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The Jagged Edge. Glass Fragment Identification. Forensic Materials Science. Scientific Working Group for Materials Science SWGMAT provides Guidelines/Best Practices that have application to Fibers, Glass, Paint, Hair, and Tape. 

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the jagged edge
The Jagged Edge

Glass Fragment Identification


Forensic Materials Science

  • Scientific Working Group for Materials Science
  • SWGMAT provides Guidelines/Best Practices that have application to Fibers, Glass, Paint, Hair, and Tape. 
  • SWGMAT also develops standards to accredit laboratories and expert witnesses.

Evidence – Class vs. Individual

  • Class evidence shares physical characteristics with a group of objects or individuals.
  • Individual evidence originates with a single person or source.
  • Some categories of physical evidence can be only be class evidence while others can be either class or individual evidence, including glass.

Glass Evidence: Class or Individual?

  • Individual: Broken glass pieces can be fitted together like a puzzle. A specific fragment can be uniquely placed at a crime scene.
  • Class: Small fragments of glass can transfer to a victim or perpetrator of a crime or their vehicle.
  • After a hit and run accident, glass fragments consistent with a vehicle class can be identified, even if the specific vehicle is not known.

Glass Chemistry

  • Glass is made by heating silica (sand) with soda ash (sodium oxide, Na2O) and lime (calcium oxide, CaO) to a molten mass, then cooling it so quickly that large crystals do not form.
  • Glass is processed by rolling it into sheets or by blowing or molding to desired shapes.

Specialty Glass

  • Metal oxides are added to make colored glass.
  • Frosted glass has surfaces treated with acid or a plastic film.
  • Tempered glass is stronger than normal glass. It is made by a rapid heating and cooling process.
    • Pyrex® baking dishes
    • Corelle® dinnerware.

Polymer Glass

  • Polymer glasses are strong transparent plastics which can replace silica glass in eyeglasses, drinking glasses, windows or vehicle tail lights.
  • Polymer glass is molded from several different plastics
    • Acrylic
    • Polycarbonate
    • Polyethylene terephthalate

Windshield Glass

  • Car windshields are made with laminated safety glass.
  • Safety glass has a layer of plastic between two pieces of ordinary glass.
  • Windshields are placed in cars using gaskets to keep them rigidly in place.
  • Modern windshields are designed not to fall out of the vehicle even if they shatter.
  • The laminated glass can break if an object is thrust into the windshield.

Collection of Glass at Crime Scene

  • Every effort should be made to collect all the glass found if any possibility exists that glass fragments may be pieced together.
  • Typically two or more glass fragments are compared to determine if they originated from different sources.
  • Unless there is an exact fit between two pieces of broken glass, it isn’t possible to prove the glass pieces came from the same source.

Analyzing Glass Fragments

  • Forensic glass comparison requires the scientist to identify and measure properties that will match one glass fragment with another while minimizing or eliminating other glass sources.
  • Forensic scientists primarily examine two physical properties:
    • Density
    • Refractive index

Comparing Glass Densities

  • Density is mass per unit volume (g/cm3).
  • When two samples have the same volume, their weights will differ if the chemical elements that make up the material are different.
  • Glass with different elemental compositions will have different weights.

Comparing Glass Densities

  • The flotation method is a precise and rapid method for comparing glass densities.
  • A glass fragment is immersed in a series of liquids of varying densities. The glass chip will neither sink nor float in the liquid medium of the same density.

Measuring Refractive Index

  • The Refractive Index (RI) of a substance is a measure of the speed at which light travels (v) through that medium.
  • When light travels through two media with differing RIs, the light becomes refracted, or bent.
  • This occurs because when the speed of the wave of light changes, the direction of that wave also changes.

Measuring Refractive Index

  • As light passes the border between media, depending upon the relative RIs of the two media, light will either be refracted to a lesser angle, or a greater one. These angles are measured with respect to the normal line.
  • In the case of light traveling from air into water, light is refracted towards the normal line, because the light is slowed down in water; light traveling from water to air refracts away from the normal line, since light speeds up.

Measuring Refractive Index

  • Snell's Law states:

For a given pair of materials,

sine of angle of incidence θ(in material 1)sine of angle of refraction θ(in material 2)

is equal to v1 / v2, and equal to n2/ n1.

  • Sine (sin) is a trigonometric function. It is the ratio of the length of the side opposite an angle in a triangle to the length of the hypotenuse.
  • A scientific calculator will easily convert an angle into its sine.
measuring refractive index
Measuring Refractive Index


Normal line

refracted ray



incident ray


snell s law
Snell’s Law



The higher the n, the more the light bends


Measuring Refractive Index

The Becke line is a bright halo near the border of a particle that is immersed in a liquid of a different refractive index.

When the two RI are the same (the match point) the Becke line disappears and minimum contrast between liquid and particle is observed.

becke line

nglass >nmedium

nglass < nmedium

nmedium  = 1.525 nglass    = 1.60

nmedium = 1.525 nglass    = 1.34

Becke line:
becke line1
Becke Line

Kendall/Hunt Publishing Company


Refractive Indices for Common Items

Vehicle Headlight 1.47 – 1.49

Window 1.49 – 1.51

Bottle 1.51 – 1.52

Contact Lens 1.52 – 1.53


Refractive index of household liquids

Refractive index at 20°C

  • Baby oil: 1.45
  • Canola oil: 1.465-1.467
  • Olive oil: 1.467-1.4705
  • Soybean oil: 1.470-1.472
  • Grape Seed Oil: 1.471-1.478,
  • Castor Oil: 1.4750 - 1.4850
  • Corn Oil: 1.4735 - 1.4785
  • Xylene: 1.505
  • Clove Oil: 1.543

FBI Refractive Index vs Density Data

  • The FBI has compiled density and refractive index data for glass from around the world.
  • The FBI has identified a relationship between their refractive indices and densities for 1400 glass specimens that is better at classification.

Measuring Refractive Index

  • Glass Refractive Index Measurement (GRIM) system employs a phase-contrast microscope with a temperature-controlled hot stage that allows for precise heating and cooling of the sample being analyzed.
  • The GRIM method uses a microscope slide containing glass fragments placed in special heatable immersion oil. This slide is placed on the heated stage and the microscope is focused and aligned.

Measuring Refractive Index

  • The temperature of the microscope stage is set so the RI of the oil is higher than that of the glass sample.
  • The temperature is then automatically lowered and the contrast between the glass shard and the oil is monitored. The match point (temperature of minimum contrast) is recorded.
  • This is then repeated by gradually heating the slide and this match point is also recorded.
  • The two numbers are averaged and this number can be compared between two fragments of glass to determine if they match or are differ.

X-ray Diffraction (XRD)

  • In XRD analysis a glass sample is bombarded with X-rays, and the atomic composition of the glass is determined through detection of the characteristic scattering of those X-rays by the electron clouds of the individual atoms comprising the sample.
  • The scattering pattern is then analyzed by specialized computer software to determine these atoms.

Elemental Analysis

  • Laser Ablation- Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)
  • A small portion of the sample is removed from the surface by laser irradiation.
  • This removed sample portion is then converted to atomic ions (charged atomic particles).
  • An instrument called a mass spectrometer is then used to separate and analyze the characteristic ions produced based upon the mass-to-charge ratio of those ions.

Elemental Analysis

  • The elemental analysis of the evidence glass is then compared to the known glass from a crime scene (such as a broken window).
  • If no known glass exists, the evidence can be compared with other types of glass (headlamp, window, drinking glasses).
  • It is also possible that the company that made the glass or even the geographical location where the glass was made can be determined from the composition.