Iodine Fuming

The iodine fuming technique has historically been recognized as one of the earliest techniques available to the investigator for developing latent prints. Prior to the introduction of chemical methods, such as ninhydrin, iodine fuming was a preferred and recommended method for use on paper products. While frequently placed in the 'chemical techniques', the development of latent prints with iodine fumes is not a chemical action, but a physical one. Chemical reactions are those in which the application of a chemical will bring about a change in the physical properties of the latent residue, e.g., ninhydrin interacts with the physical qualities of amino acids in the latent residue. However, in iodine fuming, natural body fats and oils in sebaceous material of a latent print temporarily absorb the iodine vapors. This results in a change in color, from clear to a dark brown, until the effect fades with time. The color change can be made permanent by the application of certain materials, but the developed latent can usually be photographed at the greatest intensity of color change and then allowed to fade. The fixing of the iodine print with other applications can preclude additional processing by other chemical and physical means.

While iodine fuming is often considered archaic and not used in many cases, it should not be completely discarded. Iodine fuming is one technique that can be effective for examinations at the crime scene and in the laboratory environment. One of the oldest 'technical' tools is the iodine fuming gun. This simple device is illustrated in many of the early works on latent print development. It is simply a glass tube assembled in such a manner that blowing through the tube that contains iodine crystals and a breath moisture inhibitor generates iodine fumes. The use of this 'scientific' device at early crime scenes would have the same fascination to onlookers as the highly technical instruments in use today.

Another advantage is that iodine fuming is essentially a non-destructive technique. Its use does not inhibit other techniques and so it should always be the first attempt to develop latent prints. Also, if there is a need to conduct a covert examination in which a document would not appear to have been examined, the use of the iodine method can be considered. However, while the effects of iodine vapor do fade, the composition of certain paper could result in a more lasting discoloration and test exposures are recommended.

Iodine fumes are sensitive to different latent residues than other methods. It is possible that latent prints that would have been developed by iodine fumes will not be developed by other means, whose application will then destroy the residue needed for iodine development. When the nature of surface is questionable as to a particular technique, or sequence of techniques, iodine fuming should definitely be considered. That was the case some years involving a piece of surgical tape.

The white surgical tape had been used to bind a victim in a sexual assault incident. The victim was able to eventually free herself and tear off the tape. A piece of the tape was recovered from water in a toilet bowl where it had come to rest. The tape was submitted to the laboratory for examination. This was before the development of the special adhesive tape powders (Sticky-Side Powder™ and WetWop™). It was decided to use an iodine fuming device on the adhesive side of the surgical tape. Figure 1 illustrates the result. An excellent fingerprint, almost resembling a 'rolled' fingerprint, was developed, photographed and subsequently identified to the left thumb of the assailant. Early techniques may be old, but still viable. All techniques should be considered in their proper sequence in efforts to locate and recover latent prints of value to an investigation.

Iodine fumes are considered toxic and corrosive. You should take every precaution to avoid inhaling iodine fumes. Examinations should be conducted in a fume hood or used in a well ventilated area. Additional information on iodine processing can be found in Tech Notes at www.redwop.com.

Complete Iodine Fuming Kit
Catalog #1-2800 ..........$59.95

Iodine Crystals, 100 g.
Catalog #1-2805 ..........$29.95