Fibers are class evidence because a questioned fiber cannot be matched conclusively against a known specimen. In the absence of a gross physical match–a torn questioned specimen that exactly matches the missing portion of a known specimen–the most that a forensic scientist can say with certainty is that the questioned specimen is consistent in every way with a known specimen.
Although we have found no such cases in the literature, there is no obvious reason why wool or other animal hair fibers could not be tested for an mtDNA match. We speculate that mtDNA testing is not used on animal-hair fibers from fabrics for two reasons: First, there is much less genetic variation among heavily-bred and selected livestock than among humans. Within relatively few generations, hundreds or thousands of individual animals may trace their lineage to a single champion female ancestor. Second, most animal-hair fabrics are made up of hair obtained in bulk from hundreds or thousands of animals in one batch. (Think sheep shearing.) It’s entirely possible, even probable, that one animal-hair fiber from a piece of fabric comes from a different animal than the fibers immediately adjacent to it in the fabric.
Although the chemical fiber tests described in the preceding lab sessions are important tools for initial screening among types of fibers, morphological examination can tell the forensic scientist a great deal about the individual characteristics of a questioned fiber. Morphological matching of questioned fibers to known fibers may provide compelling (although not definitive) evidence, particularly if one or more of the questioned fibers is relatively uncommon.
For example, assume that a murder victim has been found, wrapped in an old blanket, and dumped in a ditch along a back road. Upon examination, fibers are found adhering to the victim and to the blanket. It is determined that those fibers come from a particular type of carpet in a particular color that was used only in a certain make and model of car over a two-year period, and only in those cars that used a particular color of exterior paint.
The police have many potential suspects, but one of those suspects owns a car of the correct make, model, year, and color to match the questioned fibers found on the victim. After obtaining a search warrant, the police impound the car and submit it to forensic testing. Carpet fibers from the car are consistent in all respects with the fibers found on the victim.
By itself, this match is interesting, but not conclusive. After all, thousands of cars may have this particular carpet. But those thousands of cars are probably relatively evenly distributed across the country, so only a small percentage are likely to be found locally, which narrows things down considerably.
And we aren’t yet finished with the fiber evidence. Remember the blanket in which the body was wrapped. If fibers from that blanket can also be matched to fibers present in the car trunk, it becomes extremely likely that that car was used in commission of that crime.
Probabilities can be multiplied. For example, if it is determined that, say, only 0.01% (0.0001) of cars have that particular carpet and that only 0.1% of blankets use that particular fiber in that particular color, the probability that both fibers would be found randomly becomes (0.0001 * 0.001) = 0.0000001 or one in ten million. Furthermore, if fibers from the victim’s clothing can be matched to fibers in the car trunk–particularly if there are several different types of clothing fibers–it is extremely likely, nearly certain, that that victim was at some time present in that car trunk.
Although real forensics labs use instrumental analysis for definitive characterization of fibers, morphology remains an important aspect of fiber analysis. In this lab session, we’ll use many of the same techniques we used earlier in the chapter for human and animal hair to examine the morphology of natural and artificial fibers.
We’ll also use one additional technique, refractive index testing with polarized light, to reveal additional information about our artificial fiber specimens. In plane-polarized light, some artificial fibers are isotropic, which means they have only one refractive index, regardless of the polarization plane of the light used to view them. Other artificial fibers are anisotropic (also called birefringent), which means their refractive indices differ as the polarization plane is rotated relative to the specimen.
Of course, determining the refractive index of a single fiber is not a trivial matter, particularly using only the equipment practical for a home lab. Fortunately, there’s a work-around that, although it doesn’t provide a numeric value for refractive index, does allow us to determine the relative refractive index (RRI) of a fiber as we change the polarization plane.
The first step in this procedure is to determine if the fiber is isotropic or anisotropic, which can be done simply by viewing the wet-mounted fiber using light polarized in one plane and then rotating the polarizing filter 90Â°. If the fiber is isotropic, its appearance does not change as you rotate the polarization plane. If the fiber is anisotropic, its appearance changes as you rotate the polarization plane.
If the fiber is determined to be anisotropic, the next step is to determine the RRI. “Relative” in this case means relative to the refractive index of the wet-mounting medium. By choosing a wet-mounting medium with a refractive index close to that of the fiber (typically, about 1.54), you can easily determine if the RI of the fiber is higher or lower than that of the mounting medium. If you have time, you can mount the fiber with several mounting media of differing RI and determine an actual numeric value for the RI of the fiber.
Forensic scientists use expensive polarizing microscopes (also called petrographic microscopes, because such microscopes are commonly used by petroleum geologists) to determine RRI for fibers. We’ll use our standard compound microscope with an inexpensive piece of polarizing film to make the same determination.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- stereo microscope, loupe, or magnifier
- compound microscope (100X and 400X, with ocular micrometer)
- microscope slides and coverslips (as required)
- polarizing filter (see Substitutions and Modifications)
- disposable pipette
- ruler (graduated in mm)
- forceps or tweezers
- mounting fluid(s) (see Substitutions and Modifications)
- known and questioned fiber specimens
The Maker Shed carries a variety of chemistry supplies and equipment. Check out the Science Room section to see what’s currently available.
Although none of the activities in this lab session present any significant risks, as a matter of good practice you should always wear splash goggles, gloves, and protective clothing when working in the lab, if only to avoid contaminating specimens. Obviously, you may need to work without goggles when using a microscope or magnifier to examine specimens.
Substitutions and Modifications
- You can use just about any linear polarizing film or filter for this lab session. You can use a polarizing camera filter, but make sure it’s a linear polarizer rather than a circular polarizer.
- For the wet mounts, use a mounting medium with an RI close to that of artificial fibers, typically about 1.54. Clove oil (RI ~ 1.54) is a good choice. If you want to determine a numeric value for the refractive index of the fibers, you’ll need a set of RI fluids, which you can make yourself as described in Chapter 4.
This lab has three parts. In Part I, we’ll do a macroscopic examination of each specimen. In Part II, we’ll wet-mount the specimens and examine them microscopically. In Part III, we’ll determine if each fiber is isotropic or anisotropic and determine the RRI for each anisotropic specimen.
Part I – Macroscopic examination
- Examine each of your specimens, by eye and with the stereo microscope or other low-magnification optical aid.
- Record the following information about each specimen in Table 6-13: source, color, length, degree of curl, and general appearance of the fiber.
Table 6-13. Morphology of Natural and Artificial Fibers – observed macroscopic data
Part II – Microscopic Examination
The four major classes of fibers – animal, plant, mineral, and artificial/reconstituted – each share common class characteristics that determine the type and amount of information that can be gained by microscopic examination.
- Animal fibers are simply processed animal hair, and can be characterized on the same basis
- Plant fibers are often ribbon-shaped, may have twists at regular or irregular intervals, and often have little or no visible internal structure
- Mineral fibers appear rock-like or glass-like and often have jagged or fractured tips
- Artificial and reconstituted fibers are notable for their regular appearance and absence of variability in structure, and often have a multi-lobular structure and/or regular longitudinal striations that are never found in nature.
- Wet-mount each fiber as described in Laboratory 6.2, using clove oil (or another mounting fluid with an RI near 1.54) as the mounting medium.
- 2. Examine each specimen by transmitted and reflected light at medium (~100X) and high (~400X) magnification. For animal fibers, observe and note the characteristics described in lab sessions 6.2 and 6.4 in your lab notebook and/or in Table 6-14, as well as any of the other characteristics described below. For other fibers, observe, measure, and record the following microscopic characteristics about each specimen:
- Light transmission (transparent, translucent, or opaque)
- The color of the fiber by transmitted and reflected light, and whether the color is a pigment (discrete color bodies) or a dye (continuous color)
- The presence or absence of any delustrant (non-reflective particles, usually white) or other surface treatment
- The longitudinal morphology of the fiber (e.g., smooth, striated, serrated, twisted, kinked, or other), and a description of its surface texture (if any) and any embedded features such as air bubbles or voids.
- The appearance of the ends, such as square-cut, angle-cut, tapered, frayed, crimped, melted, etc.
- If you can determine the cross-sectional shape of the fiber from the longitudinal view, note it (round, oval, square, pentagonal, lobed, etc.) and measure the diameter of any visible lobes.
Figures 6-18, 6-19, and 6-20 show typical plant (cotton), animal (wool), and synthetic (polyester) fibers, respectively, at 100X magnification. Cotton fibers, as shown in Figure 6-18, are flat ribbons with a regular twist.
Figure 6-18. A cotton fiber at 100X magnification
As you might expect, wool fibers show features similar to those of other mammalian hairs, but are often dyed. The wool fiber shown in Figure 6-19 (from a Pendleton woolen scarf) has clearly been dyed and shows the prominent, broken medulla and diameter variations typical of wool.
Figure 6-19. A wool fiber at 100X magnification
The polyester fiber shown in Figure 6-20 is obviously too regular in every respect to be anything other than artificial.
Figure 6-20. A polyester fiber at 100X magnification
Part III – Refractive Index and Polarization Examination
In this part of the lab, we’ll determine the RRI of each fiber specimen, as well as its birefringence.
- Insert one of your wet-mounted fiber specimens into the slide holder.
- Place a polarizing filter under the microscope condenser, with its polarization plane oriented parallel (N||) to the longitudinal direction of the mounted fiber specimen.
- Close the substage condenser diaphragm to provide axial illumination.
- If the fiber is flat, focus critically on the edges of the fiber. If the fiber is roughly cylindrical, focus critically on the top surface of the fiber.
- While observing the fiber at 400X, use the fine-focus adjustment to increase the separation between the objective lens and the fiber very slightly (if your microscope is tube-focusing, raise the tube; if it is stage-focusing, lower the stage.)
If the refractive index of the fiber is higher than the RI of the mounting fluid, you will see a bright line (the Becke Line) move toward the center of a cylindrical fiber as you increase separation between the objective lens and the fiber. If the RI of the fiber is lower than the RI of the mounting fluid, the bright line will spread as you increase focus distance and the center of the fiber will become darker. For flat fibers, the Becke Line movement occurs at both edges of the fiber, but the movement is the same in either case: a shift toward the medium with the higher RI as the focus distance is increased.
You use only one polarizing filter in this lab session, because the fiber itself effectively acts as the second polarizing filter.
Figure 6-21 and Figure 6-22 show the movement of the Becke line toward the center of a cotton fiber (RI = 1.52+) wet-mounted in glycerol (RI = 1.47) as we opened focus.
Figure 6-21. At focus, the Becke line appears near the edge of the fiber
Figure 6-22. As focus is opened, the Becke line moves toward the center of the fiber
- As you observe the fiber, slowly rotate the polarizing filter by 90° (until the plane of polarization is perpendicular (N†) to the length of the fiber) and note the effect on the contrast between the fiber and the mounting fluid. If there is no apparent change, the fiber is isotropic, which means it has only one RI in polarized light. If the fiber appears to darken or lighten relative to the mounting fluid, that fiber is anisotropic (or birefringent), which means it has two or more refractive indices in polarized light.
- Note the degree of relief visible at each position. High relief means the fiber is distinctly visible with high contrast against the mounting fluid, and indicates that the refractive indices of the fiber and mounting fluid differ significantly. Low relief means that the fiber tends to blend into the mounting fluid, showing only a low-contrast image, and indicates that the refractive indices of the fiber and mounting fluid are similar. (If the refractive indices are identical, a colorless fiber disappears entirely; a colored fiber is visible only as a streak of color.)
- If the fiber shows birefringence, repeat steps 4 and 5 with the polarizing filter perpendicular (N†) to the length of the fiber to determine relative RIs for the fiber.
- Note your observations in your lab notebook and/or in Table 6-16.
- Repeat these tests for each of your known and questioned fiber specimens.
Figure 6-23 and Figure 6-24 show birefringence exhibited by a cotton fiber with the polarizing filter oriented to provide minimum contrast (Figure 6-23) and maximum contrast (Figure 6-24).
Figure 6-23. A cotton fiber viewed by plane-polarized light showing minimum contrast with the mounting fluid
Figure 6-24. A cotton fiber viewed by plane-polarized light showing maximum contrast with the mounting fluid
Table 6-15. Optical characteristics of fibers
|acetate||1.47 – 1.48||1.47 – 1.48||weak|
|acrylic||1.50 – 1.52||1.50 – 1.52||weak|
|cotton||1.58 – 1.60||1.52 – 1.53||strong|
|polyester||1.63 or 1.71 – 1.73||1.53 – 1.54||very strong|
|rayon||1.54 – 1.56||1.51 – 1.53||strong|
|triacetate||1.47 – 1.48||1.47 – 1.48||weak|
|vinyon||1.53 – 1.54||1.53||weak|
|wool||1.55 – 1.56||1.55||weak|
Table 6-16. Morphology of Natural and Artificial Fibers – refractive index data
Q1: What one morphological characteristic most easily discriminates a natural fiber from a manufactured fiber?
Q2: Would you expect an acetate fiber to exhibit low or high relief when wet-mounted in glycerol (RI = 1.47)? What if it were wet-mounted in clove oil (RI = 1.54)? What degree of relief would you expect for an acrylic fiber wet-mounted in these two fluids? Why?
Q3: You are examining a birefringent fiber by polarized light. With the polarizer oriented to provide maximum relief, you slowly rotate the polarizer by 90°. The relief decreases until it reaches a minimum at about 45°. The relief then begins to increase again as you continue rotating the polarizing filter, reaching another maximum at about 90°. What do you conclude about the refractive indices of the mounting fluid relative to the perpendicular and parallel refractive indices of the birefringent fiber?