Unit 2: Impression Evidences

Impression evidence is any two- or three-dimensional mark left when one object presses against, contacts or drags across a receiving surface, transferring its shape and surface detail. It is a cornerstone of the locard exchange principle ("every contact leaves a trace") and links a suspect, tool or object to a scene through class and individual characteristics.

• Class characteristics: Features shared by all members of a group — a shoe size, tread pattern, tyre model, tool blade width. They narrow the source pool but never individualise.
• Individual characteristics: Random, acquired features unique to one object — a nick in a blade, a worn heel, an accidental cut in a tread. These permit a positive identification.
• Two-dimensional (2-D) impressions: Latent or patent marks with negligible depth — a dusty shoeprint on tile, a lip print on glass.
• Three-dimensional (3-D) impressions: Marks with measurable depth in a pliable medium — a footwear cast in soil, a tool gouge in wood, a bite in skin or food.
• Positive vs negative image: The impression is a negative (mirror-reversed, inverted relief) of the object; a cast reverses it back to a positive replica.
• Documentation before recovery: Photograph with a scale (ABFO No. 2 or L-scale) and oblique lighting at ~30° before any lifting or casting, because recovery is destructive.

II. Locating and Identifying the Type of Impression Evidence

Detection, classification and preliminary examination

The purpose is to systematically search a scene for impressions and correctly categorise each so the right recovery method follows.

A. To locate and identify the type of impression evidence

Impressions are located by controlled lighting and search patterns, then classified by dimensionality, substrate and depositing object.

• Oblique (grazing) light: A torch held at a low 5–15° angle throws shadows across surface relief, revealing latent dust and residue prints invisible under direct light.
• Search methodology: Grid, spiral or line search of floors, entry/exit routes, soft ground and points of forced entry; mark each find with an evidence placard.
• Classification by receiving object:
  • Footwear and tyre: On soil, snow, dust, blood — often 3-D outdoors, 2-D indoors.
  • Lip prints (cheiloscopy): On glass, cups, cigarette ends, cloth.
  • Bite marks (forensic odontology): On skin, foodstuffs, chewing gum.
  • Toolmarks: On door frames, safes, window latches, cut wire.
• Patent, latent or plastic:
  • Patent: Visible without treatment (bloody shoeprint).
  • Latent: Invisible, needs powder, chemical or light enhancement.
  • Plastic: 3-D indentation in a mouldable medium.
• Preserving fragile finds: Cover with a raised box against wind and rain; never step on the route until searched.

III. Casting of Footwear and Tyre Impressions

Capturing three-dimensional detail as a durable positive replica

Casting converts a 3-D depression in soil, sand or snow into a solid model reproducing tread pattern and wear.

A. To perform casting of footwear and tyre impression

The method fills the void with a setting compound that hardens into a positive cast retaining class and individual detail.

• Photograph first: Scaled, oblique-light exposures directly overhead and from the sides record the impression before the destructive cast.
• Preparatory fixing: Apply a light dusting of potassium sulphate or a commercial fixative spray to loose soil, or snow print wax (grey/red aerosol) to insulate and colour a snow impression before casting.
• Casting medium — dental stone:
  • Preferred over plaster of paris for higher compressive strength and finer detail.
  • Typical mix: ~2 parts dental stone to 1 part water by weight, e.g. 900 g stone to 450 ml water, mixed in a resealable bag to a pancake-batter consistency.
• Pouring technique: Pour onto a baffle (spatula or off to one side) so the stream does not strike and erode the fragile detail; let it flow across the impression.
• Setting and lifting: Allow 20–30 minutes to cure; lift only after it is hard, then air-dry 24–48 hours before brushing off adhering soil.
• Tyre impressions: Because a tyre circumference (~2 m) prints over a long track, cast in overlapping sections, each labelled with sequence and direction of travel.
• Marking the cast: Inscribe case number, date, initials and a directional arrow into the back of the cast while still soft.

B. Comparison and limitations

• Known standard: Take test impressions from the suspect footwear or tyre in the same medium for side-by-side comparison of pattern, size and wear.
• Limitations: Coarse or waterlogged soil loses fine detail; casts record only what the substrate could hold.

IV. Lip Print Analysis (Cheiloscopy)

Classifying labial groove patterns for identification

Lip prints are impressions of the wrinkles and grooves (sulci labiorum) on the vermilion border, held to be unique and stable through life.

A. To analyze the lip print pattern found at crime scene

Analysis records the print, then classifies its groove pattern, most commonly by the Tsuchihashi (Suzuki–Tsuchihashi) system.

• Recovery of the print:
  • Visible (lipstick) prints: Photograph, then lift with fingerprint powder and lifting tape or a gel lifter.
  • Latent prints: Develop with lysochrome dyes (Sudan black, oil red O) or aluminium/magnetic powder.
• Tsuchihashi classification (Type I–V):
  • Type I: Clear vertical grooves running the full lip width.
  • Type I′: Vertical grooves that do not cover the whole lip.
  • Type II: Branched (Y-shaped) grooves.
  • Type III: Intersecting (crisscross) grooves.
  • Type IV: Reticular (net-like) pattern.
  • Type V: Undetermined / other, not fitting the above.
• Zonal recording: The lip is divided into compartments (e.g. quadrants) and the dominant type recorded per zone, since patterns differ across the lip.
• Interpretive value: Also indicates presence at scene and, from lipstick smears, can carry DNA and cosmetic trace for corroboration.

V. Comparative Analysis of Bite Marks

Matching dentition to injury patterns in forensic odontology

A bite mark records the arrangement, size and peculiarities of the biter's teeth in skin or a foodstuff, compared against a suspect's dentition.

A. To perform comparative analysis of bite marks

Comparison overlays the suspect's dental characteristics onto life-size scaled images of the mark.

• Documentation: Photograph with an ABFO No. 2 reference scale flat in the plane of the bite to permit true 1:1 reproduction and correct for distortion.
• Swabbing and impression: Swab for salivary DNA before cleaning, then take a cast of a 3-D bite (in food) with dental impression material.
• Class features measured:
  • Arch shape and inter-canine distance distinguishing upper from lower arch.
  • Individual features: Rotations, spacing (diastema), fractured or missing teeth, wear facets.
• Overlay comparison methods:
  1. Hand-traced overlay: Biting edges traced from a cast onto acetate — simple but examiner-dependent.
  2. Computer-generated overlay: Digitised dental scan superimposed on the scaled wound image — more objective and reproducible.
• Distortion caveats: Skin is elastic and curved, and swelling or postural change shifts the pattern, so bite-mark matching supports rather than singularly proves identity.

VI. Analysis of Toolmark Impressions

Linking a tool to the surface it acted upon

A toolmark is a scratch, gouge, indentation or shear left when a harder tool contacts a softer surface, carrying the tool's microscopic edge detail.

A. To perform analysis of toolmark impressions

Analysis distinguishes the type of mark, recovers a replica of it, and compares striations or contours against a test mark from the suspect tool.

• Categories of toolmark:
  1. Impressed (compression) marks: The tool presses in without lateral motion — a hammer strike, a pry indentation; reproduces the tool's negative shape.
  2. Striated (friction/scrape) marks: The tool drags sideways, cutting parallel striae — a screwdriver sliding on a latch, cut bolt-cutter edges.
• Recovery:
  • Preferably submit the whole marked object; where impossible, cast the mark with a silicone/polyvinylsiloxane replicating compound (e.g. Mikrosil) that captures micron-level detail.
• Producing the known standard: Make test marks with the suspect tool on a like material at varying angles, since edge geometry changes the striation pattern.
• Comparison microscopy: A comparison microscope places questioned and test marks in one split field; a match is declared when striae or contours align in "sufficient agreement" beyond what chance could produce.
• Individualising detail: Manufacturing and wear defects on the working edge produce a reproducible striation signature unique to that single tool.

B. Significance and limitations

• Evidentiary role: Places a specific tool at a point of entry or on a severed object, corroborating other physical evidence.
• Limitations: Soft or crumbling surfaces, corrosion and post-event damage degrade detail; edge wear over continued use can alter the signature between the crime and seizure.