Carnegie Mellon University


While closing up at the end of the day, jewelry store owner, Miles Johnson discovered that five very expensive rings were missing from a still­ locked display case.  Police suspected an "inside job" since the lock on the case did not appear to have been tampered with. All employees had access to the key.

While examining the case, police discovered a small piece of red fiber that had gotten caught on a sharp piece of metal on the framework of the glass display case. Mr. Johnson said that most of the employees wore red clothing that day because a promotional photo for a Valentine's Day sale was to be taken.  Police contacted the photographer who supplied a color print of the photo.

Mr. Johnson identified the employees who were wearing red clothing. Police obtained search warrants and recovered the clothing worn on the day of the theft. Sample fibers were removed from each garment.  The manufacturer's label was used to identify each type of fabric.

Compare the fabric found in the jewelry case to the samples taken from the employees' clothing.  Who is the thief?


Fibers used in clothing can be classified into three general categories based on their origin; animal, plant, and synthetic.

Animal fibers include wool sheared from the coats of sheep, hair from llamas, cashmere goats, Angora goats, and rabbits, and silk from the cocoons of silkworms.   After cleaning, wool and hair fibers are spun into thread, while silk requires a different kind of processing.

Silk was first made in China around 2700 B.C. and introduced into Europe by the Romans about 550 A.D. A very strong fiber, silk threads are nearly as strong as iron wire of the same diameter.  Because the strands are very smooth and elastic, dirt does not cling to silk very well.
Silk worms are raised on farms. The female moth lays 200-500 eggs which are hatched in large incubators. The worms feed on mulberry leaves for five weeks and then each is placed in a tiny cell in a special rack.  The worm spins a cocoon with a glue-like fluid from glands in its upper lip.  When the fluid hits the air, it hardens into a silk thread.  The worm spins a continuous thread surrounding its entire body.

The cocoons are then unwound, sometimes by hand, and the threads are soaked in water to loosen the gum-like substance that was holding the cocoon together. The clean thread is wound onto reels and is then manufactured into cloth.

Plant fibers are made from the bark, stems, leaves, nuts, or stalks of many different kinds of plants.  Examples include linen, made from the stem of flax plants, and cotton, probably the most widely used fiber in the world. Cotton was used by the Aztecs to make cloth over 8000 years ago. The cotton fibers, called lint, are picked from the seed pod, cleaned of seeds and debris, and then spun into thread.

The third category of fibers is synthetic fibers, produced by the chemical industry.  These fibers often have superior qualities compared to natural fibers.  The first synthetic fiber was actually made from the plant material cellulose, derived from cotton and wood pulp.

Cellulose is a chemical substance that forms in the cell walls of plants. It helps make roots and stems rigid and stiff and helps the plant to support itself. Liquid cellulose is forced through tiny openings in devices called spinnerets to form filaments which are twisted together to form threads.
The process was developed in France in 1884 and first used in the United States in about 1910. The fiber produced was called "rayon"; "ray'' because of the shiny appearance of the fiber and "on" because it was cotton-like.

Other synthetic fibers are produced from petroleum products.  These fibers are strong, resilient, light-weight, and may dry quickly or be stain­ resistant. Synthetic fibers include nylon, polyester, polypropylene, and acrylic.  They are also generally produced by the spinneret process.

Forensic analysis of fibers involves visual and physical examination of some of their properties including the following:

  • Thermal - melting point, flame resistance
  • Optical - color and luster
  • Surface - roughness, machine marks made during processing
  • Chemical - resistance to acids, bases, etc.

Under a microscope, most natural fibers have a distinctive rough appearance. Silk and most synthetic fibers which are produced by drawing out and solidifying a liquid have smooth surfaces.  This makes them difficult to distinguish from one another using just a microscope.  Additional testing is usually required.

We will use a simple flame test to examine the characteristics of different fibers. In a flame test, the fiber is brought into contact with a flame. Different types of fibers may react differently, such as melting or burning, or producing a light or dark colored ash.

By using both sets of observations (microscopic and flame testing), it should be possible to differentiate between the fibers being examined.