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Clear as Glass Issues in Analyzing Physical Evidence.

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Presentation on theme: "Clear as Glass Issues in Analyzing Physical Evidence."— Presentation transcript:

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? What are some possible sources of glass evidence at a crime scene? Broken head light at hit and run Broken head light at hit and run Broken window at break in Broken window at break in Broken bottle at bar fight Broken bottle at bar fight Broken vase at struggle in home Broken vase at struggle in home Broken glasses at mugging Broken glasses at mugging

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

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

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

6 1 2 2

7 Radial and Concentric Fracture Lines radial concentric

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

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

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

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

12 Were the lights on?

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

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

15 Extensive Physical Properties Mass Mass Weight Weight Length Length Volume Volume Absorption Absorption Resistance Resistance

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

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

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

19 What is Glass? Mixture of silicon oxides and other metal oxides Mixture of silicon oxides and other metal oxides Hard, brittle, amorphous Hard, brittle, amorphous Sand (SiO 2 )+ soda (Na 2 CO 3 ) lowers melting point and viscosity, making mix easier to work Sand (SiO 2 )+ soda (Na 2 CO 3 ) lowers melting point and viscosity, making mix easier to work LIme (CaO) is added to prevent it dissolving in water LIme (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 aluminum Traces of magnesium and aluminum Pyrex and auto headlights add boron oxide to form borosilicates Pyrex and auto headlights add boron oxide to form borosilicates

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

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

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

23 How you measured density in chemistry Use a balance to find the weight of the sample Use a balance to find the weight of the sample Determine the volume of the sample Determine the volume of the sample Measure the volume directly (if liquid) Measure the volume directly (if liquid) Water displacement if irregular solid Water displacement if irregular solid Why doesnt this work on forensic samples? Why doesnt this work on forensic samples?

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

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

26 Density of Glass Crown2.500 g/cm 3 Crown2.500 g/cm 3 Lead Crystal3.100 g/cm 3 Lead Crystal3.100 g/cm 3 Densest Flint7.200 g/cm 3 Densest Flint7.200 g/cm 3 Fused Silica2.200 g/cm 3 Fused Silica2.200 g/cm 3 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 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 The bouyant force is equal to the weight of the liquid that is displaced To float glass you need high density liquids To 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 two 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 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 Object will sink to the point where its average density equals the density of the fluid Use calibrated reference samples as check Use calibrated reference samples as check Pure crystals with known density Pure crystals with known density Solves the problem of tiny, irregular samples Solves the problem of tiny, irregular samples

30 Density Gradient Column Layers of liquids of different composition Layers of liquids of different composition Each has a slightly different density Needs to be temperature stabilized to minimize convection 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 n = c / v Ratio of speed of light in vacuum to speed of light in the material Ratio of speed of light in vacuum to speed of light in the material n always greater than 1 n always greater than 1 Light cant go faster than its speed in a vacuum Light cant go faster than its speed in a vacuum Depends on wavelength of light (dispersion) Depends on wavelength of light (dispersion) Why you get a rainbow from a prism Why you get a rainbow from a prism

33 Snells Law N=1.52 The higher the n, the more the light bends n=1.335 n=1.50

34 Snells Law Mathematically N 1 x sin(θ 1 ) = N 2 x sin(θ 2 ) Animation

35 Properties of Glass Crown2.500 g/cm Crown2.500 g/cm Lead Crystal3.100 g/cm 3 Lead Crystal3.100 g/cm 3 Densest Flint7.200 g/cm Densest Flint7.200 g/cm Fused Silica2.200 g/cm Fused Silica2.200 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 Manufacturing changed in late 1970s making glass more uniform A rough statistical estimate of the likelihood of finding glass of that refractive index (2337 samples)

37 Consequences Need to measure refractive index to 5 significant figures (1 part in 10,000) Need to measure refractive index to 5 significant figures (1 part in 10,000) Measured at sodium D line (single wavelength) Measured at sodium D line (single wavelength) Flat glass samples only Flat glass samples only Window, autos, display cases, mirrors from actual criminal investigations 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? 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? 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? 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 Laser Ablation Inductively Coupled Plasma Mass Spectrometry Detect 46 trace elements in glass Detect 46 trace elements in glass FBI and Oak Ridge National Laboratory FBI and Oak Ridge National Laboratory Developing a national database to determine likelihood of a match 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 displayIs it easy to tell the samples apart?


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