Iodine and Silver Nitrate Processes

This article is the sixth in a comprehensive series of articles on latent print development techniques written by Pat A. Wertheim, Director of Training for Forensic IdentiŽcation Training Seminars, Ltd. Watch for future articles in each issue of Minutiæ and save these issues in a binder to compile a manual of Žngerprint development techniques.

Two of the earliest chemical methods used to develop latent prints are processing with iodine fumes and treatment with silver nitrate solution. Although both methods have largely been supplanted by more sophisticated chemical techniques, each still has a place in the repertoire of the well-trained latent print technician.

Iodine fumes concentrate in the sebaceous residue of Žngerprints as a brownish deposit. This method works on both porous and nonporous surfaces. In fact, prior to the discovery of Ninhydrin, iodine fuming was the preferred method of latent print development on paper. Although it is generally considered obsolete today, it still offers the advantage of developing prints without permanently altering or damaging the evidence. However, iodine is listed as both toxic and corrosive, so handling precautions are mandatory to avoid either personal exposure to the crystals or the fumes, or escape of the fumes into the laboratory environment.

Iodine may be used on any surface, but it is most commonly used on porous surfaces. There are several methods of exposing an item to iodine fumes. All involve the use of pure iodine crystals. At temperatures slightly above room temperature, the iodine crystals sublimate. Processing may be accomplished in a fuming cabinet in which a small dish of iodine crystals is placed on a heating device. Typically, such a cabinet is only large enough to hang letter-sized sheets of paper and has a glass window. The evidence is hung in the cabinet, the iodine is place on the heating device, and the cabinet is closed. Prints that develop are photographed immediately. Fixer is available to make the prints permanent, or simply leaving the evidence exposed to air will allow the prints to fade, thus returning the evidence to its unaltered, original state.

As an alternative to a fuming cabinet, a zip-lock bag may be used. A small amount of iodine crystal powder is placed in a bottom corner of the bag. The item (usually a sheet of paper) is placed in the bag and the bag is sealed. The bottom corner of the zip-lock with the crystals inside is held tightly in the palm of the hand to heat the crystals. Prints develop on the paper and can be seen and photographed through the plastic. Care should be taken not to let the paper come in direct contact with the iodine.

Silver nitrate is used on porous surfaces in a solution with distilled water. It is also classiŽed as toxic and corrosive, but does not give off dangerous fumes, as iodine does. A standard silver nitrate solution would contain approximately 3% silver nitrate. For example, three grams of silver nitrate would be dissolved in 100 milliliters of distilled water. While this process works on paper, it ultimately destroys the paper, making Ninhydrin much preferable for use on paper. However, on raw or unŽnished wood, silver nitrate generally performs better than Ninhydrin. Many people prefer Physical Developer over silver nitrate for wood, but Physical Developer only works well on smooth wood where silver nitrate can give good results on smooth and rough wood.

Silver nitrate reacts chemically with the salt (sodium chloride) in Žngerprint residue, resulting in silver chloride. This silver chloride breaks down under intense light, or with time, to leave elemental silver, which oxidizes to a black or dark brown stain. Unreacted silver nitrate remaining in the wood also breaks down, but at a much slower rate. Therefore, once latent prints develop, they must be photographed while at peak contrast with the background.

Some examiners recommend the use of clearing solutions following silver nitrate processing. Several such solutions are described in Scott's Fingerprint Mechanics.

As a reminder, both iodine and silver nitrate are toxic and corrosive. Protective clothing should be worn, along with nitrile gloves and plastic goggles. With iodine, care should be taken to avoid introduction of the fumes into the lab. Once processed, items should be treated as contaminated to avoid accidental contact with residue remaining on the evidence.

In the next issue of Minutiæ, Pat will write about Sticky-Side Powder and Gentian Violet processing of tapes.