When the body’s natural oils and sweat are transferred onto another surface, latent prints are generated. Latent prints can be found on a variety of surfaces, but they aren’t always visible, necessitating the use of different types of powders and other methods to detect them, which are:
- Fingerprint powders,
- Chemical reagents, or
- Alternate light sources to detect them.
Mainly two methods are used to produce latent fingerprints at a crime scene.
- Physical Method
- Chemical Method
Iodine fuming is one of the fast and inexpensive chemical methods for developing latent prints. It’s an old but efficient technique. It is quite simple to use. This technique is Low-cost and non-destructive.
This technique used on both non porous and porous surfaces . This work great on fresh prints(few days). It mostly used on papers, raw wood, leather etc.
Iodine fuming should done before ninhydrin treatment.
Iodine fuming can be done in two ways; the principle is the same in both, but the procedure is different. The first is a laboratory procedure, and the second is for a crime scene (field use).
Process of Iodine Fuming In Forensic Laboratory:
The iodine crystals and the fuming chamber must be set up first.
► At normal temperature, iodine crystals are sublime (immediately transform from solid to gas), and chamber can also be filled with boiling water.
► To suspend the specimen into the chamber, use a paper clip and a sturdy wire.
► Wait 5 to 10 minutes until the violet fumes from the iodine crystals filled the chamber.
► Examine the specimen.
► On the specimen, brown (yellowish brown) hue fingerprints were produced. The fat and oil deposits of the latent fingerprints absorb iodine fumes. The brown to blackish-colored prints that are developed with iodine are not permanent unless fixed.
► Because fingerprints are not permanent, they should be photographed as soon as possible.
► Starch is used to fix fingerprints, giving them a blue-black appearance and keeping them fixed for weeks to months.
► The use of benzoflavone for permanent fixation darkens the print. benzoflavone was introduced as an after-treatment for iodine-developed prints. Prints treated with benzoflavone are effectively permanently fixed.
Process of Iodine Fuming at Crime Scene:
For field use experts should have following:
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
Step 1. We start with wool, then calcium carbonate, then wool again, Iodine crystals, and wool again, one by one.
(Some forensic firms provide a partially assembled set with rubber stoppers on both sides.)
Step 2. The next step is to blow the tube from one side to warm the iodine crystals (via vinyl tube or rubber pump in some cases). breath vaporized iodine and expelled iodine vapor from the other end of the tube, which was aimed at the surface to be treated. Iodine crystals emit vapours that are fumed to the targeted print from the other end of the tube.
Step 3. Make a one-inch space between the tube end and the target area.
Step 4. Grab the tube tightly so that some of your hand heat is transferred to the tube. (Some modern versions use battery power for heat, while others use disposable iodine fuming guns.)
Step 5. Now observe it. Iodine vapors are absorbed by fats contained in the fingerprints left on the paper. Iodine reveals the fingerprints by coloring them characteristic brown to blackish.
Caution when working with Iodine: Iodine is toxic and irritating and strongly corrosive. Always wear gloves and take necessary precautions like goggles, protecting clothing. Perform the experiment near window or under exhaust fan.
Images Courtesy: Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.