Clear as Glass Issues in Analyzing Physical Evidence.
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Transcript of Clear as Glass Issues in Analyzing Physical Evidence.
Clear as GlassClear as Glass
Issues in Analyzing Physical Issues in Analyzing Physical EvidenceEvidence
A “Simple” Example: A “Simple” Example: GlassGlass
What are some possible sources of glass What are some possible sources of glass evidence at a crime scene?evidence at a crime scene? Broken head light at hit and runBroken head light at hit and run Broken window at break inBroken window at break in Broken bottle at bar fightBroken bottle at bar fight Broken vase at struggle in homeBroken vase at struggle in home Broken glasses at muggingBroken glasses at mugging
Some Obvious Properties Some Obvious Properties to Matchto Match
ThicknessThickness ColorColor UniformityUniformity CurvatureCurvature Surface ConditionSurface Condition
SoilSoil TintingTinting TextureTexture
Distinguishing Plate Distinguishing Plate GlassGlass
Plate glass is made by floating the glass Plate glass is made by floating the glass on liquid tin as it coolson liquid tin as it cools
Some tin will diffuse into the hot glassSome tin will diffuse into the hot glass Tin atoms will fluoresceTin atoms will fluoresce under UV light under UV light
Only the side next to the tin will fluoresceOnly the side next to the tin will fluoresce
Glass is elastic—it initially bends away in Glass is elastic—it initially bends away in response to the forceresponse to the force
Glass is weaker under tension than Glass is weaker under tension than compressioncompression Once elastic limit is reached, Once elastic limit is reached, radialradial cracks form first cracks form first
on the side on the side oppositeopposite the force the force Continued force places the front surface in Continued force places the front surface in
tensiontension ConcentricConcentric cracks form second on the side cracks form second on the side nearest nearest
the forcethe force
How does glass break?How does glass break?
How does glass break?How does glass break?
1
2
2
www.channel4.com/.../science/images/fracture.gif
Radial and Concentric Radial and Concentric Fracture LinesFracture Lines
radial
concentric
Stress MarksStress Marks
RadialRadial cracks form cracks form rightright angles on the angles on the reversereverse
Almost parallel
Almost perpendicular
Conchoidal fracturelines
Use Marks and ScratchesUse Marks and Scratches
Cross hatching where wiper blades overlap
Diagonal marks where wiper blades do not overlap
Vertical scratches on side windows from grit
Which bullet was fired Which bullet was fired first?first?
Crack Crack propagation propagation is stopped by is stopped by earlier cracksearlier cracks
Which side was the bullet Which side was the bullet fired from?fired from?
Exit side is Exit side is widerwider than entry sidethan entry side
Stress lines for Stress lines for radialradial cracks form cracks form a a rightright angle on angle on the the reversereverse side of side of the forcethe force
ExitEntry
Were the lights on?Were the lights on?
http://www.michigan.gov/msp/0,1607,7-123-1593_3800-15961--,00.html
Trace EvidenceTrace Evidence
Where ever did that little piece of Where ever did that little piece of glass come from?glass come from?
Classifying PropertiesClassifying Properties
Physical v. ChemicalPhysical v. Chemical The property is physical if the composition of the The property is physical if the composition of the
substance does not change during the test (eg. substance does not change during the test (eg. density)density)
Extensive v. IntensiveExtensive v. Intensive The property is extensive if it depends on the The property is extensive if it depends on the
amount of the substance (eg. mass)amount of the substance (eg. mass) Class v. IndividualClass v. Individual
The property is a class property if it is common to all The property is a class property if it is common to all samples in a categorysamples in a category
Extensive Physical Extensive Physical PropertiesProperties
MassMass WeightWeight LengthLength VolumeVolume AbsorptionAbsorption ResistanceResistance
Intensive Physical Intensive Physical PropertiesProperties
DensityDensity ColorColor OdorOdor LusterLuster DuctilityDuctility MalleabilityMalleability HardnessHardness Thermal ConductivityThermal Conductivity
Electrical Electrical ConductivityConductivity
Refractive Index Refractive Index (function of (function of wavelength)wavelength)
Optical absorption Optical absorption coefficient (function coefficient (function of wavelength)of wavelength)
TestingTesting
Destructive v. NondestructiveDestructive v. Nondestructive Nondestructive preserves the Nondestructive preserves the
evidenceevidence Chemical is invariably destructiveChemical is invariably destructive
Do nondestructive tests firstDo nondestructive tests first Must leave untouched sample for Must leave untouched sample for
possible defense analysispossible defense analysis
Characteristics of GlassCharacteristics of Glass
Class characteristicsClass characteristics DensityDensity Refractive IndexRefractive Index Chemical composition Chemical composition
Not specific enough!Not specific enough!
Individual characteristicsIndividual characteristics Reams and striations from manufacturingReams and striations from manufacturing Irregular edges that can be pieced togetherIrregular edges that can be pieced together
What is Glass?What is Glass? MixtureMixture of silicon oxides and other metal oxides of silicon oxides and other metal oxides Hard, brittle, Hard, brittle, amorphousamorphous Sand (SiOSand (SiO22)+ soda (Na)+ soda (Na22COCO33) lowers melting point ) lowers melting point
and viscosity, making mix easier to workand viscosity, making mix easier to work LIme (CaO) is added to prevent it dissolving in waterLIme (CaO) is added to prevent it dissolving in water
60-75% silica, 12-18% soda, and 5-12% lime 60-75% silica, 12-18% soda, and 5-12% lime
Traces of magnesium and aluminumTraces of magnesium and aluminum PyrexPyrex and auto headlights add boron oxide to form and auto headlights add boron oxide to form
borosilicatesborosilicates
Amorphous StructureAmorphous Structure
Silicon atoms are Silicon atoms are gold and have gold and have four bondsfour bonds
Oxygen atoms Oxygen atoms are red and have are red and have two bondstwo bonds
Random network, Random network, no repeating no repeating structurestructure
Bond distances Bond distances are uniformare uniform
http://www.pilkington.com/resources/floatstructure.jpg
Other Types of GlassOther Types of Glass
Tempered glassTempered glass Induced stress by repeated heating Induced stress by repeated heating
and coolingand cooling Dices when broken rather than Dices when broken rather than
splinterssplinters Used in side and rear car windows, Used in side and rear car windows,
shower doors, sliding glass doorsshower doors, sliding glass doors
Laminated glassLaminated glass Layer of plastic between two sheets Layer of plastic between two sheets
of glassof glass Used in all US car windshieldsUsed in all US car windshields
http://www.displays2go.com/glasscase.htmhttp://www.keepsafemax.com/images/about.jpg
Density: An initial Density: An initial analysisanalysis
Density = Mass/VolumeDensity = Mass/Volume
Intensive physical propertyIntensive physical property Class characteristicClass characteristic
Varies with composition and thermal Varies with composition and thermal historyhistory
Can be measured nondestructivelyCan be measured nondestructively
How you measured How you measured density in chemistrydensity in chemistry
Use a balance to find the weight of the Use a balance to find the weight of the samplesample
Determine the volume of the sampleDetermine the volume of the sample Measure the volume directly (if liquid)Measure the volume directly (if liquid) Water displacement if irregular solidWater displacement if irregular solid
Why doesn’t this work on forensic Why doesn’t this work on forensic samples?samples?
Why choose density?Why choose density?
Can be used as a screening technique Can be used as a screening technique with large numbers of larger fragments with large numbers of larger fragments
Useful in identifying multiple sources Useful in identifying multiple sources present in the known and/or questioned present in the known and/or questioned samplessamples
NondestructiveNondestructive
The The BIGBIG Questions for Questions for Any Analytic TechniqueAny Analytic Technique
How much variation is there in what you are How much variation is there in what you are measuring?measuring? Is the range of potential values large enough that Is the range of potential values large enough that
you can you can excludeexclude lots of samples? lots of samples?
How precisely can you measure it?How precisely can you measure it? OR Gee, I knew OR Gee, I knew significant figuressignificant figures would come in would come in
useful sometime!useful sometime! What are the limitations of your instrumentation?What are the limitations of your instrumentation? What are the limitations of your sample?What are the limitations of your sample?
Density of GlassDensity of Glass
CrownCrown 2.500 g/cm2.500 g/cm33
Lead CrystalLead Crystal 3.100 g/cm3.100 g/cm33
Densest FlintDensest Flint 7.200 g/cm7.200 g/cm33
Fused SilicaFused Silica 2.200 g/cm2.200 g/cm33
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. Weast, Robert C. Handbook of Chemistry and Physics. 61st EditionHandbook of Chemistry and Physics. 61st Edition . Florida: CRC, 1981: 15-39. Florida: CRC, 1981: 15-39. .
Using Archimedes Using Archimedes PrinciplePrinciple
An object will float if its average density is less An object will float if its average density is less than or equal to that of the liquid it is than or equal to that of the liquid it is suspended insuspended in
The The bouyantbouyant force is equal to the weight of the force is equal to the weight of the liquid that is displacedliquid that is displaced
To float glass you need high density liquidsTo float glass you need high density liquids bromoform (2.85 g/mL), Bromobenzene (1.50 g/mL)bromoform (2.85 g/mL), Bromobenzene (1.50 g/mL) “ “Tune” density by making mixtures of the twoTune” density by making mixtures of the two
Fun with FlotationFun with Flotation
http://lahabra.seniorhigh.net/pages/teachers/pages/math/timeline/mathHist/archimedes.gif
Flotation: A whole new Flotation: A whole new approachapproach
Use a column with varying density, less dense Use a column with varying density, less dense on top, more dense on bottomon top, more dense on bottom
Object will sink to the point where its average Object will sink to the point where its average density equals the density of the fluiddensity equals the density of the fluid
Use calibrated reference samples as check Use calibrated reference samples as check Pure crystals with known densityPure crystals with known density
Solves the problem of tiny, irregular samplesSolves the problem of tiny, irregular samples
Density Gradient ColumnDensity Gradient Column
Layers of liquids of Layers of liquids of different compositiondifferent composition Each has a slightly Each has a slightly
different densitydifferent density
Needs to be temperature Needs to be temperature stabilized to minimize stabilized to minimize convectionconvection Little mixing between Little mixing between
layers even though they layers even though they are miscibleare miscible
Clear as GlassClear as Glass
Using Optical Properties to Identify Using Optical Properties to Identify GlassGlass
What is refractive index?What is refractive index?
n = c / vn = c / v Ratio of speed of light in Ratio of speed of light in
vacuum to speed of light vacuum to speed of light in the materialin the material
nn always greater than 1 always greater than 1 Light can’t go faster than Light can’t go faster than
its speed in a vacuumits speed in a vacuum Depends on wavelength Depends on wavelength
of light (dispersion)of light (dispersion) Why you get a rainbow Why you get a rainbow
from a prismfrom a prism
Snell’s LawSnell’s Law
N=1.52
The higher the n, the more the light bends
n=1.335n=1.50
Snell’s Law Snell’s Law MathematicallyMathematically
NN11 x x sin(sin(θθ11) = N) = N22 x x sin(sin(θθ22))
AnimationAnimation
Properties of GlassProperties of Glass
CrownCrown 2.500 g/cm2.500 g/cm33 1.521.52 Lead CrystalLead Crystal 3.100 g/cm3.100 g/cm33
Densest FlintDensest Flint 7.200 g/cm7.200 g/cm33 1.891.89 Fused SilicaFused Silica 2.200 g/cm2.200 g/cm33 1.461.46
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. Weast, Robert C. Handbook of Chemistry and Physics. 61st EditionHandbook of Chemistry and Physics. 61st Edition . Florida: CRC, 1981: 15-39. Florida: CRC, 1981: 15-39..http://www.is.kiruna.se/~cjo/d2i/REFRACTION.INDEX3.html http://www.is.kiruna.se/~cjo/d2i/REFRACTION.INDEX3.html
FBI Glass DatabaseFBI Glass Database
Manufacturing changed in late 1970’s making glass more uniform
1964-1979
1980-1997
A rough statistical estimate of the likelihood of finding glass of that refractive index (2337 samples)
ConsequencesConsequences
Need to measure refractive index to 5 Need to measure refractive index to 5 significant figures (1 part in 10,000)significant figures (1 part in 10,000)
Measured at sodium D line (single Measured at sodium D line (single wavelength)wavelength)
Flat glass samples onlyFlat glass samples only Window, autos, display cases, mirrors from Window, autos, display cases, mirrors from
actual criminal investigationsactual criminal investigations
The Key IssuesThe Key Issues
Where would you want to be on the Where would you want to be on the distribution if you wanted to convince distribution if you wanted to convince the jury using a class characteristic?the jury using a class characteristic?
Is it easier or harder to use refractive Is it easier or harder to use refractive index to match glass now than it was index to match glass now than it was 20 years ago?20 years ago?
Does it help to measure both density Does it help to measure both density and refractive index?and refractive index?
The Wave of the Future:The Wave of the Future:The search for individual The search for individual characteristicscharacteristics
Laser Ablation Inductively Coupled Laser Ablation Inductively Coupled Plasma Mass SpectrometryPlasma Mass Spectrometry Detect 46 trace elements in glassDetect 46 trace elements in glass
FBI and Oak Ridge National LaboratoryFBI and Oak Ridge National Laboratory Developing a national database to Developing a national database to
determine likelihood of a matchdetermine likelihood of a match
Focused UV Laser Vaporizes Focused UV Laser Vaporizes the Glass Sample the Glass Sample
Inductively Coupled Plasma Inductively Coupled Plasma Mass SpectrometerMass Spectrometer
Make Ions in plasma by knocking off electrons
Sort ions by mass in Mass Spec by pushing them around with electric fields
http://www.esslab.com/icp-ms.jpg
Comparing Trace Elements Comparing Trace Elements in Different Samples at ppmin Different Samples at ppm
http://www.spectroscopymag.com/spectroscopy/article/articleDetail.jsp?id=169918&&pageID=4
A jury friendly data display—Is it easy to tell the samples apart?