Crowle's Double Stain is a specialized chemical reagent primarily used for developing latent fingerprints on porous surfaces like paper, cardboard, and unfinished wood. It's a sequential process with two primary components:
- Solution A: Basic Fuchsin - This dye selectively stains fatty deposits and other residue components commonly found within latent fingerprints.
- Solution B: Malachite Green - This contrasting dye stains the background of the porous surface.
The result is a highly visible, pink-red fingerprint impression against a contrasting green background.
Equipment
- Glass beakers or bottles (for mixing and storing solutions)
- Graduated cylinder
- Scale
- Stirring rod
- Safety goggles
- Chemical-resistant gloves
- Lab coat
- Shallow trays
- Fume hood (recommended)
Ingredients and Quantities for a 1-Liter Solution
Solution A (Basic Fuchsin)
- Basic Fuchsin: 1 gram
- Ethanol (95%): 100 ml
- Distilled Water: 900 ml
Solution B (Malachite Green)
- Malachite Green: 1 gram
- Distilled Water: 1 Liter
Mixing Procedure
- Solution A:
- Dissolve 1 gram of basic fuchsin in 100 ml of ethanol.
- Add 900 ml of distilled water slowly with constant stirring.
- Solution B:
- Dissolve 1 gram of malachite green into 1 liter of distilled water, stirring until fully dissolved.
Latent Print Processing Using Crowle's Double Stain
- Submersion: Immerse the item of evidence in Solution A for approximately 10-20 seconds. The exact time may vary depending on the porosity of the item.
- Rinse: Gently rinse the item with distilled water to remove excess Solution A.
- Repeat: Repeat steps 1 and 2 with Solution B.
- Final Rinse: Rinse the item thoroughly with distilled water to remove all excess dye.
- Drying: Allow the item to air-dry completely.
Storage and Shelf Life
- Storage: To minimize oxidization, store both Solution A and Solution B in airtight, dark-colored glass bottles. A cool, dark location is ideal.
- Shelf Life: If prepared and stored correctly, Crowle's Double Stain can have a shelf life of up to a year. However, regularly inspect for any signs of precipitation or degradation.
Important Considerations
- Crowle's Double Stain is generally considered a destructive technique. Assess suitability on a case-by-case basis, as it may interfere with other analyses.
- This solution can be hazardous. Always work in a well-ventilated area, preferably a fume hood, while wearing appropriate personal protective equipment.
- Experiment on known fingerprint samples on similar surfaces to optimize development times before applying the process to actual evidence.