1. Forensic Science: Analysis of Glass Evidence
Chapter 4:

2. Glass evidence can be found at many crime scenes.
Automobile accident sites may be littered with broken headlight or windshield glass.
The site of a store break-in may contain shards of window glass with fibers or blood on them. Video
If shots are fired into a window, the sequence and direction of the bullets can often be determined by examining the glass.
Minute particles of glass may be transferred to a suspect’s shoes or clothing and can provide a source of trace evidence linking a suspect to a crime.

3. Objectives for Glass Forensics
Define and distinguish the physical and chemical properties of glass.
Explain the structure of glass
Explain the optic properties of glass
Explain the forensic methods for comparing glass fragments.
Understand how to examine glass fractures to determine the direction of impact

4. Glass & Forensics How was it broken? Link a suspect to a crime scene
Fingerprints
Blood
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5. Chemical and Physical Properties
Physical properties describe a substance without comparing with another substance. They are measurements like weight, volume, boiling point, melting point.
Chemical properties describe what happens when you combine it with something else in a chemical reaction. Examples are burning, coloring reagent tests, decomposing, synthesis of an alloy from individual elements

6. How is glass formed?
Long before humans began making glass, glass formed naturally.
When certain types of rock are exposed to extremely high temperatures, such as lightning strikes or erupting volcanoes, glass can form.
Obsidian is a type of glass formed by volcanoes.

7. Timeline of Events Prehistoric humans used obsidian as a cutting tool.
The earliest man-made glass objects (glass beads) were found in Egypt dating back to 2500 BC.
Glass blowing began sometime during the first century BC.
By the 14th century, knowledge of glass making spread throughout Europe.
The Industrial Revolution brought the mass production of many kinds of glass.

8. How is Glass Formed?
Glass is a hard, brittle, amorphous material made by melting sand (aka silica,silicon dioxide, SiO2) lime (aka calcium oxide CaO) and soda, sodium carbonate (Na2CO3) at very high temperatures.
The lime (CaO) is added to prevent the glass from being soluble in water.
The soda (Na2CO3) is added to lower the melting point of silica (sand) and make it easier to work with.
In some types of glass with special requirements, trace amounts of other elements are added. Example: Boron is added to make Pyrex glass.

9. Soda-lime Glass (amorphous solid) The atoms are arranged in a random fashion

10. How is Glass Made?
Following the mixing of the raw materials, they are transported to the furnace and heated to over 1200oC or 2200oF and changed into a molten mixture. Video
There are different formulas and assembly for different glass applications. Ex: car wind shields are 2 layers with plastic in between.

11. Types of Glass

12. Types of Glass:
Soda-lime glass: Mostly sand, sodium carbonate and calcium oxide:
Used for manufacturing most window and bottle glass
Making Window Panes
Making Glass Bottles
Rolling Glass mfg

13. Float Glass Flat glass typically used for windows.
Soda-lime glass that has been cooled on top of a bath of molten tin.
video

14. Borosilicates
The common metal-oxides found in soda-lime glass are sodium, calcium, magnesium and aluminum.
In addition, a wide variety of special glasses can be made by substituting in whole or in part other metal oxides for the silica, sodium and calcium oxides.
Automobile headlights, heat- resistant glass such as Pyrex are manufactured by adding Boron oxide to the oxide mix for Lab glassware, thermometers, cookware.
Auto glass video

15. Leaded Glass
Fine glassware and decorative art glass, called crystal or leaded glass substitutes lead oxide for calcium oxide (lime).
The addition of lead oxide makes the glass denser. As light passes through the more-dense glass, the light waves are bent, giving the glass a sparkling effect.

16. Tempered Glass
This glass is made stronger than ordinary window glass by introducing stress through rapid heating and cooling of the glass surfaces.
When tempered glass breaks, it does not shatter but rather fragments or “dices” into small squares with little splintering.
Used for side and rear windows of automobiles sold in the United States.
Glass Breakage video
video

17. Annealing
Bending Glass
When trying to make a distinction between tempered glass and nontempered glass particles a process known as annealing is used.
Annealing- slowly heating and then cooling the glass. A heat treatment that alters the microstructure of a material causing changes in properties such as strength and hardness. It is much stronger than normal glass.
The change in the refractive index value for tempered glass upon annealing is significantly greater when compared to nontempered glass and thus serves as a point of distinction.

18. Laminated Glass
This glass derives its strength by sandwiching one layer of plastic between two pieces of ordinary window glass.
The windshields of all cars manufactured in the United States are constructed from laminated glass.

19. Laminated Glass in Windshields
USA law requires that all auto windshields are made from laminated glass. This is a safety consideration because it does not break into shards of glass that can injure. The combination of tempered glass and the sandwiched plastic layer try to keep the glass together.

20. Bulletproof Glass
Bulletproof glass is a combination of two or more types of glass, one hard and one soft.
The softer layer makes the glass more elastic so it can flex instead of shatter.
The index of refraction for both of the glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear view through the glass.
Bulletproof glass varies in thickness from three-quarter inch to three inches.

21. Properties of Glass and Comparing Glass Fragments
For the forensic scientist, comparing glass consists of finding and measuring the properties that will associate one glass fragment with another while minimizing or eliminating the possible existence of other sources.
Considering the prevalence of glass in our society, it is easy to appreciate the magnitude of this analytical problem.
Obviously, glass possesses its greatest evidential value when it can be individualized to one source.
Glass can flex to a point and then it breaks. Sound waves can transmit enough energy to vibrate a glass crystal fast enough to break. video

22. Glass LABORATORY Properties
Forensic Science uses physical and chemical properties for analyzing glass in the lab.
These include refraction, reflection, dispersion, spectroscopy, density, and others.
Some physical analysis is as simple as fitting the broken pieces together.

23. Optical Properties of Glass
Glass is in widespread use largely due to most glass composition are transparent to visible wavelengths of light.
When glass is shaped differently and polished, it can make passing light behave differently. It can allow it to pass unchanged or cause it to disperse into a rainbow.
Glass has the ability to refract, reflect, and transmit light following geometrical optics, without scattering it. This is how glasses and contacts work, except we use plastics.

24. Density of Glass Determination
To determine the density of glass, it is best to use the immersion method.
Density = mass/volume
Mass is measured on a balance / scale.
Volume is determined by immersing parts of the glass and seeing the level change in a measurement glassware.

25. Comparing Densities: Flotation
A solid particle will either float, sink, or remain suspended in a liquid, depending upon its density relative to the liquid medium.
Flotation = a standard / reference glass particle is immersed in a liquid; a mixture of bromoform or bromobenzene may be used. The composition of the liquid is carefully adjusted by adding small amounts of bromoform or bromobenzene until the glass chip remains suspended in the liquid medium. At this point, the standard / reference glass sample and the liquid each have the same density. Glass chips (same size and shape as reference sample) are added to the liquid for comparison. If both the unknown and standard / reference samples remain suspended, they have the same density.

26. Flotation

27. Refraction of Light
Refraction of light is the bending of the light as it passes the boundary between two different optically dense mediums. It changes directions because it changes speed

28. Snell’s Law

29. Refractive Index
When light changes from space to air to water, it slows down. This changes the path it takes. That’s refraction.
The refractive index is the ratio of the velocity of light in space to the velocity inside a different material.
Refractive index = Vel in space
Vel in medium
Think of it as the optical density of a material. The thicker it is the slower you go

30. Examples of Refractive Index
Material
Refractive index
Speed of light
Space
1.0000
3 x 108 m/s2
Air
1.0005
2.99 x 108 m/s2
Water
1.3333
2.25 x 108 m/s2
Glass, flint
1.6600
1.81 x 108 m/s2
Glass, crown
1.5200
1.97 x 108 m/s2
NaCL
1.5440
1.94 x 108 m/s2
Quartz (SiO2)
1.4580
2.07x 108 m/s2
Diamond
2.4190
1.24 x 108 m/s2
Snell’s Law of refraction is the formula that tells us how much the light will change directions. That is covered in physics.

31. Lab Determination of Refractive Index
An automated approach for measuring the refractive index of glass is by the immersion method into oil of different temperatures.
The Grim 3 instrument is used and the FBI has a database of refractive indexes from different manufacturers glass.

32. Refractive Index

33. Dispersion of Light
Dispersion of light is when white light passes through a prism and is separated into the different colored wavelengths by the refraction of the prism.

34. Birefringence
Asymmetric crystals will refract the light in two directions because of their shape and the way the light goes through them. You will see a double image or rainbow.

35. Becke line
The Becke line is a bright line observed close to the boundary between two grains in plane polarized light. If the microscope focus is raised, increasing the distance between the objective lens and the sample, the Becke line will move into the material with the higher refractive index.

36. Forensics of Broken Glass
When broken glass is found at a crime scene it is gathered and checked for several things:
Puzzle pieces fitting together.
Fracture pattern.
Blood or any DNA source
Fingerprints
Composition or type of glass
Density – mass per unit of volume.
Determining refractive index.
Any unique characteristics: ex. Paint, scratches

37. Proper Collection of Glass Evidence
Standard reference glass should be taken from the crime scene (1 in2)
Package in solid containers to prevent breakage
Preserve garment (shoe, pants, shirt) with glass on it
All broken glass must be recovered and submitted for analysis when direction of impact is desired.
Whenever possible, the exterior and interior surfaces of the glass must be indicated. The presence of dirt, paint, grease or putty may indicate the exterior surface of the glass.

38. Jigsaw Effect – Most Beneficial
When the suspect and crime-scene fragments are assembled and physically fitted together.
Comparisons of this type require piecing together irregular edges of broken glass as well as matching all irregularities and striations on the broken surfaces. The possibility that two pieces of glass originating from different sources will fit together exactly is so unlikely as to exclude all other sources from practical consideration.
Unfortunately, most glass evidence is either too fragmentary or too minute to permit a comparison of this type

39. Glass Fracture Patterns
When a force is applied to the amorphous glass structure it spreads it out as much as possible over the surface.
When the limit of its elasticity is reached, the glass fractures.
The observed patterns of fracture tells the forensic investigator many things about the force magnitude, direction, and glass type.

40. Projectile Breaking Glass

41. Analyzing Glass Fracture Patterns

42. Radial Fractures of Glass
Projectiles (like bullets) will often create such a high impulse force that the glass reaches its elastic limit and breaks into a “Radial” pattern and is called a Primary Radial Fracture. It looks like the spokes of a bicycle radiating out from the center of the projectile hole.
The cracks will form on the opposite side of the glass from the force application side.

43. Secondary Fractures Secondary fractures may also form.
These fractures take the form of concentric circles around the point of impact.
Concentric circles are circles that have the same center.
Concentric circles form on the same side of the glass as the point of impact.

44. Concentric Fractures of Glass

45. Concentric Fractures
For concentric fractures, the perpendicular end of the stress marks happen on the surface on which the force originated. Note that this is the opposite of radial fractures !

46. 3 R Rule- Determining Side of Impact
Radial Cracks form a Right Angle on the Reverse side of the force.

47. 3R Rule
Radial cracks are at Right angles to the Rear (side opposite the impact)
Exceptions
–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”

48. Multiple Hole Fracture Patterns
If there have been multiple penetrations of glass, it is possible to determine the sequence of events by tracing the migration patterns out to points of termination where the crack ends in another crack.

49. Successive Penetrations
A fracture always terminates at an existing line of fracture
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51. Breakage of Glass from a Fire
During a fire, glass may break as a result of heat fracturing.
Heat fracturing produces breakage patterns on glass that are different from breakage patterns caused by impact.
Wavy fracture lines develop in glass that has been exposed to high heat.
Glass will tend to break toward the region of higher temperature.
If the glass was not broken before the fire, there will be no radial or concentric circle fracture patterns in glass that is broken by high heat.

52. Exit vs Entry Holes in Glass
The exit hole of a projectile going through a glass is always larger than the entry hole because of the shock waves traveling out in a concentric pattern from the impact point.

53. Direction of Penetration
A projectile hole is inevitably wider at the exit side
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54. Glass Fracture Examples
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55. Glass Fracture Examples
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