Using Fingerprints to Investigate Arson
According to most professional criminal investigators, fingerprints obey three fundamental principles. These principles are:
- A fingerprint is an individual characteristic. It is yet to be found that prints taken from different individuals possess identical ridge characteristics.
- A fingerprint will remain unchanged during an individual's lifetime.
- Fingerprints have general characteristic ridge patterns that permit them to be systematically classified.
Paul Lemur's NotesContents
Read the FBI's introduction to classifying fingerprints here. This will tell you a little about how fingerprints are identified by their basic ridge patterns.
- VISIBLE PRINTS are prints made by fingers touching a surface after the ridges have been in contact with a colored material such as blood, paint, grease, or ink.
- PLASTIC PRINTS are ridged impressions left on a soft material such as putty, wax, soap, or dust.
- TRUE LATENT PRINTS are invisible print impressions caused by the perspiration on the ridges of one's skin coming in contact with a surface and making an invisible impression on it. Perspiration contains water, salt, amino acids, or oils and easily allows impressions to be made.
Paul Lemur's NotesContents
- Dusting with Carbon Powder on white or light colored surfaces.
- Dusting with Lanconide Powder for black surfaces.
- Dusting with Aluminum Powder for hard or dark colored surfaces as well as mirrors and metal surfaces.
- Use of Cyanoacrylate(Super-glue) fuming.
- Use of Iodine fuming techniques.
- Use of Ninhydrin.
- Use of Silver Nitrate.
- Use of Gentian violet.
- Use of Laser technology.
Paul Lemur's NotesContents
I. Preparation of sample evidence
One group of four students will prepare "evidence" for another group to be analyzed and identified. This evidence will consist of three right thumb prints on various surfaces. These will be labeled "Exhibit A", "Exhibit B", and "Exhibit C". Each group member will be responsible for preparing one set of three prints. Once prepared, each member, should place their set of three pieces of evidence into a separate small baggy. The four separate sets should then be placed into an evidence box for exchanging with another group at a later time.
Materials Needed (per individual):
One piece of dark colored ceramic tile, one microscope slide, and a table knife.
A. Preparation of "Exhibit A."
- Obtain a clean microscope slide. Carefully handle it only by the edge. If it is not clean or print free, wash and rinse it with distilled water. Wipe it dry, making sure that no unwanted fingerprints appear on the glass surface.
- Hold the slide in a paper towel or cloth and place a distinct RIGHT thumb print on the surface of the slide. If the print is smudged, not clear, or barely visible, wipe off the slide and try it again. In order to take a good print, place the side of the thumb on the slide and slowly, but with slight pressure, roll it across the slide once. Do not roll it back and forth since the print will smudge. To check if the print is usable, hold the slide up to your mouth and exhale across it. The moisture from your breath will momentarily show the existence of the print. If the print did not take, attempt again with a cleaned slide. This time, rub your thumb over an oily portion of your face or hair. Blot your thumb so that the excess oil will not smudge the print, and then place a good print on the slide as described before.
- Use a grease pencil to label the slide as "Exhibit A."
B. Preparation of "Exhibit B"
- Obtain a piece of dark ceramic tile. Wash it as previously described above. Transfer a RIGHT thumb print by using the same procedure.
- Use a grease pencil to label the tile as "Exhibit B."
C. Preparation of "Exhibit C."
- Obtain a table knife. Wash it as previously described . Use the same procedure to place two RIGHT thumb prints on the knife: one on the handle and one on the flat part of the blade.
- Use a grease pencil to label the knife as "Exhibit C."
D. Preparation of "Exhibit D."(one per laboratory group of four).
- Obtain a spoon. Wash it as above. Select one member and, by the same procedure, have them place a RIGHT thumb print on the open bowl part of the spoon.
- Use a grease pencil to label the spoon as "Exhibit D."
Each person should have placed their 3 pieces of evidence(Exhibits A, B, C) into a ZiplocTM baggy. Exhibit D also should be placed into a separate baggy by itself. All five baggies of evidence should be placed into an evidence box for later distribution.
II. Preparation of Direct Prints for a Class File
Materials Needed: Modus Operandi(MO) sheets for each member, ink pad, cleanser.
- Each group member will fill out an MO sheet using a fictitious name (Barb Wyre, Mike R. Fone, Polly Merz, etc.) but writing his/her real name on the back of the sheet.
- One person from the group will act as the "officer on duty" and will take the fingerprints of each member using the ink pad. Both right and left hands are taken (10 prints). Lightly inking the fingertips and carefully rolling them one at a time, neatly and without smudging, onto the proper finger locations on the MO sheet is an important task. A good print to compare with is a necessity in fingerprint identification. The inked fingers may be cleaned with a good detergent or cleanser such as Comet ® after the procedure.
- When they dry, place the MO sheets from all members of the group into the box containing the "evidence" prepared in Part I.
III. Analysis of the "evidence"
Each group should exchange their box of "evidence" and their MO sheets with another group. Working within their own group, the students will analyze the fingerprints and determine the identity of the person who made them. For the three separate pieces of evidence, students should place the lifted prints on a "3 x 5" index card and determine whose prints they were by using the included MO sheets. Students should check their results with the other group and with their teacher . The group will next use the "super glue" method to analyze the prints on the spoon and again match with the separate MO's.
A. Dusting for and Lifting Prints from a Smooth, Non-Porous Surface: Exhibits A, B, and C.
Materials Needed: Exhibits A, B, and C, 3-different dusting brushes, 3-different powders(Carbon, Lanconides, Aluminum), newspapers, cellophane tape, index cards, magnifying glass, MO sheets.
This procedure should be accomplished over a large sheet of newspaper so as to facilitate clean-up afterwards.
Obtain a brush for dusting the print. Make sure it is clean and the bristles are separated from each other. Small quantities of the three powders may be place in separate petri dishes and distributed to each laboratory set of four members. Use a different brush for each powder.
Various dusting powders can be used depending on the type of surface to be examined . Use very fine carbon black powder on a white or transparent surface. Use a white powder, like Lanconide, for prints on dark surface. Aluminus powder can be used on either type of surface . Use either the carbon black or aluminum powder to dust for the print on the slide.
CAUTION: METALLIC DUST CAN BE HARMFUL TO LUNGS IF INHALED!
- After the evidence boxes are exchanged between groups, each individual member will select a different baggy of evidence. Each should select an exhibit(A, B, C) from the baggy, the proper powder, and an appropriate brush . A small amount of the proper powder is placed in a small petri dish. The brush is dipped into the powder and the area containing the print is lightly dusted. After all of the print is developed, the excess powder is removed by gently blowing or brushing the powder off in one direction. If a brush is used, the excess powder can be brushed back into the petri dish for later use. Be extremely careful not to destroy the print with too hard a brush stroke. If necessary, sprinkle a small amount of the powder on the suspected print and lightly blow off the excess. This method may also help to develop a print that is difficult to obtain.
- If a camera is available, try to photograph the print. A Polaroid with black and white film works well.
- Lift the print . To lift the print and to transfer it to an index card, unroll about 3" to 4" of cellophane tape, place the loose end of the tape to the right of the latent print on the objects, and carefully allow the tape to cover the whole print. Slide your thumb over the face of the tape and smooth it down over the print to force out all air bubbles. The print can be removed by pulling up on the roll end of tape and then placing the removed print on the Index card in the same manner as the tape was placed over the print. Make sure the tape is secure. Cut the tape from the roll.
- Everyone in the group should observe the lifted print under the magnifying glass and compare it to the right thumb prints on the MO's. Identify the owner by using the FBI classification chart here (Note: This is a pdf file that will download to your computer). Decide which basic ridge pattern each print has in order to help you identify it more quickly. Record the name of the owner on the student data sheet and then check with them to verify your conclusion.
- Use the same procedure to analyze the other two exhibits(not the spoon). Select the proper powder and brush for the material to be examined. Lift the prints in the same manner, as previously described, placing them next to the other prints on the same index card. Identify each print by using the enclosed MO sheets. Attach the index card to the Student Data Sheet and record all pertinent data. Once the suspect is identified, check with that person to verify your conclusion.
Paul Lemur's Notes
B. Using Cyanoacrylate (from superglue) to Visualize Prints (do one per lab group)
CAUTION: Super glue will adhere to your skin and possibly ruin the object being examined.
Materials needed: Large jar, Foil, "Super-glue", string, Exhibit "D"
- Obtain a large container-e.g., large old fruit jar.
- Cover the bottom of the jar with foil.
- Add 10-20 drops of superglue spread evenly across the bottom.
- Suspend the spoon (Exhibit D) by a string into the jar. It should not touch the bottom or the sides of the jar. Cover the top tightly with foil and place a heavy object such as a book over the jar to make it air tight.
- In 24 hours, a permanent print should appear.
- Identify the print as before and record the results on the Student Data Sheet.
C. Extension Exercise
Once the procedures above are properly accomplished within your own lab group, try to make and lift prints off other objects in the room such as a beaker, a window, a mirror, a black hard table surface, and a chrome plated water or gas pipe. The added practice in dusting, lifting, and identifying various prints will definitely help in the future arson investigation.
The FBI's introduction to fingerprint identification
Newton's Apple Murder Mystery: a little more on forensic science and what is involved in gathering evidence at a murder scene. This page has an activity for making plaster casts of footprints!
Newton's Apple DNA Fingerprinting Activities: learn the basics of using DNA evidence to identify a criminal.
Paul Lemur's NotesContents