This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.

Many latent fingerprint development methods are useful on a broad range of surfaces. Others, such gentian violet development of prints on adhesive surfaces, are optimum for one particular special type of surface or a narrow range of surfaces. In this lab session, we'll look at another specialized development technique, this one used for raising latent fingerprints on brass cartridge cases.

There are actually two common methods for developing latent fingerprints on brass cartridge cases, both of which depend on the fact that the oils and fats in fingerprint residues coat the brass and prevent aqueous solutions from contacting it. One of those methods uses gun bluing solution, which is readily available from sporting goods stores. This solution darkens exposed brass, leaving the brass covered by the fingerprint residues bright and shiny. The second method uses even more readily-available solutions, ordinary vinegar and 3% hydrogen peroxide, available from any drugstore.

Acidified hydrogen peroxide may be used with or without preliminary light super glue fuming. In this lab session, we'll use the acidified hydrogen peroxide method to develop latent fingerprints on brass cartridge cases, using both methods.

Required Equipment and Supplies

• goggles, gloves, and protective clothing
• magnifying glass, loupe, or stereo microscope to examine specimens (optional)
• camera (optional)
• graduated cylinder, 100 mL
• forceps, hemostat or tweezers
• small beaker or similar container
• vinegar, ~ 21 mL per specimen
• hydrogen peroxide, 3%, ~ 30 mL per specimen
• brass cartridge case specimens (see Substitutions and Modifications)

The MAKE Forensic Fingerprinting Kit includes all of the specialty items needed for this lab session. Alternatively, you can purchase these items individually from Maker Shed or other laboratory supplies vendors.

CAUTIONS

Wear splash goggles, gloves, and protective clothing.

Substitutions and Modifications

• Make up the acidified hydrogen peroxide reagent fresh before each use. Use the graduated cylinder to measure 30 mL of 3% hydrogen peroxide and transfer it to the beaker. Add 21 mL of vinegar to the beaker and stir or swirl the solution to mix it thoroughly. Use this solution immediately, and discard it after use. You can simply flush it down the drain.


• If you don't have a brass cartridge case, you can substitute any brass object as long as it is not lacquered or otherwise coated.

Procedure

1. If you have not already done so, put on your splash goggles, gloves, and protective clothing.
2. Make up the acidified hydrogen peroxide reagent in the beaker.
3. Using the forceps, transfer a brass cartridge case upon which you have placed a latent fingerprint to the beaker.
4. Swirl the beaker frequently to expose fresh solution to the latent print, or use the forceps to swirl the cartridge case in the solution, as shown in Figure 8-20. As development proceeds, the brass that is not protected by the fingerprint residues begins to assume a dull yellowish color. Continue development until the latent print is clearly visible against the background.

Figure 8-20. Treating cartridge cases with acidified hydrogen peroxide

5. When development is complete, use the forceps to remove the case from the beaker, rinse it thoroughly with running water, and allow it to dry.
6. Examine the case carefully under strong oblique light with the magnifier or loupe. You should see the fingerprint revealed in considerable detail. If you have a camera, shoot an image of the developed fingerprints for your records (or images; on the round cartridge case, the entire print may not be visible in a single image). Record the pertinent details for the specimen in your lab notebook.
7. Put a latent print on a fresh cartridge case.
8. Fume the cartridge case very lightly with super glue. A few seconds of fuming is sufficient.
9. Repeat steps 2 through 6 to develop the print on the cartridge case, and compare the two cases to determine what advantage, if any, is conferred by preliminary super glue fuming.

Dennis Hilliard comments

We have had had no success in developing prints of value on fired cartridge casings. We theorize that any residue is vaporized when the cartridge is fired. Also, the surface is round and partial prints have limited value. Whenever possible, it is more useful to develop prints on the weapon.

Dennis is absolutely correct, as usual. To verify, we handled several cartridges to make sure all had latent prints, and then fired several of the cartridges. We were able to develop the latent prints on the unfired cartridges, but not those on the fired cartridges. However, this method still has some value, because criminals who remember to wear gloves while handling the weapon (or wipe it clean before discarding it) often forget to handle cartridges as carefully. Unfired cartridges in the magazine or cylinder of the weapon may yield usable prints.

Figure 8-21 shows a partial latent fingerprint on a cartridge case, revealed by development with acidified hydrogen peroxide.

Figure 8-21. Partial latent fingerprint on a cartridge case revealed by acidified hydrogen peroxide development

In September 2008, a new method for revealing fingerprints on cartridge cases was published. John Bond, a forensic scientist with the Northamptonshire Police in the UK, discovered that the copper and brass alloys used in many cartridge cases are corroded very slightly by fingerprint residues, leaving a very faint fingerprint impression. Even when such impressions are much too faint to be revealed by chemical methods or an ALS, Bond discovered that they could be revealed by subjecting the cartridge case to high voltage and treating it with fine metal-based particles similar to laser printer toner. For more information about Bond's technique, visit http://www.technologyreview.com/communications/21331/.

Review Questions

Q1: Latent fingerprints on cartridge cases are sometimes the only way to tie a criminal to the weapon he used. Propose an explanation.