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ELEMENTARY, MY DEAR WATSON:
The Forensic Examination of Glass

12/22/04

Lab GraphicRemember that old "minute mystery" about poor Jack and Jill, their bodies discovered in the library with only broken glass and a pool of water left behind as evidence of the crime?

We're happy to tell you that Everything You Ever Wanted To Know about the forensic examination of glass is now available in this month's issue of Forensic Science Communications--68 pages worth.

So put on your Crime Scene Investigation hat and pick up some of that broken glass next to Jack and Jill to figure out whodunit.

1. Review definitions and map out a plan of analysis. This gives you basic definitions (e.g., glass is an inorganic product of fusion that has cooled to a rigid condition without crystallizing) and suggests you use a variety of analytical methods (see below) to yield a maximum of different kinds of information.

2. Collect, handle, and identify the evidence. Here you learn how to retrieve/collect the evidence from the floor (and possibly from the clothing of Jack and Jill's murderer)...how to select samples and how to clean and analyze them.

3. Make your initial examination. If possible, you'll want to determine the color, fluorescence, surface features, curvature, and thickness of the glass.

4. What about fractures? Are they radial or concentric? Crater or hackle? A fracture match is an absolute means of identification.

5. Now measure the density of the sample. Density tells you something about the composition and thermal history of the glass, telling you if you've got different sources of glass in the sample. You need a relatively large piece of glass for this test.

6. Can you measure the refractive index of the evidence? Refractive index is the most commonly measured property in forensic glass analysis. It can tell you the same sorts of things as density, but you only need a very small piece of glass. Testing both density and refractive index tells you more than just checking one or the other.

7. Now let's determine major, minor, and trace elements in the glass, even though it can destroy the sample. So be sure you've finished all the nondestructive exams first...and satisfied any legal considerations for an upcoming trial. Your choice: you can use scanning electron microscopy-energy dispersive X-ray spectrometry...X-ray fluorescence spectrometry...inductively coupled plasma-optimal emission spectrophotometery...inductively coupled plasma-mass spectrometry...laser ablation-inductively coupled plasma-mass spectrometry,,,or even atomic absorption spectrophotometery. The good and bad points of each are outlined.

The upshot: all fingers point to the butler! He was dusting when he knocked the fish bowl off the library table. Poor Jack and Jill.

Links: FBI Laboratory