This article incorporates, in modified form, material from the soon-to-be-published Illustrated Guide to Home Forensic Science Experiments.
You’ll need the following items to complete this lab session. (The standard kit for this book will be available from http://www.thehomescientist.com by the time the book is out, and includes the items listed in the first group.)
Materials from Kit
- fingerprint brush
- fingerprint powder, black
- fingerprint powder, white
Materials You Provide
- Camera with macro capability (optional)
- Desk lamp
- Index cards or paper (white and black)
- Scanner (optional)
- Tape, packing or similar transparent
- Ultraviolet (UV) light source (optional)
- Specimens: objects with fingerprints
We’ll begin with the oldest fingerprint development method, dusting. With the exception of using magnetic powers to treat recently-touched paper, dusting is used almost exclusively on nonporous surfaces, and can provide excellent results if it’s done skillfully. If not done skillfully, dusting can easily damage or destroy any latent fingerprints present, as we found out and you probably will, too. In this lab session, we’ll dust various specimens, using dark or light dusting powder as appropriate for the color of the surface. We’ll then do a tape lift to preserve the fingerprints we’ve developed.
Before you get started, you need to create some specimens to be tested. Choose several nonporous items, such as a drinking glass, beverage can, and so on. Use a soft cloth to polish the item clean, removing any contamination. For your first attempts at dusting, you may find it difficult enough to get usable prints even under perfect conditions. You can increase the likelihood of having usable prints on your specimens by rubbing your forefinger against your nose or forehead and then carefully pressing your finger into contact with the surface, making sure not to smear the prints. Once you’re comfortable with this training-wheels version of developing prints by dusting, you can try dusting random objects from around the house to get a better idea of the highly variable quality of real latent fingerprint specimens.
In addition to creating latent fingerprints on several non-porous objects, create at least one set of latent fingerprints on an ordinary sheet of paper. You’ll use that specimen to determine how suitable dusting is for developing prints on non-porous surfaces.
Procedure IV-1-1: Dusting Latent Fingerprints
- Wearing gloves and handling the questioned object by the edges or otherwise as required to avoid damaging any latent prints, observe the object by oblique lighting from the desk lamp or other directional light source. Record your observations in your lab notebook, and note the approximate location of any latent prints that are made visible by the oblique lighting.
- If you have a black light or other UV light source, repeat step 1 using that light source.
- Place the object on a clean, flat surface with the suspected location of the latent prints accessible. (Don’t forget to wear gloves.)
- Choose the dark or light fingerprint dusting powder, according to which will provide better contrast with the color of the surface.
- Transfer a small amount of the dusting powder into the lid or work directly from the jar that contains the powder. Dip just the tips of the bristles of the brush into the powder so that a small amount of powder is retained by the bristles. Tap the brush gently to return excess powder to the container.
- Under a strong light, use a circular, twirling motion to sweep the brush gently over the area to be treated, as shown in Figure IV-1-1, allowing the bristles to just barely contact the surface. Continue depositing powder lightly until the latent fingerprint begins to develop, concentrating on that area as it becomes clearer where the latent prints are on the surface. If necessary, add more powder to the brush using the procedure in step 5. When the ridges begin to appear, change the direction of motion to follow the direction of the ridges. Once the fingerprint is developed clearly, stop dusting immediately. Beginners tend to overdevelop prints, which almost invariably causes loss of detail if not loss of the entire print.
Figure IV-1-1. Barbara dusting a specimen for fingerprints
- Use the brush or a puffer bulb gently to remove any excess powder. (Okay, we admit it; we used our mouths to puff off excess powder, but that’s a horrible practice.) You can also use canned air if you do so extremely carefully, keeping the canned air nozzle far enough away from the dusted print to avoid blowing away everything, including the print. If you have a camera, shoot an image of the revealed print.
- Repeat steps 1 through 7 for each of your other specimens, including at least one set of latent fingerprints on a sheet of paper.
You’ll probably find that your first efforts are poor but you improve rapidly with practice. Of course, getting really good at dusting prints requires lots of practice. Professional fingerprint technicians can work wonders when dusting latent prints, but then they have years of experience in doing it. Figure IV-1-2 shows a specimen after dusting but before lifting the fingerprints.
Figure IV-1-2. Dusted prints on the surface of the specimen
Procedure IV-1-2: Lifting Developed Fingerprints
- Choose one of your better developed prints. (If possible, shoot an image of a print before attempting to lift it. Accidents happen.)
- Wearing gloves, lift the free end from the roll of lifting tape and smoothly pull out about 6 cm to 7.5 cm (2.5 to 3 inches) of tape from the roll. Don’t touch the sticky surface of the tape, and do not cut the tape from the roll.
- Press the free end of the tape into contact with the surface, starting 5 to 6 cm from the nearest part of the dusted print. Make sure the tape adheres firmly to the surface.
- Beginning at the free end, use your fingers to carefully press the tape down onto the surface, making sure that no air bubbles are trapped.
- Continue pressing tape onto the surface, unrolling more as necessary, until you have covered the entire print with tape and continued for a couple of centimeters past the print.
- Using the roll as a handle, peel the tape from the surface using one smooth motion, as shown in Figure IV-1-3. It helps to put one finger on the free end of the tape to make sure the tape doesn’t curl back on itself.
Figure IV-1-3. Barbara lifting a dusted fingerprint from a specimen
- Stick the free end of the tape near one edge of a transfer card of a color that contrasts with powder you used to dust the print. Make sure the free end adheres tightly to the transfer card, and then carefully press the tape into contact with the transfer card, making sure to avoid air bubbles.
- Cut the used tape from the roll and press the free end into contact with the transfer card. Label the transfer card with your initials, the date and time, and the object from which the print was lifted.
- Repeat steps 2 through 8 for your other specimens.
This procedure works well for most lifts, but what if the specimen isn’t flat? If the contour of the surface makes it impossible to follow these directions exactly, modify the procedure as necessary, for example by removing the tape from the roll before pressing it into place.
Figure IV-1-4 shows a transferred print.
Figure IV-1-4. A partial fingerprint after transfer
What if it ain’t flat?
The procedure described works well for most lifts, but there are times when the contour of the surface makes it impossible to follow these directions exactly. In such situations, you may need to modify this procedure slightly, for example by removing the tape from the roll before pressing it into place.
Q1: Should dusting be the first or last method attempted to raise latent fingerprints?
Q2: Is dusting better suited for porous or nonporous surfaces?
August 16, 2009