1. Issues in Analyzing Physical Evidence
Clear as Glass
Issues in Analyzing Physical Evidence

2. A “Simple” Example: Glass
What are some possible sources of glass evidence at a crime scene?
Broken head light at hit and run
Broken window at break in
Broken bottle at bar fight
Broken vase at struggle in home
Broken glasses at mugging

3. Some Obvious Properties to Match
Thickness
Color
Uniformity
Curvature
Surface Condition
Soil
Tinting
Texture

4. Distinguishing Plate Glass
Plate glass is made by floating the glass on liquid tin as it cools
Some tin will diffuse into the hot glass
Tin atoms will fluoresce under UV light
Only the side next to the tin will fluoresce

5. How does glass break?
Glass is elastic—it initially bends away in response to the force
Glass is weaker under tension than compression
Once elastic limit is reached, radial cracks form first on the side opposite the force
Continued force places the front surface in tension
Concentric cracks form second on the side nearest the force

6. How does glass break? 2 1 2

7. Radial and Concentric Fracture Lines

8. Stress Marks Radial cracks form right angles on the reverse
Conchoidal fracture
lines
Almost parallel
Almost perpendicular

9. Use Marks and Scratches
Diagonal marks where wiper blades do not overlap
Vertical scratches on side windows from grit
Cross hatching where wiper blades overlap

10. Which bullet was fired first?
Crack propagation is stopped by earlier cracks

11. Which side was the bullet fired from?
Exit side is wider than entry side
Stress lines for radial cracks form a right angle on the reverse side of the force
Exit
Entry

12. Were the lights on?

13. Where ever did that little piece of glass come from?
Trace Evidence
Where ever did that little piece of glass come from?

14. Classifying Properties
Physical v. Chemical
The property is physical if the composition of the substance does not change during the test (eg. density)
Extensive v. Intensive
The property is extensive if it depends on the amount of the substance (eg. mass)
Class v. Individual
The property is a class property if it is common to all samples in a category

15. Extensive Physical Properties
Mass
Weight
Length
Volume
Absorption
Resistance

16. Intensive Physical Properties
Density
Color
Odor
Luster
Ductility
Malleability
Hardness
Thermal Conductivity
Electrical Conductivity
Refractive Index (function of wavelength)
Optical absorption coefficient (function of wavelength)

17. Testing Do nondestructive tests first Destructive v. Nondestructive
Nondestructive preserves the evidence
Chemical is invariably destructive
Do nondestructive tests first
Must leave untouched sample for possible defense analysis

18. Characteristics of Glass
Class characteristics
Density
Refractive Index
Chemical composition
Not specific enough!
Individual characteristics
Reams and striations from manufacturing
Irregular edges that can be pieced together

19. What is Glass? Mixture of silicon oxides and other metal oxides
Hard, brittle, amorphous
Sand (SiO2)+ soda (Na2CO3) lowers melting point and viscosity, making mix easier to work
LIme (CaO) is added to prevent it dissolving in water
60-75% silica, 12-18% soda, and 5-12% lime
Traces of magnesium and aluminum
Pyrex and auto headlights add boron oxide to form borosilicates

20. Amorphous Structure Silicon atoms are gold and have four bonds
Oxygen atoms are red and have two bonds
Random network, no repeating structure
Bond distances are uniform

21. Other Types of Glass Tempered glass Laminated glass
Induced stress by repeated heating and cooling
Dices when broken rather than splinters
Used in side and rear car windows, shower doors, sliding glass doors
Laminated glass
Layer of plastic between two sheets of glass
Used in all US car windshields

22. Density: An initial analysis
Density = Mass/Volume
Intensive physical property
Class characteristic
Varies with composition and thermal history
Can be measured nondestructively

23. How you measured density in chemistry
Use a balance to find the weight of the sample
Determine the volume of the sample
Measure the volume directly (if liquid)
Water displacement if irregular solid
Why doesn’t this work on forensic samples?

24. Why choose density?
Can be used as a screening technique with large numbers of larger fragments
Useful in identifying multiple sources present in the known and/or questioned samples
Nondestructive

25. The BIG Questions for Any Analytic Technique
How much variation is there in what you are measuring?
Is the range of potential values large enough that you can exclude lots of samples?
How precisely can you measure it?
OR Gee, I knew significant figures would come in useful sometime!
What are the limitations of your instrumentation?
What are the limitations of your sample?

26. Density of Glass Crown 2.500 g/cm3 Lead Crystal 3.100 g/cm3
Densest Flint  g/cm3
Fused Silica  g/cm3
For most samples the range will be about 10%
Need to measure in parts per thousand or better
How uniform is density across a pane?
Weast, Robert C. Handbook of Chemistry and Physics. 61st Edition. Florida: CRC, 1981:

27. Using Archimedes Principle
An object will float if its average density is less than or equal to that of the liquid it is suspended in
The bouyant force is equal to the weight of the liquid that is displaced
To float glass you need high density liquids
bromoform (2.85 g/mL), Bromobenzene (1.50 g/mL)
“Tune” density by making mixtures of the two

28. Fun with Flotation

29. Flotation: A whole new approach
Use a column with varying density, less dense on top, more dense on bottom
Object will sink to the point where its average density equals the density of the fluid
Use calibrated reference samples as check
Pure crystals with known density
Solves the problem of tiny, irregular samples

30. Density Gradient Column
Layers of liquids of different composition
Each has a slightly different density
Needs to be temperature stabilized to minimize convection
Little mixing between layers even though they are miscible

31. Using Optical Properties to Identify Glass
Clear as Glass
Using Optical Properties to Identify Glass

32. What is refractive index?
n = c / v
Ratio of speed of light in vacuum to speed of light in the material
n always greater than 1
Light can’t go faster than its speed in a vacuum
Depends on wavelength of light (dispersion)
Why you get a rainbow from a prism

33. Snell’s Law The higher the n, the more the light bends n=1.50 n=1.335

34. Snell’s Law Mathematically
N1 x sin(θ1) = N2 x sin(θ2)
Animation

35. Properties of Glass Crown 2.500 g/cm3 1.52 Lead Crystal 3.100 g/cm3
Densest Flint  g/cm
Fused Silica  g/cm
For most samples the range will be about 5%
Need to measure in parts per thousand or better
How uniform is refractive index across a pane?
Weast, Robert C. Handbook of Chemistry and Physics. 61st Edition. Florida: CRC, 1981:

36. FBI Glass Database
A rough statistical estimate of the likelihood of finding glass of that refractive index (2337 samples)
Manufacturing changed in late 1970’s making glass more uniform

37. Consequences
Need to measure refractive index to 5 significant figures (1 part in 10,000)
Measured at sodium D line (single wavelength)
Flat glass samples only
Window, autos, display cases, mirrors from actual criminal investigations

38. The Key Issues
Where would you want to be on the distribution if you wanted to convince the jury using a class characteristic?
Is it easier or harder to use refractive index to match glass now than it was 20 years ago?
Does it help to measure both density and refractive index?

39. The Wave of the Future: The search for individual characteristics
Laser Ablation Inductively Coupled Plasma Mass Spectrometry
Detect 46 trace elements in glass
FBI and Oak Ridge National Laboratory
Developing a national database to determine likelihood of a match

40. Focused UV Laser Vaporizes the Glass Sample

41. Inductively Coupled Plasma Mass Spectrometer
Make Ions in plasma by knocking off electrons
Sort ions by mass in Mass Spec by pushing them around with electric fields

42. Comparing Trace Elements in Different Samples at ppm
A jury friendly data display—Is it easy to tell the samples apart?