Answer these Q’s

• What is glass made of?

• How many types of glass can you name?

• Name as many objects as you can that are composed of glass.

• How can glass be transferred?

• How do you think forensic scientist use glass in an investigation?

Forensic Analysis of Glass

Forensic Examination of Glass

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

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

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

– Chemical 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 & density

What 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 make– Right angles on the – Reverse side from where the force was

applied

Exceptions to the Three R RuleTempered 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

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.

Rolled Glass Float Glass

Float Glass Process

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.

Glass Transfer Evidence• When 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 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.

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 dependent on sample mass).

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

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Density

Type of Glass Density

window 2.46-2.49

headlight 2.47-2.63

pyrex 2.23-2.36

lead glass 2.9-5.9

porcelain 2.3-2.5

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 of liquid of varying density.

Flotation Methodprecise and rapid method for comparing glass densities.

1. a glass particle is immersed in a liquid.

2. The density of the liquid is carefully adjusted – by the addition of small amounts of an appropriate

liquid – until the glass chip remains suspended in the liquid

medium.

3. At this point, the glass will have the same density as the liquid medium

– can be compared to other relevant pieces of glass which will remain suspended, sink, or float.

• A shattered glass jar was found at a burglary scene and was determined to be made of soda-lime glass (density = 2.47 g/mL). A glass fragment recovered from a suspect’s home had a mass of 4.652 g. When the glass fragment was placed into a graduated cylinder with 20.00mL of water, the level of the water rose to 21.53 mL.

• Does the glass fragment link the suspect to the crime scene?

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Determination ofRefractive Index

Immersion method—lower fragments into liquids whose refractive index is different.

Match point—when the refractive index of the glass is equal to that of the liquid

Becke line—a halo-like shadow that appears around an object immersed in a liquid. It disappears when the refractive index of the liquid matches the refractive index of the glass fragment (the match point)

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 match point, temperature is noted and refractive index of the liquid is read from a calibration chart

• 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.

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The Becke Line

The Becke line is a “halo” that can be seen on the inside of the glass on the left, indicating that the glass has a higher refractive index than the liquid medium. The Becke line as seen on the right is outside of the glass, indicating just the opposite.

Refractive Index By Immersion

Glass has higher refractive index-note white line inside

• Glass has lower refractive index-note white line outside

Becke Lines

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Refractive Index

Liquid RI Glass RIWater 1.333 Vitreous silica 1.458

Olive oil 1.467 Headlight 1.47-1.49

Glycerin 1.473 Window 1.51-1.52

Castor oil 1.82 Bottle 1.51-1.52

Clove oil 1.543 Optical 1.52-1.53

Bromobenzene 1.560 Quartz 1.544-1.553

Bromoform 1.597 Lead 1.56-1.61

Cinnamon oil 1.619 Diamond 2.419

• refractive index = velocity of light in vacuum velocity of light in medium

Snell’s Law

N=1.52

The higher the n, the more the light bends

N=1.33

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Fracture Patterns

Radial fracture lines radiate out from the origin of the impact; they begin on the opposite side of the force

Concentric fracture lines are circular lines around the point of impact; they begin on the same side as the force

3R rule—radial cracks form a right angle on the reverse side of the force.

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Glass as Evidence

Class characteristics; physical and chemical properties such as refractive index, density, color, chemical composition

Individual characteristics; if the fragments can fit together like pieces of a puzzle, the source can be considered unique

Considerationsfor Collection

The collector must consider that fragments within a questioned sample may have multiple origins. If possible, the collector should attempt an initial separation based on physical properties.

The collector must consider the possibility there may be a physical match to a known sample (e.g., a piece of glass to a fractured vehicle headlamp). When an attempt to make a physical match is made at the site of collection, the collector should take precautions to avoid mixing of the known and questioned samples.

Any glass samples collected should be documented, marked (if necessary), packaged, and labeled.

Collecting the Sample The glass sample should consist of the largest amount that can be

practically collected from each broken object and packaged separately. The sample should be removed from the structure (e.g., window frame, light assembly). The inside and outside surfaces of the known sample should be labeled if a determination of direction of breakage or reconstruction of the pane is desired.

When multiple broken glass sources are identified, it is necessary to sample all sources.

A sample should be collected from various locations throughout the broken portion of the object in order to be as representative as possible.

The sample should be collected with consideration being given to the presence of other types of evidence on that sample (e.g., fibers, blood).

Important Physical Properties

• Light waves travel in air at a constant velocity until they penetrate another medium, such as glass or water, at which point they are suddenly slowed, causing the rays to bend.

• The bending of light waves because of a change in velocity is called refraction.

• Refractive index is the ratio of the velocity of light in a vacuum to that in the medium under examination.

Important Physical Properties• 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 and will serve to characterize a substance.

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

Why use RI?

• precise refractive indices can be measured rapidly on the small fragments typically found in casework

• It can aid in the characterization of glass

• it provides good discrimination potential

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

Density vs RI• The glass fragment must be scrupulously clean

and free of inclusions

• accurate density measurements require a sample that is 2-3 mm in diameter

• refractive index determination are more rapid and can be performed with smaller samples so most laboratories choose to preform refractive index first.

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.

Learning Check

1. Which unique chemical component would be found in each type of glass shown below?

(beaker) (windshield) (crystal)

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

Glass Composition Analysis

• most methods of glass composition analysis are destructive

• require glass samples larger than those found in forensic casework

• many of the techniques used require expensive equipment

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

• LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) is a powerful analytical technology that enables highly sensitive elemental and isotopic analysis to be performed directly on solid samples.

• powerful analytical technology that enables highly sensitive elemental and isotopic analysis to be performed directly on solid samples.

Use the table to determine if lead borosilicate glass can be distinguished from borosilicate glass by density, refractive index, or both.