Gun Bluing, primarily composed of selenium dioxide, is traditionally used to restore the blue-black finish of firearms. However, in forensics, it serves a different purpose. When applied to metals like brass (a common component of cartridge casings), it acts as a preferential etching agent. This means the latent print residue – oils, sweat, etc. – protects the metal from the bluing solution, resulting in a reversal where the print appears lighter against a darker background.
Equipment
- Fume Hood or Well-Ventilated Area: Gun Bluing solutions emit fumes.
- Non-Reactive Trays: Glass or plastic beakers or trays.
- Non-Reactive Forceps: Stainless steel or plastic to avoid contaminating the solution.
- Distilled Water: For diluting the Gun Bluing concentrate.
- Graduated Cylinder: To measure ingredients accurately.
- Gloves and Safety Glasses: Essential personal protective equipment.
Ingredients and Mixing a 1-Liter Solution
Here's a common recipe for a Gun Bluing solution. You might see slight variations between labs:
- Gun Bluing Concentrate: (Example: Birchwood Casey Super Blue Liquid Gun Blue) – approximately 500 mL
- Distilled Water: 500 mL
Mixing Procedure
- Safety First: Don your gloves and safety glasses. Work in a fume hood or well-ventilated space.
- Measure and Dilute: In a clean glass beaker, measure 500 mL of distilled water. Slowly add 500 mL of the Gun Bluing concentrate while gently stirring.
- Label and Date: Affix a label to the container indicating "Gun Bluing Solution," the ingredients, the preparation date, and any relevant safety information.
Latent Print Processing with Gun Bluing
- Pre-Treatment (Optional): Items may be fumed with cyanoacrylate (super glue) before applying Gun Bluing. This can help stabilize latent prints.
- Immersion: Submerge the item (e.g., cartridge casing) in the Gun Bluing solution for several seconds to minutes. Monitor the development closely.
- Rinse: Once latent prints become visible, thoroughly rinse the item with distilled water to stop the etching process.
- Dry: Carefully air dry or blot the item with soft tissue.
- Photograph: Document the developed latent prints immediately.
Storage Requirements
- Cool, Dark Location: Store Gun Bluing solutions in an airtight container away from direct sunlight and excessive heat.
- Separate Storage: Keep Gun Bluing solutions separate from other fingerprint development chemicals to avoid cross-contamination.
Shelf Life
The shelf life of a Gun Bluing solution can vary, typically ranging from 6 months to a year. However, if you notice discoloration or precipitation in the solution, it's time to prepare a fresh batch.
Important Considerations and Notes
- Gun Bluing success varies with different types of brass and how it's been manufactured.
- Gun Bluing may not be effective on heavily oxidized surfaces.
- Always perform testing and quality control with known latent prints before processing evidence.
- Gun Bluing should be used in a well-equipped forensic laboratory.