Using Silver Nitrate for Latent Print Development
Silver Nitrate has been used for over 75 years as a latent print development technique. In 1937, M. Edwin O’Neill published, “The Development of Latent Fingerprints on Paper,” in which the author mentions the Silver Nitrate processing method.
Latent print powders were the most accepted means of latent development back then, but developing latents on paper was a difficult task. Porous surfaces such as paper and cardboard tend to absorb latent print residues rendering powders virtually useless, unless the prints were reasonably fresh.
Silver nitrate is a somewhat light sensitive chemical when dissolved in water or ethyl alcohol and
will darken over several days unless it is stored in amber bottles or locked away in a light-tight cabinet. But its light sensitivity can be turned to advantage when searching for latents on porous materials like paper.
Silver nitrate combines with the latent print moisture present on a document. The result is that when salt, sodium chloride, is present in the perspiration, which it usually is, the silver combines with the chloride atom of salt to form silver chloride—a highly light sensitive substance. When this occurs, latent prints appear on the paper with dark brown to black ridge detail.
Ordinary fluorescent lighting will trigger the development process, but bright sunlight or shortwave UV will greatly accelerate it.
Silver nitrate enjoyed widespread popularity until the 1950s when ninhydrin was introduced to the crime labs around the world. Ninhydrin had a number of advantages over silver nitrate, among which is the fact that once silver nitrate is applied to a surface, it continues to darken even under low light conditions; and cleaning off silver nitrate residues from laboratory apparatus was a major chore.
Another problem was that the evidence thus processed would continue to darken each time it was exposed to light, and eventually the developed latents would indeed be difficult to discern from the darkening background.
Few crime labs today use silver nitrate since DFO and ninhydrin are far more effective and do not pose long term cleanup problems. But don’t write off the use of silver compounds—the next PrinTips post will feature Physical Developer.
Learn more from the Technical Bulletin: “Overview of Latent Print Development Techniques”
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