1. Chapter 14 Glass
“There is no den in the wide world to hide a rogue. Commit a crime and the earth is made of glass.”
—Ralph Waldo Emerson

2. Kendall/Hunt Publishing Company
Glass Analysis
Students will learn:
The difference between physical and chemical properties.
How glass can be used as evidence.
How individual evidence differs from class evidence.
The nature of glass.
How to use the properties of reflection, refraction, and refractive index to classify glass fragments.
Kendall/Hunt Publishing Company

3. Kendall/Hunt Publishing Company
Glass Analysis
Students will be able to:
Make density measurements on very small particles.
Use logic to reconstruct events.
Use technology and mathematics to improve investigations and communications.
Identify questions and concepts that guide scientific investigations.
Kendall/Hunt Publishing Company

4. Kendall/Hunt Publishing Company
Glass as Evidence
Investigators can use glass fragments found at a crime scene to place a suspect at the scene.
If it can be pieced back together like a puzzle, then it can be individualized and the source can be considered unique.
Usually, it cannot and must be considered class evidence
Kendall/Hunt Publishing Company

5. Kendall/Hunt Publishing Company
Chapter 14
Glass as Evidence
Forensic scientists must use as many physical and chemical properties of glass as possible to characterize fragments and link them to a crime scene.
Characteristics such as refractive index, density, color, and chemical composition can be analyzed.
Kendall/Hunt Publishing Company
Kendall/Hunt

6. Characteristics of Glass
Chapter 14
Characteristics of Glass
Hard, amorphous solid (noncrystalline: atoms arranged randomly)
Usually transparent
Primarily composed of silica (sand) with various amounts of element oxides
Brittle
Exhibits conchoidal fracture
Kendall/Hunt Publishing Company
Kendall/Hunt

7. Kendall/Hunt Publishing Company
Common Types
Soda-lime—used in plate and window glass, glass containers, and light bulbs
(soda= sodium carbonate)
Soda-lead—fine table ware and art Borosilicate—heat resistant, like Pyrex
Silica—used in chemical ware
Tempered—used in car side windows
Laminated—used in the car windshields
Kendall/Hunt Publishing Company

8. Composition of Glasses
Bottles/ Lab/ Crystal High temp
widows Bakware
Silica SiO2 73% % 35% 96%
Soda Na2O
Lime CaO
Potash K2O
Magnesia MgO
Alumina Al2O
Boric Oxide B2O
Lead Oxide PbO
Most glass starts by melting sand (silica) and ashes (soda). Soda reduces the melting point of the silica. Lime is added to make the glass water insoluble.
Boric oxide increases temperature stability (good for ovenware). Lead increases density and refractive index so that the glass sparkles. Adding various elements can change color. (pg. 312)
Kendall/Hunt Publishing Company

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
Fluorescence
Markings —striations, dimples, etc
Kendall/Hunt Publishing Company

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

11. Kendall/Hunt Publishing Company
Refractive Index
When light strikes most glass, there is a change in its velocity and therefore, a change in direction.
This phenomenon is called refraction.
The refractive index (RI or n) is a comparison of the speed of light in a vacuum to the speed of light in another substance.
Kendall/Hunt Publishing Company

12. Kendall/Hunt Publishing Company
Calculating RI
Ex: Speed of light in water is 225,000,000 meters per second. It’s refractive index is calculated as follows: (note: 3 X108 = speed of light in air)
3.00 X = 1.33
2.25 X 108
Kendall/Hunt Publishing Company

13. Immersion Method of Determining Refractive Index
If a clear material, like glass, is immersed in a liquid that has the same refractive index, the glass will appear to disappear.
Investigators use this principle when characterizing glass.
Kendall/Hunt Publishing Company

14. Immersion Method of Determining Refractive Index
Immerse a glass fragment into a drop of liquid with a known refractive index.
Transmit light through the sample to the eyepiece of a microscope.
If the liquid has a higher or lower index of refraction than the glass, the piece of glass will be visible.
Kendall/Hunt Publishing Company

15. Kendall/Hunt Publishing Company
The Becke Line
A “halo” of light, called the Becke line will be visible around the perimeter of the glass.
Kendall/Hunt Publishing Company

16. Kendall/Hunt Publishing Company
Match Point
If the refractive indexes are the same, the Becke line disappears and the glass boundaries appear to be invisible.
This is called the match point.
Kendall/Hunt Publishing Company

17. Kendall/Hunt Publishing Company
If the Becke line appears on the inside perimeter of the glass, then the investigator will need to try another liquid with a higher refractive index to try to reach the match point.
If the Becke line appears on the outside perimeter of the glass, then a liquid with a lower refractive index will be required.
Kendall/Hunt Publishing Company

18. Kendall/Hunt Publishing Company
The Becke Line
The Becke line is a “halo” that can be seen on the inside of the glass on the left, indicating that the glass has a higher refractive index than the liquid medium. The Becke line as seen on the right is outside of the glass, indicating just the opposite.
Kendall/Hunt Publishing Company

19. Kendall/Hunt Publishing Company
Another way to determine the refractive index using just one liquid is to use a liquid such as silicone oil.
The refractive index of a high boiling liquid, such as silicone oil, changes with temperature
Kendall/Hunt Publishing Company

20. Determination of Refractive Index
This occurs in an apparatus called a hot stage which is attached to a microscope. Increasing the temperature allows the disappearance of the Becke line to be observed
At match point, temperature is noted and refractive index of the liquid is read from a calibration chart.
Kendall/Hunt Publishing Company

21. Kendall/Hunt Publishing Company
Refractive Index
Liquid RI
Glass
Water
1.333
Vitreous silica
1.458
Olive oil
1.467
Headlight
Glycerin
1.473
Window
Castor oil
1.82
Bottle
Clove oil
1.543
Optical
Bromobenzene
1.560
Quartz
Bromoform
1.597
Lead
Cinnamon oil
1.619
Diamond
2.419
Note: this chart is on pg. 318 not 314 as your notes say.
Kendall/Hunt Publishing Company

22. Kendall/Hunt Publishing Company
Fracture Patterns
Investigators can sometimes reconstruct a crime using glass fracture patterns.
How cracks form, their shape, and if the breakage came from inside or outside can be clues as to what happened at the crime scene.
Kendall/Hunt Publishing Company

23. Kendall/Hunt Publishing Company
When a projectile such as a bullet or a rock breaks glass, it will form two distinct types of fractures:
Radial fractures
Concentric fractures
Kendall/Hunt Publishing Company

24. Kendall/Hunt Publishing Company
Fracture Patterns
Radial fracture lines radiate out from the origin of the impact; they begin on the opposite side of the force
Concentric fracture lines are circular lines around the point of impact; they begin on the same side as the force
3R rule —radial cracks form a right angle on the reverse side of the force.
Kendall/Hunt Publishing Company

25. Kendall/Hunt Publishing Company
Sequencing
A high velocity projectile always leaves a hole wider at the exit side of the glass.
Cracks terminate at intersections with other. This can be used to determine the order that the fractures occurred.
Kendall/Hunt Publishing Company

26. Considerations for Collection
The collector must consider that fragments within a questioned sample may have multiple origins. If possible, the collector should attempt an initial separation based on physical properties.
—Forensic Science Communications
Kendall/Hunt Publishing Company

27. Considerations for Collection
The collector must consider the possibility that there may be a physical match to a known sample (e.g., a piece of glass to a fractured vehicle headlamp).
When an attempt to make a physical match is made at the site of collection, the collector should take precautions to avoid mixing of the known and questioned samples.
Kendall/Hunt Publishing Company

28. Considerations for Collection
Any glass samples collected should be documented, marked (if necessary), packaged, and labeled.
Kendall/Hunt Publishing Company

29. Kendall/Hunt Publishing Company
Collecting the Sample
The glass sample should consist of the largest amount that can be practically collected from each broken object and packaged separately.
—Forensic Science Communications
Kendall/Hunt Publishing Company

30. Kendall/Hunt Publishing Company
Collecting the Sample
The sample should be removed from the structure (e.g., window frame, light assembly).
The inside and outside surfaces of the known sample should be labeled if a determination of direction of breakage or reconstruction of the pane is desired.
Kendall/Hunt Publishing Company

31. Kendall/Hunt Publishing Company
Collecting the Sample
When multiple broken glass sources are identified, it is necessary to sample all sources.
A sample should be collected from various locations throughout the broken portion of the object in order to be as representative as possible.
Kendall/Hunt Publishing Company

32. Kendall/Hunt Publishing Company
Collecting the Sample
The sample should be collected with consideration being given to the presence of other types of evidence on that sample (e.g., fibers, blood).
Kendall/Hunt Publishing Company