Chapter 22: Impression Evidence

Impression Evidence – evidence at a crime scene that results from the criminal leaving marks or prints of some kind on surfaces or materials like soil or dust.

22.1: Types of Impression Evidence

• Donors – contains 3D markings.
  • Includes shoe soles and heels, tire treads, fingerprints, and other friction ridges such as footprints and lip prints, tools that leave markings on the objects on which they are used, and tools that leave markings on the objects on which they are used.
• Recipients – made of a material that can form and hold a negative image of the donor markings.
  • Includes soft plastics, soil, putty, paint, dust, metals, and some soft plastics.

22.2: Footwear Impressions

• Footwear impressions can indicate the type, manufacturer, model, and often, the exact size of the footwear.

• When footwear touches the ground…

  • A static electric charge can be applied to the impression if the shoe is clean and dry.
  • If the surface is soft or pliable, the pressure exerted by the foot will cause the surface to deform and take on the contours of the surface of the footwear.
  • If the surface is dirty or snowy, the impression may be long-lasting.
  • If the surface is grass or carpet, the impression may be quite transitory.

• Imprint – where there is enough residue on the footwear to leave an impression on the recipient's surface.

• Positive Impression – the residue is on the surface of the footwear that touches the surface.

• Negative Impression – If the shoe sole is clean and the recipient surface contains a lot of dust or residue.

Footwear Impressions at Crime Scenes

• Oblique lighting and physical methods of development may be useful for discovering hard-to-see images.
• Avoid altering an impression until examination-quality photographs have been taken.
• If at all possible, the impression and the object on which it is found should be physically removed and transported to the laboratory, where there are usually better facilities for additional photographic or other treatments.
• If removal is not practical or possible, then a cast should be made if the impression is three-dimensional, or it should be lifted if two-dimensional.
• If lifting isn’t possible, then the impression should be enhanced to the maximum degree possible and more examination photographs taken.
• Casting materials – dental plasters and stones; are denser and have more uniform, smaller particle sizes; they dry more quickly and show more detail.
• Footwear impression in the snow – snow print wax; is sprayed on the snow print and dries in a few minutes; it shows excellent detail but is fragile.
• Lifting imprints – electrostatic lifting devices; a large, static electricity charge will strongly attract dust and other fine powders.
  • A contrasting color film attracts the particles from the impression, thus affecting a transfer.

22.3: Tire Impression Evidence

• Tread – the part of the modern tire that is in contact with the road.

• Lifts of two-dimensional tire impressions are made in the same way as footwear impressions.

• Casting for three-dimensional impressions is the main advantage and can be easily seen.

• Wheelbase – the distance from the center of the front hub to the center of the rear hub of a motor vehicle.

• Stance – the distance from the centerline of the right tire to the centerline of the left tire.

22.4: Bite Mark Evidence

• Known bite marks are generally obtained by the use of an impression of the teeth in dental stone or a similar medium. The material used for the impression must be fine-grained so that small details on the surface of the teeth will show up.
• One promising development in the analysis of bite mark evidence is the use of three-dimensional impressions and bite marks.

22.5: Serial Numbers Restoration

• When serial numbers are at issue in a crime, it is often because someone attempted to obliterate them from the object, making it more difficult to identify its owner or source.

• It is possible and quite common for a forensic chemist to restore an obliterated serial number impression.

• When a serial number is stamped into the metal, two things happen to the metal under the number.

  • First, it is compressed, making it denser than the surrounding metal.
  • Second, the metal–metal bonds in the stamped area are disrupted, and the metal structure becomes weakened.

• To restore the serial number, the metal surface that has been abraded is polished with a fine abrasive and then slowly treated with corrosive acid.

• The best way is to observe the restoration process using a low-power stereomicroscope.

• Once a serial number is restored, it will eventually disappear and then will be gone forever. It is good practice to have a camera ready to take pictures of each number as it is restored, so there will be a permanent, visual record of the restoration.

• Plastics present difficult problems when it comes to serial numbers.

• Not all serial numbers are stamped into surfaces. Some are applied with decals or etched. Others are embossed and raised above the surface of the object. These types of serial numbers cannot be restored using the same methods as are used with stamped numbers.

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