Unit 5: Development of Impressions

Impression evidence is any three-dimensional or two-dimensional mark left when one object contacts another with sufficient force to transfer detail. This unit treats the field discipline of finding, recording, recovering and comparing such marks — footwear, tyre, tool, fabric, glove and bare-foot impressions — so that class and individual characteristics survive from scene to laboratory.

• Impression vs. print: an impression is three-dimensional (indented in a soft substrate), a print is two-dimensional (residue deposited on a surface).
• Class characteristics: features shared by all items from one mould or batch — pattern, size, tread design; they narrow the source population.
• Individual characteristics: randomly acquired damage — cuts, wear, embedded stones, air bubbles; they permit unique identification.
• Locard's principle: every contact leaves a trace, the governing assumption behind searching for impressions at all.
• Order of operations: photograph before any physical recovery, because lifting and casting are destructive.
• Positive vs. negative: the cast reverses the mould, so a recovered cast is the "positive" replica of the "negative" scene impression.

II. Methods of Collection of Various Impressions

Matching the recovery technique to the substrate and dimensionality of the mark.

Collection begins only after the impression is located, protected and photographed with a scale.

A. Two-dimensional impressions

• Dust and dry-residue marks: lifted electrostatically or with gelatine/adhesive lifters; the substrate stays at the scene.
• Wet or contaminated marks (blood, mud): enhanced chemically then photographed; physical lift often impossible.

B. Three-dimensional impressions

• Soft substrates (soil, sand, snow): recovered by casting, since the detail is depth-based and cannot be lifted flat.
• Snow-specific: treated with an aerosol wax (e.g. Snow Print Wax) to add contrast and a rigid shell before dental-stone casting.

C. Substrate-driven choice

• Hard non-porous (tile, glass): photography plus lifting.
• Porous (paper, cardboard): electrostatic detection apparatus or oblique light, no casting.
• Yielding (carpet, upholstery): oblique light photography; casting rarely feasible.

III. Casting of Impressions

Producing a durable positive replica of a three-dimensional impression.

Casting captures depth, contour and fine detail that a photograph flattens.

A. Materials and preparation

• Dental stone (die stone): the standard casting medium; high compressive strength, minimal shrinkage.
• Mix ratio guide: roughly 2 parts stone to 1 part water by weight, mixed to a pancake-batter consistency in a resealable bag.
• Fixative: loose soil sprayed lightly with fixative or lacquer to prevent grains washing away when the mix is poured.

B. Pouring technique

• Baffled pour: stone poured onto a spatula or off to the side of the impression so it flows in, avoiding air-jet damage to fine detail.
• Depth: built to about 2–3 cm thickness for strength; reinforcement (wire, sticks) laid across large casts.
• Curing: left undisturbed ~30 minutes minimum before lifting; labelled on the back while still soft with case number, date, initials, and a directional arrow.

C. Cleaning and handling

• Delayed cleaning: adhering soil left in place for 24–48 hours until fully cured, then rinsed gently under a slow water stream — never scrubbed.

Worked sequence: photograph → spray fixative → build a retaining frame → pour dental stone via baffle → cure 30 min → lift, label, air-dry → clean after 24 h.

IV. Electrostatic Lifting of Impressions

Recovering dry dust and residue impressions using an electrostatic charge.

An electrostatic dust lifter (EDL) transfers a two-dimensional dry impression onto a metallised lifting film without contact damage.

A. Principle

• Charge attraction: a high-voltage supply charges the black lifting film; oppositely charged dust particles migrate onto it, reproducing the impression.
• Voltage range: typically several kilovolts at negligible current, so it is safe on most surfaces.

B. Procedure

• Film placement: the metallised (black) side laid dust-side up over the impression, ground plate connected.
• Activation: the charge applied and the film pressed with a soft roller to ensure intimate contact.
• Recording: the lift photographed immediately under oblique light against a dark background, since the image is fragile.

C. Applications and limits

• Best on: dry surfaces — paper, tile, wood, human skin (post-mortem), fabric.
• Fails on: wet, greasy or heavily contaminated marks; conductive metal surfaces short the charge.
• Non-destructive advantage: the underlying substrate is preserved for further examination.

V. Utilization of Light Source for Impression Evidence Visualization

Using directed and wavelength-specific light to reveal impressions invisible under flat lighting.

Light manipulation makes faint impressions photographable without any physical treatment.

A. Oblique (grazing) light

• Mechanism: light held at a low angle (5–30°) casts shadows into the ridges and valleys of a 3-D or dust impression, raising contrast.
• Rotation: the source swept around the mark; the impression photographed at the angle giving maximum relief.

B. Alternate light source (ALS) and coloured light

• Fluorescence: selected wavelengths (e.g. 450 nm blue-green) with matching barrier filters make treated or biological residues fluoresce against a dark surface.
• Reflected UV/IR: ultraviolet or infrared reveals impressions where residue and substrate differ in reflectance but not in visible colour.

C. Coaxial and transmitted light

• Coaxial (episcopic): light directed straight down through a beam-splitter to detect impressions on smooth, glossy surfaces.
• Transmitted: light passed through translucent substrates (thin paper, film) to silhouette an impression.

VI. Generation of Control Standards for Comparison

Creating known exemplars from a suspect item under controlled conditions.

A control (test) standard is a deliberately produced impression of a suspect shoe, tyre or tool, made so that its features can be matched against the questioned mark.

A. Footwear and tyre standards

• Inked/powder prints: the sole coated and pressed onto a print lifter or paper to record the two-dimensional tread.
• Test impressions in medium: the shoe or tyre pressed into a substrate matching the scene (soil, wax, foam) to reproduce three-dimensional detail.
• Multiple exemplars: several taken to capture variation in pressure and weight transfer.

B. Tool-mark standards

• Reproduction of action: the suspect tool used on a soft test medium (lead, wax, modelling clay) replicating the questioned action — a cut, pry or strike.
• Angle control: varied because striae change with tool orientation.

C. Documentation

• Provenance: each standard labelled with source item, examiner, date and the conditions of production so it is admissible and repeatable.

VII. Comparison of Control and Questioned Impression

Systematically evaluating whether a control standard and a scene impression share a common source.

Comparison proceeds from general to specific, from class characteristics down to individual detail.

A. Class-characteristic comparison

• Screening step: overall pattern, dimensions and design compared; disagreement here excludes the source outright.
• Example: a size-9 diamond-tread standard cannot have produced a size-11 chevron-tread questioned mark.

B. Individual-characteristic comparison

• Overlay and side-by-side: transparent overlays, split-screen comparators and 1:1 photographs aligned to test whether wear, cuts and inclusions coincide in position and shape.
• Physical fit: for tool marks, striations aligned under a comparison microscope for continuous match.

C. Conclusion scale

• Identification: sufficient agreement in individual detail with no unexplained difference.
• Exclusion: disagreement in class or individual features.
• Inconclusive: insufficient detail to decide.
• Anchoring rule: any genuine, unexplainable difference outweighs many agreements and forces exclusion.

VIII. Proceedings to be Followed at Crime Scene for Handling Such Impressions

The field protocol that preserves impression evidence from discovery to laboratory transfer.

Scene handling determines whether an impression retains evidentiary value.

A. Protection and search

• Cordon and pathway: the scene secured and a common approach path designated to avoid trampling ground marks.
• Systematic search: oblique light passed low across floors and soil to locate faint impressions early.

B. Documentation before recovery

• Photography: overall, mid-range and close-up frames; the close-up taken square-on with a scale in the same plane and identifying data.
• Sketch and notes: location fixed by measurement to two fixed reference points.

C. Recovery and packaging

• Technique selection: casting for 3-D, electrostatic or gel lifting for dry 2-D, chemical enhancement for residue marks.
• Packaging: casts cushioned in rigid boxes; lifts kept flat, dark and dry; each item sealed and initialled across the seal.

D. Chain of custody

• Continuous record: every transfer logged with time, person and purpose so integrity is demonstrable in court.
• Reference exemplars: known substrate samples (clean soil, blank tile) collected for laboratory calibration and control.