Clear as Glass

1
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 elasticit 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
www.channel4.com/.../science/images/fracture.gif
7
Radial and Concentric Fracture Lines
concentric
radial
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?
http//www.michigan.gov/msp/0,1607,7-123-1593_3800
-15961--,00.html
13
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

• 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

http//www.pilkington.com/resources/floatstructure
.jpg
21
Other Types of Glass

• Tempered 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

http//www.displays2go.com/glasscase.htm http//ww
w.keepsafemax.com/images/about.jpg
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 doesnt 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 7.200 g/cm3
• Fused Silica 2.200 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 15-39.

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

http//lahabra.seniorhigh.net/pages/teachers/pages
/math/timeline/mathHist/archimedes.gif
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
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 cant go faster than its speed in a vacuum
• Depends on wavelength of light (dispersion)
• Why you get a rainbow from a prism

33
Snells Law
n1.50
n1.335
N1.52
The higher the n, the more the light bends
34
Snells Law Mathematically

• N1 x sin(?1) N2 x sin(?2)
• Animation

35
Properties of Glass

• Crown 2.500 g/cm3 1.52
• Lead Crystal 3.100 g/cm3
• Densest Flint 7.200 g/cm3 1.89
• Fused Silica 2.200 g/cm3 1.46
• 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 15-39.
• http//www.is.kiruna.se/cjo/d2i/REFRACTION.INDEX3
  .html

36
FBI Glass Database
A rough statistical estimate of the likelihood of
finding glass of that refractive index (2337
samples)
1964-1979
Manufacturing changed in late 1970s making glass
more uniform
1980-1997
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 FutureThe 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
http//www.esslab.com/icp-ms.jpg
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 displayIs it easy to tell
the samples apart?
http//www.spectroscopymag.com/spectroscopy/articl
e/articleDetail.jsp?id169918pageID4