impression evidence

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  • Impression Evidence
    • what is it?
      • when objects or materials retain the characteristics of other objects or materials through direct contact they leave a unique impression
        • footwear - tyre tracks - tool/instruments
    • tyre marks
      • impressions left by tyres on the surface onto which a vehicle drove
        • sometimes surfaces are not suitable to retain the impression
          • but if the surface is sort i.e. mud/dirt/snow, the tyres will often leave an impression (3D mark)
      • can be matched back to a particular vehicle due to the thread of the tyres and the wear
      • recovery technique
        • cast using dental stone - same as footwear
          • dental stone gives finer detail
        • photograph and enhance through the use of light and chemicals
      • similar to footwear examination - just a larger scale
        • photograph
          • cast
            • test prints on inked sheets
      • similar conclusions as footwear
      • limitations
        • complex structure
        • variation in patterns
      • examination and comparison
        • pattern
        • pattern size
        • wear
        • damage
      • elimination prints
        • ambulances, police vehicles, fire service and other legitimate access
          • not as easy as footwear to recover and eliminate - limitation
    • tool marks
      • found at various crime scenes
        • burglary
        • criminal damage
        • fires
      • wide range of tools available
        • screwdrivers
        • jemmys
        • bolt croppers
      • impressed detail
        • pressing detail
        • crushing detail
        • physical impression of tool
        • can show class and individual characteristics
      • striation detail
        • dynamic scrapping action
        • cutting action
      • recovery technique
        • photograph
        • cast with silicone material with hardener i.e. mikrosil and hardener
        • cut out mark if able too
      • comparison
        • use comparator
        • tool vs cast
        • examine for matching detail using split screen stero microscope
          • cast from scene and cast from tool
          • make notes and photograph
      • examination
        • shape, size, apparent detail and measurements


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