Forensics Lab 8.2: Revealing Latent Fingerprints Using Iodine Fuming

This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.

Iodine fuming has been used since the turn of the 20th century to develop latent fingerprints on porous surfaces, particularly paper. Iodine fuming is still widely used because it is inexpensive and easy, sensitive, and is non-destructive because the stains it produces are ephemeral. If it is used at all, iodine fuming is normally the first processing method attempted. Some forensics texts state that iodine fuming is used less often nowadays than formerly. That may be true in the limited sense that there are now many alternatives, but iodine fuming is still used frequently by many forensics labs. It’s cheap, fast, effective, and completely reversible. What’s not to like?

In the lab, iodine fuming is done in a chamber, but the process was adapted to field use quite early. The first iodine fuming wands were simple tubes with a small reservoir for iodine crystals. The operator warmed the tube in his hand and blew gently into one end of the wand. His breath vaporized iodine and expelled iodine vapor from the other end of the tube, which was aimed at the surface to be treated. Modern versions of the iodine fuming wand substitute battery power for body heat and the operator’s breath to avoid the risk of inhaling iodine vapor, but the principle remains the same.

Dennis Hilliard comments
I have seen iodine fuming done in a plastic zip-lock bag. The paper is placed in the bag with several iodine crystals and allowed to sit for a period of time while the crystals sublimate inside the zip-lock bag.

The fugitive nature of iodine-developed prints is a two-edged sword. On the one hand, it’s nice that iodine fuming is easily reversible. Developed prints simply disappear within a few hours or days as the iodine gradually sublimes from the prints, and that process can be accelerated by warming the prints slightly. On the other hand, there are times when it would be nice if the developed prints were a bit more permanent. The first method used to stabilize iodine-developed prints was to treat them with a starch solution. Iodine and starch combine to form a deep blue-black complex, which persists for weeks to months, depending on storage conditions. Later, benzoflavone was introduced as an after-treatment for iodine-developed prints. Prints treated with benzoflavone are effectively permanently fixed.

In this lab session, we’ll use iodine fuming to develop latent prints on paper. We’ll then treat those developed prints with a starch solution to fix them, at least temporarily.

Required Equipment and Supplies

• goggles, gloves, and protective clothing
• iodine fuming chamber (see Substitutions and Modifications)
• magnifying glass, loupe, or stereo microscope to examine specimens (optional)
• camera (optional)
• small sprayer bottle
• iodine crystals (a few crystals per run)
• starch solution (~1 g of soluble starch (or cornstarch) in 25 mL water; filter or decant off clear liquid)
• specimens of paper with latent fingerprints

The MAKE Forensic Fingerprinting Kit includes all of the specialty items needed for this lab session. Alternatively, you can purchase these items individually from Maker Shed or other laboratory supplies vendors.

Substitutions and Modifications

• You can make an iodine fuming chamber from any covered container large enough to contain your specimens. We used a 600 mL beaker with a watch glass for maximum visibility when photographing the process, but an old coffee can with a snap-on plastic lid works just as well. Although the iodine crystals sublimate at room temperature, the fuming process is much faster if you place the container in a pie plate or similar shallow container that contains hot tap water or place it on a hot plate adjusted to the lowest temperature setting. Depending on the container and the size of your specimens, you can simply roll the specimen loosely and allow it to unroll and expand to cover the inside surface of the container, or use a stiff wire and a paper clip to suspend the specimen inside the container.
• You may substitute discarded decongestant nasal sprayers or similar bottles for laboratory spray bottles. Alternatively, you can buy small spray bottles at a drugstore for about $0.50 each. If you don’t have a sprayer bottle, you can simply immerse your iodine-fumed fingerprints in starch solution.
• The DEA classifies iodine as a List I material, which means you may have to provide identification and fill out paperwork to purchase it. If you prefer not to buy iodine, you can make it from potassium iodide by following the instructions at http://homechemlab.com/iodine.html. You can also substitute a small amount of iodine tincture or Lugol’s solution for iodine crystals, although results are generally better if you use crystal iodine.
• Rather than making up starch solution, you can substitute water in which potatoes, pasta, or another starchy food has been boiled. Starch solution spoils quickly, even when refrigerated, so make up your starch solution no more than a day before using it and discard any unused solution.

Procedure

Before you get started, you need to create some fingerprint specimens to be fumed. Since iodine fuming is used almost exclusively for paper specimens, you should produce specimens on various types of paper, including ordinary copy paper, cotton bond, kraft or wrapping paper, thin card stock, the paper side of photographs, glossy magazine paper, and so on. You can also test random paper items you find around the house. (Remember, iodine fuming is reversible, so you won’t damage a specimen by fuming it.)

We found that latent fingerprints were difficult or impossible to develop by iodine fuming if we’d washed our hands soon before touching the paper. Apparently, soap removes all or most of the skin oils and other residues to which iodine fumes adhere. We obtained good results with specimens that we’d touched when our hands hadn’t been washed recently, and the best results when we intentionally rubbed our fingers against our noses or foreheads before making the latent prints.

1. If you have not already done so, put on your splash goggles, gloves, and protective clothing.
2. Set up your iodine fuming chamber and fill the pan that it sits in with hot tap water.
3. Place the specimen inside the chamber, either suspended from a stiff wire or against the inside wall of the chamber, with the print side toward the interior of the chamber.
4. If you’re not working outdoors, turn on the fume hood or exhaust fan. Sprinkle a few small crystals of iodine into the bottom of the chamber and replace the cover. The crystals should begin sublimating immediately, filling the chamber with violet vapor. If the chamber or lid is transparent, you should see latent prints becoming visible within a few seconds as orange smudges.

Figure 8-8. Latent fingerprints begin to appear seconds after the specimen is placed in the fuming chamber

5. Allow the fuming to continue until the ridge detail is evident in the developing prints. Depending on the specimen and the amount of iodine vapor present, this may anything from just a few seconds to a minute or more.
6. When development appears to be complete, quickly remove the specimen from the chamber and replace the lid. (Be careful not to inhale the iodine vapor, which is irritating and has a strong chlorine-like odor.) You can use the iodine vapor already present in the chamber to develop additional specimens, adding a few more iodine crystals as required to keep the chamber filled with iodine vapor.
7. Place the specimen on a clean, flat surface and examine it carefully under strong light with the magnifier or loupe. You should see fingerprints revealed in considerable detail, as shown in Figure 8-9. If you have a camera, shoot an image of the developed fingerprints for your records. Record the pertinent details for the specimen in your lab notebook.

Figure 8-9. Fingerprints after iodine fuming

8. Spray the specimen with the cornstarch solution to develop the prints. You needn’t drench the specimen; a gentle misting is sufficient. As the iodine reacts with the starch, the color of the developed prints changes to blue-black, as shown in Figure 8-10.
9. Record your observations in your lab notebook and tape the developed specimen into your lab notebook.
10. Repeat steps 3 through 9 for your other specimens. (Retain at least two iodine-fumed specimens that you have not treated with starch solution for later testing with other methods. We’ll use one of these for ninhydrin development and one for silver nitrate development.)

Figure 8-10. Iodine-fumed fingerprint (from Figure 8-2) after treating with starch solution

Review Questions

Q1: For what type(s) of specimen is iodine fuming best suited?

Q2: Is iodine fuming normally the first method attempted for revealing latent prints? Why?

Q3: What are two after-treatments used to preserve iodine-fumed fingerprints?

August 16, 2009