Forensic Examination of Glass

To begin identification, the glasses refractive index must be determined.  This is done by placing the glass particle on a slide of a microscope equipped wth a heating element.  The particle is then covered with a drop of silicone oil and heated until the refractive index of the glass is the same as that of the silicone drop.  At this point the Becke line (a characteristic of glass that produces a halo) vanishes and the glass particles refractive index value can now be calculated.  After the refractive index has been recorded, the next step in identification is to determine the particles physical density.

Glasses physical density is rather easy to determine.  The glass particle does not need to be weighed; instead it is submerged into a liquid whose density is already known.  If the particle neither floats nor sinks, but remains suspended in the liquid, then the glasses density is the same as that of the liquids.  If neither one of these techniques are able to reveal much about the samples, new technology such as spectroscopy or neutron activation is now available. 

Spectroscopy involves the use of a carbon arc or laser beam to burn the glass sample releasing specific wavelengths of light relevant to the particle.  When the wavelengths are analyzed, specific elements present in the glass can then be identified.  A more recent technology involving neutron activation is also being used because unlike spectroscopy, it does not destroy the sample being analyzed.  Neutron activation is conducted by bombarding the sample with neutrons until it becomes radioactive.  The sample will then emit gamma rays, which have their own energy levels, each of which can then be measured.  After both the refractive index and the physical density are known, the painstaking task of piecing together the samples is undertaken.

When glass is fractured, distinctive patterns such as circles, ridges, and radial lines can form.  When compared closely under a microscope, an experienced forensic examiner can use these unique characteristics to piece together the particles similar to that of a puzzle.  In cases such as hit-and-runs, larger pieces of glass are likely to be found.  These pieces must be reassembled with the knowledge that their shape and the make of the car from which they came can be determined.  Since glass is actually a liquid and not a solid, shell-like fractures tend to form on the opposite side of that which the breaking force was applied.  Because of this also, under a microscope it can be of great use when fitting the broken flakes together.

With the ever changing field of forensic science, the techniques used to examine glass will be forever changing too.  Even though we may not be able to say with 100% conviction that a particular sample came from a particular source, this ability will more than likely be in our future.  But, for right now, we must use the techniques available to piece together the story the crime scene is trying to tell.  “Crime leaves a trial like a water-beetle, like a snail it leaves its shine” (Malay Proverb).