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?