1. Glass
2018

2. 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
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. What is Glass?
An amorphous fusion of mineral compounds that produces a transparent solid when cooled.

4. Types of Glass by Composition
Soda-Lime Glass
Most common and least expensive
Windowpanes and bottles
Made up of:
quartz sand, sodium carbonate and “lime” or calcium oxide.

5. Types of Glass by Composition
Lead glass (“crystal”)
Decorative glassware, radiation protection
Lime replaced with >20% lead oxide (PbO)
– increases refractive index
– reduces viscosity and melting point

6. Types of Glass by Composition
Borosilicate Glass (Pyrex®)
Cookware, laboratory glassware, light bulbs
Lime replaced with boron oxide (B2O3)
– increases resistance to thermal shock

7. Types of Glass by Manufacturing
Laminated glass
Two sheets of glass with plastic between them.
Used in windshield and skylights

8. Types of Glass by Manufacturing
Tempered glass
Made to be stronger than normal window glass by introducing stress through rapid heating and cooling of the glass surfaces.
Designed to shatter into tiny pieces rather than fracture
Used in auto side windows, high stress areas

9. Physical Characteristics
Density—mass divided by volume
Refractive index (RI)—the measure of light bending due to a change in velocity when traveling from one medium to another
Fractures
Color
Thickness
Markings—striations, dimples, etc
Kendall/Hunt Publishing Company

10. Why Measure Density?
Can be used as a screening technique with large numbers of fragments.
Useful in identifying multiple sources present in the known and/or questioned samples.
It is nondestructive and an intensive property (not dependant on sample mass).
Need to measure very precisely in parts per hundred or thousand or better.

11. Kendall/Hunt Publishing Company
Density of Glass
Type of Glass
Density
window
headlight
pyrex
lead glass
Kendall/Hunt Publishing Company

12. Why Measure Refractive Index?
Refractive index: ratio of the velocity of light in a vacuum to the velocity of light in any other medium
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.
Like density, refractive index is an intensive property but it can be measured very precisely (±0.0002) and does not destroy the sample.
Refractive index of glass varies with small changes in composition or by how it is manufactured.

13. Refractive Index By Immersion

14. Refractive Index By Immersion
Immersing a glass particle in a liquid medium (silicone oil) whose refractive index can varied 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.
The Becke line is a bright halo near the border of a particle that is immersed in a liquid of a different refractive index.

15. Becke Lines Glass has higher refractive index – note white line inside
Glass has lower refractive index – note white line outside

16. 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.

17. Types of Fractures by Projectiles
Bullets are a projectile force (load) that can pass through glass.
Load side = entrance side
unloaded side = exit side.
Low-speed projectiles: rib marks may indicate where breaking force was applied
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.

18. How Do Glass Windows Break?
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.

19. 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.

20. How Does Glass Break?
Edges of broken pieces of glass will show “stress” marks – Wallner lines.
In a radial crack, the stress marks are perpendicular to unloaded side and parallel to loaded side. The arrow shows the side that received the impact.
3R rule:
Radial cracks form at
Right angles on the
Reverse side of the force

21. Stress on Flat Glass

22. More Glass

23. Exceptions to the Three R Rule
Tempered glass
“dices” without forming ridges
Very small windows held tightly in frame
can’t bend or bulge appreciably
Windows broken by heat or explosion
no “point of impact”
curved, smooth edges at break points

24. 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?

25. 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.

26. Learning Check
In the figure below & left, which impact occurred first?
In the figure above & right, from which side did the impact occur?

27. Collection of Glass Samples
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.

28. Which side was the bullet fired from?
Fractures by Projectiles
Which side was the bullet fired from?
Exit (unloaded) side is wider than entry (load) 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.