Carnegie Mellon University

The Jewelry Store Robbery: Teacher's Notes


  • fiber samples in zip-lock bags candle
  • candle holder
  • matches
  • microscope
  • microscope slides and cover slips
  • lamp or flexible flashlight if needed
  • forceps
  • dropper bottle of water
  • safety glasses

Any available types of fibers can be used.  All of the fibers should, of course, be the same color. Red was required here for the plot, but by changing the story a little, any color could be used. Red and blue seem to be easy to match and are readily available in a great many types of fibers. Fibers can be purchased as sewing or craft thread, or can be pulled from fabric samples.

A completed data sheet is included as part of the teacher's notes.  As can be seen, for the particular samples examined, there is enough of a difference between them to differentiate one from another.

This lab exercise relies critically on student observation skills. While any of the fibers may be chosen as the unknown (evidence), polyester gives a distinctive black bead on the end of the fiber after the flame test.  For less experienced students, this is probably the best one to choose as the unknown. Alternately, cotton, with its coarse appearance under the microscope, is also a good choice.

You should examine all of the fibers you choose and perform flame tests to determine their characteristics, which may differ from those on the (completed) sample data sheet.

Some of the fibers do not self-extinguish when removed from the flame and will continue burning until completely consumed.    Remind the students to quickly blow out the flame in order to be able to observe the burned end of the fiber.

Choose a candle that will not easily tip over. Tea light candles or a wide-based votive candle work well.  It should be placed in something to contain any dripping wax.  A votive candle holder is an obvious choice, although a small petri dish also works well.

Any inexpensive microscope can be used to examine the fibers. If room light is not sufficient to illuminate the sample, a small flexible flashlight or a tensor light can be added to the set-up.