes markings produced when one object comes in contact with another

Fracturing

Occurs when the two marked fractures are cut through a surface (Saw/drill)

Impression

Occurs when the tool makes an impression on the surface (Hammer/breechface impression)

Striated

Two objects move, laterally past one another with enough force to leave marks (Bullet striations through barrel)

Pinching

Tools which grab, pull, squeeze, or grasp (pliers/tweezers)

scraping

Occurs when a tool is drugged across another surface (key on car paint)

Shearing

Tools tears or split the layers by opposed jaws whose cutting blades are offset to avoid one another in the cutting process (scissors/wire cutters)

Slicing

Slicing movement of a sharp object through another softer object to produce individual parts or pieces (knives/swords)

Impression photography

Tripod necessary, 90° angle, scale, parallel plane, fill the frame oblique, lighting or bracketing

Types of impression evidence

Patent: visible impression, an impression left behind on a surface in some deposited material

plastic: 3D, normally made in a soft material like soil or clay

latent: invisible to the naked eye, need chemical or physical enhancement

Recovery of patent impressions

ALS, chemical enhancement, gel lifter

recovery of plastic impressions

Cast

Casting methods

Mikrosil or AccuTrans: toolmarks

Dental Stone: tire and footwear

Mikrosil

For use on hard substrates (metal wood plastic)

Add silicone and hardener to piece of paper, stir with stick, smooth mixture onto tool mark, keep stick in mold to use as handle

Dental stone

1. Prepare the impression: remove large debris with tweezers

2. Fixative treatment: gentle spraying, slightly hardens the impression

3. Fence the area: beneficial in hilly areas

4. Prepare the dental stone: 2 pounds of dental stone to 12 ounces of water, knead to pancake consistency

5. Poor dental stone: begin an area, not including detail, let the material flow into the impression

6. Allow time to set check in after 30 minutes

7. Photo and package the cast: bubble wrap/box

Snowy impressions

Dental stone is an exothermic reaction allow it to cool slightly before casting

Measurement considerations with footwear and tire impressions

Footwear: stride distance, and stance width

Tire: tire track (outside of driver to outside of passenger tire), wheelbase (center of front to center of rear tire), turning radius (arc)

Factors that affect the quality of a tool mark

The material/surface used on

Use or misuse of tool - Increased likelihood of leaving individual characteristics

Known standards for tire impressions (collection and info needed)

Roll entire inked impression/circumference of tire

Note manufacturer, brand, tire model size, tread, and depth

Collecting a known standard of two marks + footwear

Wrap head of tool in butcher paper to prevent scraping and damage

Photo sole with scale, biofoam box or inked impression

Database for footwear impression

Solemate FPX 3.0

Snowy impression collections

Pros/cons to snow wax technique

Dental stone / snow wax

Pros - give structure and contrast

cons - clogs + separates impression

Snowy impressions are __ ______ and can be a variety of densities and textures

Time sensitive

Impressions filled with water

Sift dry dental stone on impression

how to package Mikrosil/AccuTrans

Envelope

How to package dental stone

Bubble wrap and box

How to package a gel lifter

Cover with clear protective film, and bag, envelope, or box

Lifting for dry impressions (5 methods)

Electrostatic dust lifter

Gelatin lifter

Mikrosil

AccuTrans

Dental stone

Lifting wet impression evidence

Develop with powders, gelatin lifter, chemical enhancement

Class characteristics can tell us

shoe, size, brand, type of vehicle, helpful with matching to impression at scene, can only narrow down to a kind/size

Individual characteristics can

Specifically link a suspect

Has wear, tear, cuts, individually acquired characteristics