How Police Find Latent Fingerprints-Iodine and Superglue Fuming – Part 4

Iodine and Superglue Fuming Methods

Review from Part 1 of this series:

The three contemporary methods used to develop latent fingerprints are:

  • Latent print powders: oxide, metallic, combination, fluorescent and magnetic powders
  • Chemical development: DFO, Ninhydrin, Silver Nitrate or Physical Developer
  • Fuming: Iodine and Superglue fuming

1. A latent fingerprint is one that is generally not visible unless it is treated in some manner. The print itself is composed of moisture-mostly water-but it also contains small amounts of the constituents of perspiration (sweat), like amino acids, lactic acids, creatinine, choline, sugars and uric acid.

2. The palms of the hands and soles of the feet are covered with friction ridges, and these ridges have sweat pores along their surface. Sweat from these two areas has one single kind of sweat glands: Eccrine glands. Only water and the substances listed above are exuded from these glands.

3. Other parts of the body have two other sweat glands: Sebaceous and Apocrine glands. Sebaceous glands exude fatty acids, glycerides and hydrocarbons. In other words these secretions are oily in nature. Sebaceous glands are associated with hair follicles. The Apocrine glands secret water along with ions of sodium, potassium, and iron as well as proteins, carbohydrates and cholesterol.

4. When the hands come into contact with the face and hair they become contaminated with this mix.

Iodine Fuming: Iodine crystals have a unique characteristic-when exposed to air above room temperature they begin a process known to scientists as sublimation. This process is when a solid like iodine crystals converts directly into a gas-iodine fumes. All it takes is a little heat to accelerate the sublimation process.

Iodine fuming is a process used to develop latent fingerprints on porous surfaces such as paper, cardboard and raw wood. Criminalists and crime scene investigators have found this to be a valuable tool for developing latent prints at the crime scene or the crime lab. But iodine fuming has a few shortcomings.

Iodine Fuming

The iodine method of latent print development is generally the first step in the attempts to develop latent prints that are thought to be reasonably fresh. The advantage of this method is that it is non-destructive to subsequent testing using other chemicals.

Iodine fuming requires no sophisticated apparatus in order to deliver its fumes onto a porous surface. Iodine fumes react to the sebaceous sweat secretions (oil) that contaminate the fingertips. Touch your nose, cheek, ears, arms and chest and you have these secretions on your fingers.

The iodine fumes develop latent prints that are orange to brown in color. This process works best on prints that are known to be fresh; like a ransom note, a holdup note handed to a bank teller or even a suicide note.

But as mentioned, iodine fuming has its shortcomings-mainly that the prints that do appear are fugitive-they begin to fade shortly after development; and iodine fuming only works on prints that are thought to be reasonably fresh. You will hear more on this in a moment.

SPECIAL NOTE: Iodine fumes are toxic. Take all possible precautions.

The fuming procedure is relatively simple:

1. Iodine crystals are placed in a confined area along with a document or other porous items. At the crime scene this can be a zip-top plastic bag. In the crime lab it can be a converted fish tank, or a fuming chamber specifically built for this purpose.

2. A low level heat source is used to begin the sublimation process. This can be simply your hand wrapped around the zip-top plastic bag containing iodine crystals or an inexpensive coffee warmer.

3. The iodine fumes are usually visible-a kind of purple haze. Once the fumes are seen, the heat source may be removed. Prints will be visible in seconds.

4. The next step is to remove the evidence being processed, and then photograph any visible latent prints. Be certain to include a scale that is visible in each shot. If the prints are reasonably fresh, they should remain visible for 15-20 minutes or longer.

5. Once photographs are completed, you may apply an iodine enhancer/fixative to the developed prints, which provides a permanent image.

Cyanoacrylate Fuming:

Back in the late 1970s members of the US Army Crime Laboratory in Japan were shown an interesting method for developing latent fingerprints on non-porous surfaces such as metal, painted wood, plastic and glass. Instead of using powders, members of the Japanese National Police Force used a material called cyanoacrylate to develop latent prints on most nonporous surfaces.

At that period of time this material was popularly known as Super Glue. The name Super Glue was once trademarked, but through common usage, the term superglue has become generic and several cyanoacrylate manufacturers use it to describe their product.

If you have ever used it, you know that cyanoacrylate has a very strong, noxious odor. You may have seen commercials long ago showing how a drop of this glue could bond an automobile to the hook on a crane, which lifted it off the ground. In any case, the glue does form a tight bond-provided there is a very thin film of moisture (water) on both surfaces to be bonded. And of course most surfaces on this planet have moisture on them as a result of humidity.

So how can this develop latent prints? It is really very simple. A process known as chemical “fuming” is used. The crime lab technicians will carefully place several items of evidence into an enclosure. Early fuming tanks were nothing more than fish tanks with a cover of some sort.

Several drops of glue are measured out into a fuming tray (usually an aluminum cup-like object). This is placed in the chamber and the chamber is sealed up. Development without any sort of acceleration takes several hours, but a small coffee warmer can be used under the glue to accelerate the process.

It is also possible to accelerate the fuming process by adding a few drops of the glue onto a cotton pad. To protect the surface of the floor in the fuming chamber, place the cotton pad on a piece of aluminum foil or one of the cups mentioned above. This is one very good reason for NOT wearing cotton gloves. Considerable heat is generated during the fuming process and may cause burns to the hands.

The glue fumes will circulate inside the chamber and will come in contact with the various items of evidence. The fumes will polymerize (turn into a solid) on any moisture on the objects-like fingerprint residue.

The resulting developed latent prints will be hard as a rock, and will conform to the ridge structure that the suspect left behind. The developed prints are white in color so visual and photographic contrast may be added on light colored surfaces using fingerprint powders or dye stains.

Many crime labs suggest that CSIs fume evidence with cyanoacrylate to prevent damage or loss of the latents during transport to the lab.

If you would like to learn more about crime scene investigative techniques, an abundance of information is available from this website.

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