How Police Find Latent Fingerprints: Iodine Smoke and Super Glue – Part 4

Smoking methods with iodine and superglue

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 development: 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 treated in some way. The impression itself is made up of moisture, primarily water, but it 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. Sweat from these two areas has only one type of sweat pads: eccrine pads. 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. Apocrine glands secrete water along with sodium, potassium, and iron ions, as well as proteins, carbohydrates, 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 such as iodine crystals is converted directly into gas-iodine vapors. All it takes is a little 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. Criminologists and crime scene investigators have found this to be a valuable tool for developing latent prints at the crime scene or in the 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 onto a porous surface. Iodine vapors react with sebaceous (oil) sweat 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 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 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 begin to fade shortly after development; and iodine fuming only works on prints believed to be reasonably recent. You’ll 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 together with a document or other porous items. At the crime scene, it can be a zip-lock plastic bag. In the crime lab, it can be a converted fish tank or a smoke chamber built specifically for this purpose.

2. A low level heat source is used to start the sublimation process. This can simply be your hand wrapped around the zippered 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 visible 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 in each shot. If the prints are reasonably fresh, they should remain visible for 15 to 20 minutes or more.

5. Once the photos are complete, you can apply an iodine fixer/replenisher 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 of 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 is used by various cyanoacrylate manufacturers to describe their product.

If you have ever used it, you know that cyanoacrylate has a very strong and noxious smell. You may have seen commercials long ago that showed how a drop of this glue could attach a car to a crane hook, 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 are damp as a result of moisture.

So how can this reveal latent prints? It is 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 fishbowls with some kind of cover.

Several drops of glue are measured into a smoking tray (usually an aluminum cup-like object). This is placed in the chamber and the chamber is sealed. Development without any acceleration takes several hours, but a small coffee heater 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 a piece of aluminum foil or one of the cups mentioned 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 the hands.

The 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 revealed latent prints will be rock hard and will conform to the structure of the ridge left by the suspect. Developed prints are white, so visual and photographic contrast can be added on light colored surfaces using fingerprint powders or dyes.

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

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