Collecting footprints is a crucial process in various fields, from forensic investigations to tracking, involving a range of techniques tailored to different surfaces and print conditions. The primary goal is to capture the impression accurately while preserving its integrity.
How to Collect Footprints Safely and Effectively
Collecting footprints involves several specialized methods, each suited for different types of surfaces and the nature of the impression. Careful documentation and precise execution are paramount to ensure the evidence is valuable.
1. Casting Footprints
Casting is a common method for collecting three-dimensional impressions found in soft substrates like soil, mud, or sand.
Materials for Casting:
- Casting Material: Plaster of Paris, dental stone, or specialized snow print wax.
- Water: For mixing plaster/dental stone.
- Mixing Bowl/Bag & Stirring Tool: For preparing the casting material.
- Spray Fixative (optional): To strengthen loose impressions before casting.
- Ruler/Scale: For photographic documentation.
- Camera: To photograph the print before and after casting.
- Protective Barriers (optional): Cardboard or metal strips to create a dam around the print.
Steps for Casting:
- Document the Scene: Before any physical collection, extensively photograph the footprint. Take overall, mid-range, and close-up photos with a scale and case identification number. Note environmental conditions and the exact location.
- Prepare the Print:
- Loose Materials: Gently remove any loose debris (twigs, leaves) without disturbing the print.
- Strengthening (if needed): For very sandy or powdery prints, a light application of aerosol fixative (like hairspray from a distance) can help stabilize the surface, though this should be done with extreme caution as it can alter the print.
- Build a Dam: If the print is on a slope or you need to contain the casting material, carefully place cardboard or metal strips around the impression to create a small wall.
- Mix the Casting Material: Follow the manufacturer's instructions for mixing plaster of Paris or dental stone with water to achieve a creamy consistency. Avoid air bubbles.
- Pour the Cast:
- Slowly pour the mixed material into the deepest part of the impression, allowing it to flow naturally and fill the print from one end to the other. This minimizes air entrapment.
- Ensure the entire impression is covered with sufficient material (typically 1-2 inches thick).
- Allow to Cure: Let the cast set and fully harden according to the material's instructions. This can take anywhere from 30 minutes to several hours, or even longer in cold or damp conditions. Do not disturb the cast during this time.
- Retrieve and Clean the Cast:
- Once the material is fully cured, carefully dig around the edges of the hardened cast.
- Gently remove surrounding loose material, such as sand or soil, until sufficient depth is created around the cast.
- Carefully lift the entire lump of material, including the hardened plaster cast and the surrounding sand or soil, using your hands.
- Place this lump onto a smooth, stable surface, ensuring the plaster face (the side with the impression) is oriented downwards.
- Using a soft brush, meticulously remove the remaining sand or soil from the back and sides of the cast to fully reveal the footprint details.
- Avoid cleaning the impression surface immediately, as fine details can be damaged. Allow the cast to dry completely for 24-48 hours before any further cleaning or examination.
Special Considerations for Snow Prints:
Collecting footprints from snow requires a specific approach due to the cold temperatures and fragile nature of the impression. Sulfur casting or specialized snow print wax aerosols are typically used. Sulfur casts create a very detailed, hard replica, while snow wax sprays build up a layer that can then be cast with dental stone.
2. Lifting Latent Footprints
Latent footprints are those that are not readily visible to the naked eye, often found on smooth surfaces like floors, paper, or plastic.
Techniques for Lifting Latent Prints:
- Electrostatic Dust Lifter (ESDL): Ideal for collecting dust prints on surfaces like linoleum, wood, or even carpet. It uses an electrostatic charge to transfer dust from the surface to a black lifting film.
- Procedure: Place the ESDL film over the print, connect the power pack, and apply charge. The dust adheres to the film.
- Gel Lifters: These are flexible sheets with a low-tack adhesive surface, suitable for lifting dusty or faint impressions from various surfaces, including textured ones. They come in black or white to contrast with the print.
- Procedure: Gently press the gel lifter onto the print, smooth it out, then slowly peel it back.
- Adhesive Lifters/Tapes: Similar to gel lifters but with stronger adhesive, used for lifting visible prints or prints that have been enhanced with powder from non-porous surfaces.
- Procedure: Apply the adhesive tape over the powdered or visible print, rub gently, and lift.
- Fingerprint Powders: Used to enhance latent prints on non-porous surfaces (e.g., glass, metal, plastic). The powder adheres to residues left by the foot.
- Procedure: Apply powder with a brush, then lift with an adhesive lifter.
3. Photographic Documentation
Photography is an essential first step for all types of footprint collection and can sometimes be the only practical method.
- Importance: Provides a permanent visual record of the print's original state, context, and location before any collection attempts.
- Procedure:
- Overall Photos: Capture the general scene showing the print's relationship to the surroundings.
- Mid-Range Photos: Zoom in to show the print's location within a smaller area.
- Close-Up Photos: Take multiple detailed shots of the print itself, always including a ruler or scale parallel to the print and at the same elevation.
- Lighting: Experiment with oblique (side) lighting to enhance contrast and reveal three-dimensional characteristics. Flash photography can also be useful.
- Angles: Take photos from directly above (90 degrees) and from various angles to capture all features.
Tools and Equipment for Footprint Collection
A well-equipped collection kit is crucial for success:
| Category | Essential Items |
|---|---|
| Documentation | Digital Camera, Macro Lens, Tripod, Scale/Ruler, Measuring Tape, Crime Scene Log, GPS |
| Casting | Plaster of Paris/Dental Stone, Water Bottles, Mixing Bowls/Bags, Stirring Spoons, Shovel |
| Lifting | Electrostatic Dust Lifter (ESDL), Gel Lifters (black/white), Adhesive Lifting Tapes |
| Enhancement | Fingerprint Powders (black, white, fluorescent), Brushes (fiberglass, camel hair) |
| Protection & Safety | Gloves, Shoe Covers, Face Mask, Crime Scene Barrier Tape |
| Cleaning | Soft Brushes, Sifters |
| Packaging | Evidence Boxes, Bags, Tags, Permanent Markers |
Key Considerations for Collection
- Prioritize Photography: Always photograph the print before attempting any physical collection.
- Minimize Contamination: Wear gloves and use clean tools to avoid introducing foreign material.
- Preserve the Scene: Avoid walking near or disturbing other potential evidence.
- Detailed Documentation: Maintain thorough notes on location, time, date, environmental conditions, collection method, and chain of custody.
- Multiple Methods: If possible and practical, use more than one method (e.g., photograph and then cast) to maximize the chance of a successful recovery and provide corroborating evidence.
Collecting footprints requires patience, precision, and adherence to established protocols to ensure the integrity and evidentiary value of the collected material. Understanding the different techniques allows for the most appropriate method to be chosen for each unique situation.
