1 of 14

Forensics Impressions

By: Jaid Gilbert, Nick Hafner, Sarah Brice, Zachary O’Grady

​

2 of 14

Impressions

  • Impressions - a pattern showing some or all of its surface characteristics
  • Impressions can not be moved from their original surface/state. They are not permanent, and can deteriorate over time.
  • Forensic scientists need to retain as much information as possible when collecting impression evidence.
  • Impressions are three-dimensional markings having length, width and depth.

3 of 14

Types of Impressions

  • Shoeprints
  • Tire Tracks
  • Tool Marks
  • Fired Bullet
  • Teeth Marks

​

  • The most common types of pattern evidence found at a crime scene are footwear and tire track imprints and impressions
  • Impressions can be found on a variety of surfaces such as dust, carpet, mud, sand, and blood.

4 of 14

Imprints – (not impressions)

  • Imprints are two-dimensional markings having only length and width.
  • Imprints can either be a positive or a negative marking.
  • Imprints maybe produced as a result of residue on the object itself or an imprint may be made in residue on a surface.

​

5 of 14

Methods of Collecting, Documenting, and Preserving Impression Evidence

  • Photographing the imprint/impression at the scene using special photographic techniques. This is one of the most important aspects of collecting and documenting evidence.
  • Making a cast of the impression using different materials and methods (e.g., dental stone, plaster of pairs, paraffin wax, etc.)
  • Lifting imprints using materials and equipment such as rubber or adhesive lifters, Electrostatic Dust Lifter, etc.
  • Enhancing areas to develop latent or low quality imprints using chemicals and alternate light sources.

​

6 of 14

Teeth Impressions

  • Teeth Impressions are three-dimensional.
  • Eighty percent of the time teeth impressions are used to identify unknown victims.
  • Teeth Impressions are almost a positive for identifying the victim if they have all their teeth in contact and have dental records.
  • Bite marks are frequently seen on the victims of attack – especially in sexual harassment and child abuse cases.

​

​

��������������������������������������������������������������������������������������fr

​

7 of 14

Tool Marks

  • The first type of tool mark evidence is called compression which is when a tool surface pushes into a softer surface
  • The second type of tool mark is sliding which is when a object slides against another one and causes a scrape
  • Cutting is a combination of the two previous examples listed above.

8 of 14

Tire Impressions

  • Tire Impressions are the tread design and dimensions of a tire.
  • Tire Tracks are the related dimensions of the 4+ wheels
  • Most of the design elements are grooves, slots, swipes and a tread wear indicator.
  • If tire design was exactly repeated around the tire, noise would result.

​

​

​

9 of 14

Shoeprints

  • Some shoe identifiers are wear lines which are dependent on angle of footfall or weight distribution.
  • You can also tell the brand and logo from shoeprints.
  • There are three types of footwear evidence.
    • Footwear Outsole Impressions
    • Footwear Insole Impressions
    • Footwear Trace Evidence

​

​

​

10 of 14

Footwear Outsole Impressions

  • Footwear outsole impressions are impressions left on an object that was caused by contact with a piece of footwear.

​

11 of 14

Footwear Insole Impressions

  • Footwear insole imprints are imprints left in the inside of footwear caused by contact from the person’s foot.
  • Analysis of the insole imprints can be used to link a person to a piece of footwear.

12 of 14

Footwear Trace Evidence

  • Footwear trace evidence is trace evidence that is recovered from footwear.
  • Types of trace evidence that can be recovered:
    • Skin
    • Glass Fragments
    • Body Hair
    • Soil Particles
    • Dust and Body Fluids

13 of 14

Casting Footwear/Tire Impressions

  1. Spray footprint with hardening spray.
  2. Mix casting material with water according to directions.
  3. Pour casting material in a flow patter over the print.
  4. Take a tongue depressor and smooth out surface or casting material.
  5. Let casting material set for about 30 minutes and then lift print.
  6. Then allow print to set for 72 hours before washing off the debris.
  7. After 72 hours, rinse all remaining dirt off of the cast.

14 of 14

Fired Bullet Impressions

  • Each gun has its own riffling pattern with different groves and lands, so when the bullet is fired it is scraped by the groves and lands leaving an unique pattern on the bullet.
  • When you are comparing bullets, you look at it under a microscope and you rotate the bullets until you find when they line up; if they do not line up they are negative.