Forensic Analysis of Glass

Forensic Examination of GlassGoals in examining glass evidence:Determine the types of glass at the sceneDetermine how the glass was fracturedUse physical characteristics to classify itIndividualize the glass to a source

Compare physical and chemical characteristics:Optical properties: color and refractive indexNon-optical properties: surface wear, striations from manufacturing, thickness, surface film or dirt, hardness, densityChemical properties: additives or trace elements

What is Glass?Fusion of sand (SiO2), soda (Na2CO3) & lime (CaO) that produces a transparent solid when cooled.A 3D network of atoms which lacks the repeated, orderly arrangement typical of crystalline materials.

Physical properties: hard, elastic, brittle, non-conductor of electricity, density, refractive index, etc.Chemical: resistant to all but fluorine and very strong bases.

The primary uses for glass are in windows, containers, light bulbs and eyewear.

Borosilicate Glass (pyrex): 5% borax (Na2B4O7) is added to resist breaking when heated or cooled.

Colored Glass: metal oxides or colloidal iron (Fe) & sulfur (S) are added to change its color.

Lead glass: Pb increases refractive index & densityWhat Types of Glass Are There?

What Types of Glass Are There?Flat glass: made by a float glass process; molten glass is floated on a pool of tin while cooling. Commonly found in doors and windows.

Laminated glass: used in windshields, two sheets of glass with plastic between them.

Tempered safety glass: used in car side windows and designed to break into tiny pieces; potassium (K) replaces sodium (Na) on the surface.

Each force causes a deformation that may leave a visible mark or fracture the glass. This can be used to determine the direction and amount of force.

Glass acts initially as an elastic surface and bends away when a force is applied. When the force increases beyond its tensile strength, it cracks.

How Do Glass Windows Break?

Radial cracks form first and are propagated in short segments on the side opposite the force.Concentric cracks come later from continued pressure on the same side as the force applied. How Does Glass Break?

How Does Glass Break? Edges of broken pieces of glass will show rib (stress) marks.

In a radial crack, the rib marks are perpendicular to unloaded side and parallel to loaded side. The arrow shows the side that received the impact.

3R rule:Radial cracks give rib marks that makeRight angles on the Reverse side from where the force was applied

Exceptions to the Three R RuleTempered glassdices without forming ridges

Very small windows held tightly in framecant bend or bulge appreciably

Windows broken by heat or explosionno point of impactcurved, smooth edges at break points

Types of Fractures by ProjectilesBullets are a projectile force (load) that can pass through glass.Load side is the entrance side; unloaded side is the exit side.Low-speed projectiles: rib marks may indicate where breaking force was appliedAs the bullets velocity increases, the central hole becomes smaller, cracking patterns become simpler, and the exit hole becomes wider than the entrance hole.

Fractures by ProjectilesStress lines on the glass edge of radial cracks form a right angle on the reverse side from the force.Stress lines on the glass edge of concentric cracks form a right angle on the same side as the force.Exit (unloaded) side is wider than entry (load) side.Which side was the bullet fired from?

Which Bullet Hole Was First?In the figure above, which impact occurred first?The sequence of impacts can be determined since crack propagation is stopped by earlier cracks.

Putting it Back Together Again?Examiners can fit together two or more pieces of glass that were broken from the same object.Because glass is amorphous, no two glass objects will break the same way.

Learning CheckIn the figure below & left, which impact occurred first?In the figure above & right, from which side did the impact occur?

Glass Transfer EvidenceWhen glass objects are broken, glass flies backward from all parts of the object where cracks appear not just from point of impact.

This creates a shower of minute glass particles and a transfer of evidence.

Glass fragment comparison depends finding and measuring properties that will associate one glass fragment with another while eliminating other sources.

Collection of Glass SamplesThe glass fragments should be packaged in boxes to avoid further breakage.

If evidence is to be examined for glass fragments, it should be taken whole and each item individually wrapped in paper and boxed.

If even the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all glass fragments.

Submit glass evidence along with a representative sample of each type of glass from the crime scene.

Forensic Examination of GlassGoals in examining glass evidence:Determine the types of glass at the sceneDetermine how the glass was fracturedUse physical characteristics to classify itIndividualize the glass to a source

Compare physical and chemical characteristics:Optical properties: color and refractive indexNon-optical properties: surface wear, striations from manufacturing, thickness, surface film or dirt, hardness, densityChemical properties: additives or trace elements

Optical Properties of GlassMake side-by-side comparisons using similar-sized fragments.

Place samples on a white surface using natural light.

Use both fluorescent and incandescent light to determine the glasss color.

Visual color analysis is very subjective.

Dyes and pigments can be almost impossible to extract.

Nonoptical Physical Properties of GlassSurface striations and markingsRollers leave parallel ream marks on sheet glassMarkings may indicate the glasss orientation when pieces are missingSurface scratches, etchings, and other markings may also be used to individualize evidence

Other PropertiesHardness=5-6 on Mohs scale; use a scratch test.Determinations of curvature can distinguish flat glass from container, decorative, or ophthalmic.

Forensic Analysis of Glass

Quantitative Properties of Glass

Why Measure Density?Can be used as a screening technique with large numbers of fragments.

Useful in identifying multiple sources present in the known and/or questioned samples.

It is nondestructive and an intensive property (not dependant on sample mass).

Need to measure very precisely in parts per hundred or thousand or better.

Glass DensityDensity can be measured by: directly determining mass and volume (usually by displacement) comparison by flotationcomparison using a density gradient column

Density gradient column method:Fragments of different densities settle at different levels in the column of liquid of varying density.Technique is not accurate for fragments that are cracked or contain an inclusion.

Density by the Flotation MethodA glass particle is immersed in a liquid. The density of the liquid is adjusted by the addition of small amounts of another liquid until the glass chip remains suspended.

At this point, the glass will have the same density as the liquid medium and can be compared to other relevant pieces of glass which will remain suspended, sink, or float.

Why Measure Refractive Index?Refractive index: ratio of the velocity of light in a vacuum to the velocity of light in any other medium

For example, at 25oC the refractive index of water is 1.333. This means that light travels 1.333 times faster in a vacuum than it does in water.

Like density, refractive index is an intensive property but it can be measured very precisely (0.0002) and does not destroy the sample.

Refractive index of glass varies with small changes in composition or by how it is manufactured.

Snells LawN=1.52The higher the n, the more the light bendsN=1.33

Refractive Index By Immersion

Refractive Index By Immersion

Immersing a glass particle in a liquid medium (silicone oil) whose refractive index can varied with temperature until it is equal to that of the glass particle.

At this point, known as the match point, the Becke line disappears and minimum contrast between liquid and particle is observed: RI oil = RI glass.

The Becke line is a bright halo near the boarder of a particle that is immersed in a liquid of a different refractive index.

Glass has higher refractive index-note white line insideGlass has lower refractive index-note white line outsideBecke Lines

Histogram = flat glassesCurve = container glassesThe measurement of RI alone can be of limited use because RI distributions of flat glasses and container glasses overlap.Problems with Refractive Index

Manufacturing changed in late 1970s making glass more uniform1964-19791980-1997A rough statistical estimate of the likelihood of finding glass of that refractive before 1970Problems with Refractive Index

The FBI has compiled density and refractive index data for glass from around the world.

The FBI has identified a relationship between their refractive indices and densities for 1400 glass specimens that is better at classification.FBI Refractive Index vs Density Data

Chemical Analysis of GlassFluorescenceUnder UV radiation, many glasses exhibit fluorescence (glow) Caused by heavy metals (including tin) from float process or organic coatings

Scanning Electron Microscopy Energy Dispersive X-ray AnalysisCan determine many elements simultaneouslySurfaces of samples (>50 mg) can be analyzed

Atomic Absorption SpectroscopyYou must first know which elements are presentCan analyze ppm levels of elements present

Learning CheckWhich unique chemical component would be found in each type of glass shown below?

(beaker) (windshield) (crystal)

2. Use Table 2.3 to determine if lead borosilicate glass can be distinguished from borosilicate glass by density, refractive index, or both.

Quantitative Properties of Glass

The Wave of the Future: Laser AblationLaser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)Laser burns off a microscopic sampleElements are ionized by plasmaDetects 46 trace elements and their isotopes simultaneously in glass at < 1 ppb

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