Name: ________________________________________ Date:_____________ Class: _________
LAB: Fingerprint Development with Iodine Fuming
Background:
The iodine fuming technique is one of the earliest techniques available to the investigator for developing latent
prints. Before the introduction of chemical methods, such as ninhydrin, iodine fuming was a preferred and
recommended method to be used on porous (paper, cardboard, material) products. While frequently classified as a
'chemical technique', the development of latent prints with iodine fumes is not a chemical reaction, but a physical one.
Chemical reactions change molecules/bonds. For example, ninhydrin interacts with and changes the molecules/bonds
of amino acids in fingerprints. However, in iodine fuming, natural body fats and oils (from sebaceous glands) of a
latent print temporarily absorb the iodine vapors.
Iodine is originally in a solid, crystal form. It sublimes, changes from a solid directly into a gas, when it is not
under the pressure of a container. These gaseous molecules stick to the fatty acids in fingerprints, resulting in brown
colored prints that will fade with time. The color change can be “fixed” (made permanent) by spraying the print with a
starch solution. The starch reacts with the iodine, turning it purple, and permanently fixes it onto the paper.
While iodine fuming is often considered archaic and not used in many cases, its use should not be completely
discarded. It still has its advantages. One advantage is that iodine fuming is essentially a non-destructive technique. Its
use does not inhibit other techniques and so it should always be the first attempt to develop latent prints. Also, if there
is need to conduct a secret or concealed examination in which a document would not appear to have been examined,
the use of the iodine method can be considered. However, while the effects of iodine vapor do fade, the composition
of certain paper could result in a more lasting discoloration.
Iodine fumes act on different molecules in fingerprints than other methods. It is possible that the application of
another development method would destroy all molecules in a fingerprint, including the ones needed for iodine
development. When the nature of surface is questionable as to a particular technique, or sequence of techniques,
iodine fuming should definitely be considered first.
Procedure:
1. Your teacher will give you a small square of clean paper. Do not contaminate it!
2. Oil up your right thumb and make a good print in the box on the back of this paper.
3. Carefully pick up this piece of paper using forceps and place in your group’s fumigation
chamber. (small jar)
4. Each group will need a volunteer to carry out the iodine fuming procedure. This person will
need to put on an apron, gloves, and goggles and complete the procedure beneath the fume
hood, under the teacher’s supervision.
5. The volunteer will quickly and carefully place a few iodine crystals into the small chamber,
and replace the lid promptly and seal tightly.
6. The volunteer will allow this chamber to sit for 5-10 minutes. If you warm the chamber by
leaving it near a window or holding it with warm hands, this will speed up the reaction.
7. Once the fingerprints are developed the volunteer will carefully remove the paper using
forceps and replace iodine in original container.
8. The volunteer will immediately spray the fingerprints with starch to stop the chemical
reaction.
9. Make an ink print of your right thumb in the space below.
10. When iodine prints dry, find the match to your print below and then tape or staple it next to
your inked print.
Inked Thumb Print
Iodine Thumb Print
Conclusions:
1. Were you able to conclusively identify your iodine print?
2. Which types of surfaces does iodine fuming work on?
3. Give 3 specific examples of evidence on which this technique could be used to develop prints.
a.
b.
c.
4. Why should iodine fuming be used as a first attempt to develop prints on these types of surfaces?
5. What does “sublime” mean?
6. Which molecules in fingerprint does gaseous iodine stick to?
7. What is used to react with the iodine and “fix” the print permanently to the paper?
8. Why is iodine fuming not really a chemical reaction, but a physical one?