1. Forensic Analysis of Glass Dr. John Woolcock – IUP Dept. of Chemistry

2. Forensic Examination of Glass
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Forensic Examination of Glass
Compare physical and chemical characteristics:
Optical properties: color and refractive index
Non-optical properties: surface wear, striations from manufacturing, thickness, surface film or dirt, hardness, density
Chemical properties: additives or trace elements

3. Forensic Examination of Glass
Goals in examining glass evidence:
Determine the types of glass at the scene
Determine how the glass was fractured
Use physical characteristics to classify it
Individualize the glass to a source

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What is Glass?
Fusion of sand (SiO2), soda (Na2O) & lime (CaO) that produces a transparent solid when cooled.
Physical properties: hard, elastic, brittle, non-conductor of electricity, density, refractive index, etc.

5. What Types of Glass Are There?
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What Types of Glass Are There?
The primary uses for glass are in windows, containers, light bulbs and eyewear.
Borosilicate Glass (pyrex): 5% borax (Na2B4O7) is added to resist breaking when heated or cooled.

6. What Types of Glass Are There?
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What Types of Glass Are There?
Colored Glass: metal oxides or colloidal iron (Fe) & sulfur (S) are added to change its color.
Lead glass: Pb increases refractive index & density

7. How Do Glass Windows Break?
Each force causes a deformation that may leave a visible mark or fracture the glass. This can be used to determine the direction and amount of force.
Glass acts initially as an elastic surface and bends away when a force is applied. When the force increases beyond its tensile strength, it cracks.

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How Does Glass Break?
Radial cracks form first and are propagated in short segments on the side opposite the force.
Concentric cracks come later from continued pressure on the same side as the force applied.

9. Types of Fractures by Projectiles
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Types of Fractures by Projectiles
Bullets are a projectile force (load) that can pass through glass.
As the bullet’s velocity increases, the central hole becomes smaller, cracking patterns become simpler, and the exit hole becomes wider than the entrance hole.

10. Which side was the bullet fired from?
Fractures by Projectiles
Which side was the bullet fired from?
Exit side is wider than entry side.
Stress lines on the glass edge of radial cracks form a right angle on the reverse side from the force.
Stress lines on the glass edge of concentric cracks form a right angle on the same side as the force.

11. Which Bullet Hole Was First?
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Which Bullet Hole Was First?
The sequence of impacts can be determined since crack propagation is stopped by earlier cracks.
In the figure above, which impact occurred first?

12. Learning Check
In the figure below, which impact occurred first?

13. Putting it Back Together Again?
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Putting it Back Together Again?
Examiners can fit together two or more pieces of glass that were broken from the same object.
Because glass is amorphous, no two glass objects will break the same way.

14. Collection of Glass Samples
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The glass fragments should be packaged in boxes to avoid further breakage.
If evidence is to be examined for glass fragments, it should be taken whole and each item individually wrapped in paper and boxed.
If even the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all glass fragments.
Submit glass evidence along with a representative sample of each type of glass from the crime scene.

15. Nonoptical Physical Properties of Glass
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Nonoptical Physical Properties of Glass
Surface striations and markings
Markings may indicate the glass’s orientation when pieces are missing
Surface scratches, etchings, and other markings may also be used to individualize evidence
Other Properties
Hardness=5-6 on Mohs scale; use a scratch test.

16. Glass Density Density can be measured by:
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Glass Density
Density can be measured by:
directly determining mass and volume (usually by displacement)
comparison by flotation
comparison using a density gradient column
Density gradient column method:
Fragments of different densities settle at different levels in the column of liquid of varying density.

17. Density by the Flotation Method
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Density by the Flotation Method
A glass particle is immersed in a liquid. The density of the liquid is adjusted by the addition of small amounts of another liquid until the glass chip remains suspended.
At this point, the glass will have the same density as the liquid medium and can be compared to other relevant pieces of glass which will remain suspended, sink, or float.

18. Why Measure Refractive Index?
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Why Measure Refractive Index?
Refractive index: ratio of the velocity of light in a vacuum to the velocity of light in any other medium
Nothing travels faster than the speed of light in a vacuum (roughly 3.0 x 108 m / s or 6.7 x 108 mph)
For example, at 25oC the refractive index of water is This means that light travels times faster in a vacuum than it does in water.
Refractive index of glass varies with small changes in composition or by how it is manufactured.

19. Refractive Index
N=1.52
N=1.33

20. Refractive Index By Immersion

21. Refractive Index By Immersion
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Refractive Index By Immersion
Immersing a glass particle in a liquid medium (silicone oil) whose refractive index can vary with temperature until it is equal to that of the glass particle.
At this point, known as the match point, the Becke line disappears and minimum contrast between liquid and particle is observed: RI oil = RI glass.
Translation – the glass disappears

22. Becke Lines Glass has higher refractive index-note white line inside
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Becke Lines
Glass has higher refractive index-note white line inside
Glass has lower refractive index-note white line outside

23. FBI Refractive Index vs Density Data
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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.

24. Chemical Analysis of Glass
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Chemical Analysis of Glass
Fluorescence
Under UV radiation, many glasses exhibit fluorescence (glow)
Caused by heavy metals (including tin) from or organic coatings
Scanning Electron Microscopy Energy Dispersive X-ray (SEM-EDX) Analysis
Can determine many elements simultaneously
Surfaces of samples (>50 mg) can be analyzed

25. Chemical Analysis of Glass
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Chemical Analysis of Glass
Atomic Absorption Spectroscopy (AAS)
You must first know which elements are present (weakness)
Can analyze ppm levels of elements present