Iodine and Super Glue Smoking Methods

Review of 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 Developer: DFO, Ninhydrin, Silver Nitrate, or Physical Developer
• Smoking: smoking iodine and super glue

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

2. The palms of the hands and the soles of the feet are covered with friction ridges, and these ridges have sweat pores along their surface. The sweat from these two areas has only one type of sweat tassels: eccrine tassels. 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. The sebaceous glands exude fatty acids, glycerides and hydrocarbons. In other words, these secrets are oily in nature. Sebaceous glands are associated with hair follicles. The apocrine glands secrete water along with sodium, potassium, and iron ions, as well as protein, carbohydrate, and cholesterol.

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

Iodine Emanation: 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 is converted directly into iodine-gas vapors. All it takes is a little bit of heat to speed up the sublimation process.

Iodine fumigation 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 a crime scene or crime lab. But smoking iodine has some shortcomings.

smoking iodine

The iodine method of latent print development is generally the first step in attempts to develop latent prints that are considered reasonably recent. The advantage of this method is that it is non-destructive for subsequent tests with other chemicals.

Iodine vaporization does not require any sophisticated apparatus to deliver its vapors over a porous surface. Iodine vapors react with sebaceous sweat (oil) secretions that contaminate the fingertips. Touch your nose, cheek, ears, arms and chest and you have these secrets in your fingers.

Iodine vapors develop latent traces 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 delivered to a bank teller, or even a suicide note.

But as mentioned, smoking with iodine has its drawbacks, mainly that the traces that appear are fugitive: they start to fade soon after developing; and iodine fuming only works on prints believed to be reasonably recent. You will hear more about this in a moment.

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

The smoking 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, it may be a plastic zip-lock bag. In the crime lab, this may be a converted fish tank or a smoke chamber built specifically for this purpose.

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

3. Iodine vapors are usually visible, a kind of purple haze. Once the fumes are seen, the heat source can be removed. Prints will be viewable in seconds.

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

5. Once the photos are complete, you can apply an iodine fixer/enhancer to the developed prints, providing a permanent image.

Smoking with cyanoacrylate:

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 used a material called cyanoacrylate to reveal latent prints on most non-porous surfaces.

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

If you have ever used it, you know that cyanoacrylate has a very strong and noxious odor. You may have seen commercials long ago showing how a drop of this glue could attach a car to the hook of a tow truck, which lifted it off the ground. In either case, the glue forms a strong bond, as long as 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 as a result of humidity.

So how can this reveal latent prints? It’s really very simple. A process known as chemical “smoking” is used. Crime lab technicians will carefully place various pieces of evidence in an enclosure. The first smoking tanks were nothing more than fish tanks with some kind of cover.

Several drops of glue are measured onto a smoking tray (usually an object similar to an aluminum mug). This is placed in the chamber and the chamber is sealed. Development without any speedup takes several hours, but a small coffee warmer can be used under the glue to speed up the process.

It is also possible to speed up the smoking process by adding a few drops of glue to a cotton pad. To protect the floor surface in the smoking chamber, place the cotton pad on top of a piece of aluminum foil or one of the cups listed above. This is a very good reason NOT to wear cotton gloves. Considerable heat is generated during the smoking process and can cause burns to hands.

Glue vapors will circulate within the chamber and come into contact with the various test items. Vapors will polymerize (turn solid) with any moisture on objects, such as fingerprint residue.

The resulting developed latent prints will be rock hard and conform to the ridge structure left behind by the suspect. Developed prints are white so visual and photographic contrast can be added to light colored surfaces using fingerprint powders or stains.

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

If you would like more information on crime scene investigation techniques, there is a wealth of information available on this website.