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How to Identify Glass in Investigations

Forensic scientists uses various techniques to identify unknown substances recovered at a crime scene or found on a crime suspect. These traces of physical evidence, including hair, clothing fibers, body fluids and other materials, when examined closely can help solve a crime. Often, broken glass such as car windows, vases, interior windows and bottles provide important clues to what took place at a burglary, homicide or auto theft. Scientists can identify glass samples by measuring density, examining the physical properties (e.g., fracture pattern, polarity), taking fingerprints from the sample, doing a chemical analysis and seeing how fragments fit together.

Things You'll Need

Glass sample
Container (not glass) for tight seal
Beaker to measure volume of sample
Scale
Chemical analysis kit
Polarizers

Instructions

- 1
  
  Carefully collect glass evidence samples.
  
  Collect the glass sample to be investigated that is closest to where the impact took place. Find the largest available sample that is intact and can be packaged separately. Mark and package it carefully in a tightly sealed container.
- 2
  
  Measure sample volume by immersing in water.
  
  Measure the density of the sample by first weighing its mass. Then submerge it in a container of water, and measure the amount of water that is displaced. Divide the mass by the volume to get the density. The density of the sample can help match it to another piece of glass found elsewhere.
- 3
  
  The way glass breaks tells about the material and type of impact.
  
  Determine the type of glass based on physical examination. Types include rolled glass, safety glass, plate glass, bottle glass, crystal and Pyrex and other cooking glass. Look at the texture: ask yourself if it is laminated, textured, colored, flattened or thickened. Also ask yourself how it broke: cleanly or in radial- or concentric-fracture patterns.
- 4
  Use a chemical kit to test whether the glass is made of soda lime silicate (e.g., light bulbs), borosilicate (e.g., cookware), lead alkali silicate or aluminosilicate, four common types.
- 5
  Determine whether the sample is isotropic or anisotropic. Isotropic glass appears dark when turned while under crossed polarizing lenses. Anisotropic materials do not display this property and are not made of glass.

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