Forensic Analysis of Glass

Forensic Examination of Glass

• Goals in examining glass evidence:– Determine • the types of glass at the scene• how the glass was fractured– Use physical characteristics to classify it– Individualize the glass to a source

• Compare physical and chemical characteristics:– Non-optical properties: surface wear, striations

from manufacturing, thickness, surface film or dirt, hardness, density

– Optical properties: color and refractive index– Chemical properties: additives or trace elements

What is Glass?

““A hard, brittle, amorphous solid that is A hard, brittle, amorphous solid that is composed of silicon oxides mixed with composed of silicon oxides mixed with various metal oxides.” various metal oxides.”

• Fusion of sand (SiO2), soda (Na2CO3) & lime (CaO) that produces a transparent solid when cooled.

• An extended, 3D network of atoms which lacks the repeated, orderly arrangement typical of crystalline materials.

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.

• Soda-Lime Glass: most common type, made from silica (SiO2), soda (Na2CO3) and lime (CaO).

• 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 & density

What Types of Glass Are There?

http://www.glasswebsite.com/video/default.asp

• 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.

• Initially, glass acts initially as an elastic surface and bends away when a force is applied.

How Does Glass 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 make– Right angles on the – Reverse side from where the force was

applied

Order of shots

Which bullet hole was made first?

Exceptions to the Three R Rule

Tempered glass“dices” without forming ridges

Very small windows held tightly in framecan’t bend or bulge appreciably

Windows broken by heat or explosionno “point of impact”curved, smooth edges at break points

Putting it Back Together Again?(Not just for Humpty Dumpty)

• Examiners may be able to 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.

Forensic Analysis of Glass

Glass Transfer Evidence

• When 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 Samples

• The 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.

Optical Properties of Glass

Make 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 glass’s color.

Visual color analysis is very subjective.

Dyes and pigments can be almost impossible to extract.

Nonoptical Physical Properties of Glass

• Surface striations and markings– Rollers leave parallel ream marks on sheet glass– Markings may indicate the glass’s orientation

when pieces are missing– Surface scratches, etchings, and other markings

may also be used to individualize evidence

• Other Properties– Hardness=5-6 on Mohs scale; use a scratch test.– Determinations of curvature can distinguish flat

glass from container, decorative, or ophthalmic.

Physical 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 thousand or better.

Glass Density

• Density can be measured by: • directly determining mass and

volume (usually by displacement) • comparison by flotation• comparison using a density gradient

column

• Density gradient column method:• Fragments of different densities

settle at different levels in the column

• Technique is not accurate for fragments that are cracked or contain an inclusion

Density by the Flotation Method

• A 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

• 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.

Light slows down when it enters a medium such as water or glass.

Snell’s Law

N=1.52

The higher the n, the more the light bends

N=1.33

Refractive Index By Immersion

Refractive Index By Immersion

• Immersing a glass particle in a liquid medium (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 edge of a particle that is immersed in a liquid of a different refractive index.

Glass has higher refractive index-note white line inside

• Glass has lower refractive index-note white line outside

Becke Lines

Histogram = flat glassesCurve = container glasses

The 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 1970’s making glass more uniform

1964-1979

1980-1997

A rough statistical estimate of the likelihood of finding glass of that refractive before 1970

Problems 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 Glass

• Fluorescence– Under 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 Analysis– Can determine many elements simultaneously– Surfaces of samples (>50 mg) can be analyzed

• Atomic Absorption Spectroscopy– You must first know which elements are present– Can analyze ppm levels of elements present

The Wave of the Future: Laser Ablation

• Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)– Laser burns off a microscopic sample– Elements are ionized by plasma– Detects 46 trace elements and their isotopes

simultaneously in glass at < 1 ppb

Types of Fractures by Projectiles

• Bullets 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 applied– As the bullet’s velocity increases, the central hole

becomes smaller, cracking patterns become simpler, and the exit hole becomes wider than the entrance hole.

Fractures by Projectiles

•Stress 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 from 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 figures above, which impact occurred first?

• The sequence of impacts can be determined since crack propagation is stopped by earlier cracks.