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.
Dusting, iodine fuming, and ninhydrin all depend on the presence of volatile organic components in latent fingerprints. Old fingerprints-and sometimes even recent ones–may lack those volatile components, either because they’ve evaporated over time or because they were present in unusually small amounts in the original latent print. Conversely, depending on environmental conditions and other factors, volatile components may persist for surprisingly long periods; we’ve developed latent prints with ninhydrin on paper that had not been touched for a decade or more.
Dennis Hilliard comments
The organic components of fingerprint residue are amino acids and fatty oils, which are not too volatile. The most volatile component is water, which is why we recommend, if possible, the treatment of evidence for prints ASAP, before they come to the lab. As the water evaporates, so do our chances to develop prints of value. Same for prints on paper, water will evaporate more rapidly on porous surfaces.
Silver nitrate development is based on the reaction of soluble silver nitrate with the sodium chloride (salt) that is present in most latent fingerprints to form insoluble and light-sensitive silver chloride. Exposing the silver chloride produced by this reaction to sunlight or an ultraviolet lamp causes the silver chloride to be reduced to metallic silver, making the latent prints visible as black or dark gray traces. Because sodium chloride is not volatile, even very old latent fingerprints retain it and can be developed by silver nitrate. Accordingly, silver nitrate development may work when iodine fuming and ninhydrin fail completely. (Note that failure of these other reagents says nothing about the age of the latent prints; even prints that are only hours or days old may respond only to silver nitrate development.)
Silver nitrate development is destructive (as is Physical Developer), so if it is to be used it must be used after all other methods have been attempted. Iodine fuming, ninhydrin, and most other development methods don’t interfere with silver nitrate, so forensics labs often use silver nitrate development as the final step, in the hope of revealing latent prints that were not revealed by the other methods.
In this lab session, we’ll use silver nitrate solution to develop latent prints on untreated paper specimens, as well as on paper specimens that have previously been treated with iodine fuming and ninydrin.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- spray bottle
- 3% silver nitrate solution (0.75 grams silver nitrate per 25 mL water)
- magnifying glass, loupe, or stereo microscope to examine specimens (optional)
- camera (optional)
- paper with latent fingerprints
Although the MAKE Forensic Fingerprinting Kit includes the specialty items needed for several other fingerprint development methods covered in related articles, we decided not to include silver nitrate in this kit because its relatively high cost would have boosted the price of the kit significantly, whether kit buyers wanted to use the silver nitrate method or not. If you do want to try silver nitrate development, you can purchase silver nitrate separately from Maker Shed or other laboratory supplies vendors.
Silver nitrate is toxic, corrosive, a strong oxidizer, and stains anything it contacts. Wear splash goggles, gloves, and protective clothing. If silver nitrate comes into contact with your skin, it produces a deep black stain that, although harmless, will persist for several days or longer.
Substitutions and Modifications
- You may substitute a discarded decongestant nasal sprayer or similar for a laboratory spray bottle.
Create some fresh fingerprint specimens on various types of paper using the procedure described in the iodine fuming lab session. You can also try making latent prints on raw (unpainted) wood. Use only items you are willing to discard after the experiment; silver nitrate stains are persistent. As you might expect, we found that latent prints made when our hands were sweaty produced the best results with silver nitrate. Washing our hands thoroughly immediately before making the latent print specimens apparently eliminated most or all of the sweat (and salt) present, and silver nitrate treatment of those prints was unsuccessful.
- If you have not already done so, put on your splash goggles, gloves, and protective clothing.
- Place the specimen print-side up on paper towels or old newspaper to protect the surface against overspray.
- Spray the specimen with silver nitrate solution sufficient to wet the surface, as shown in Figure 8-14. Don’t drench it, but make sure the entire surface is dampened with silver nitrate solution.
Figure 8-14. Spraying a questioned specimen with silver nitrate solution
- Allow the specimen to air dry for 15 to 30 minutes.
- Expose the specimen to direct sunlight or an ultraviolet lamp for, checking it every few minutes for development progress. The latent fingerprints become visible first as a pale yellow/purple color. With continued exposure, the prints gradually darken, eventually becoming dark gray or black. Complete development may require from 5 minutes to an hour or more, depending on the particular fingerprint, the type of surface, and the intensity of the light. Exposing the specimen too long will eventually cause excess silver nitrate to be reduced to metallic silver, staining the entire specimen.
- Once the prints are fully developed, bathe the specimen in water to remove the excess silver nitrate, which “fixes” the print. Dry the specimen, record your observations in your lab notebook, and tape the developed specimen into your lab notebook.
- Repeat steps 2 through 6 for your other specimens, including those that have already been iodine fumed and those that have already been iodine fumed and developed with ninhydrin.
Figure 8-15 shows latent fingerprints revealed by silver nitrate development.
Figure 8-15. Latent fingerprints revealed by silver nitrate development
Q1: With which component of fingerprint residues does silver nitrate solution react to form black elemental silver?
Q2: Is silver nitrate development used before or after other development methods? Why?
Q3: For what types of specimens would you use an ethanolic solution of silver nitrate rather than an aqueous solution? Why?