Forensic Science:Forensic Science:Analysis of Glass Analysis of Glass EvidenceEvidence

Chapter 4:

Glass evidence can be found Glass evidence can be found at many crime scenes.at many crime scenes.

Automobile accident sites may be littered with broken headlight or windshield glass.

The site of a store break-in may contain shards of window glass with fibers or blood on them. Video

If shots are fired into a window, the sequence and direction of the bullets can often be determined by examining the glass.

Minute particles of glass may be transferred to a suspect’s shoes or clothing and can provide a source of trace evidence linking a suspect to a crime.

Objectives for Glass Objectives for Glass ForensicsForensicsDefine and distinguish the physical

and chemical properties of glass.Explain the structure of glassExplain the optic properties of glassExplain the forensic methods for

comparing glass fragments.Understand how to examine glass

fractures to determine the direction of impact

Glass & ForensicsGlass & Forensics

►How was it broken?►Link a suspect to a crime scene

►Fingerprints►Blood

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Chemical and Physical Chemical and Physical PropertiesProperties

►Physical properties describe a substance without comparing with another substance. They are measurements like weight, volume, boiling point, melting point.

►Chemical properties describe what happens when you combine it with something else in a chemical reaction. Examples are burning, coloring reagent tests, decomposing, synthesis of an alloy from individual elements

How is glass formed?How is glass formed?Long before humans began

making glass, glass formed naturally.

When certain types of rock are exposed to extremely high temperatures, such as lightning strikes or erupting volcanoes, glass can form.

Obsidian is a type of glass formed by volcanoes.

Timeline of EventsTimeline of EventsPrehistoric humans used obsidian as

a cutting tool.The earliest man-made glass objects (glass

beads) were found in Egypt dating back to 2500 BC.

Glass blowing began sometime during the first century BC.

By the 14th century, knowledge of glass making spread throughout Europe.

The Industrial Revolution brought the mass production of many kinds of glass.

How is Glass Formed?How is Glass Formed?Glass is a hard, brittle, amorphous material made

by melting sand (aka silica,silicon dioxide, SiO2) lime (aka calcium oxide CaO) and soda, sodium carbonate (Na2CO3) at very high temperatures.

The lime (CaO) is added to prevent the glass from being soluble in water.

The soda (Na2CO3) is added to lower the melting point of silica (sand) and make it easier to work with.

In some types of glass with special requirements, trace amounts of other elements are added. Example: Boron is added to make Pyrex glass.

Soda-lime Glass (amorphous solid)Soda-lime Glass (amorphous solid)The atoms are arranged in a The atoms are arranged in a

random fashionrandom fashion

How is Glass Made?How is Glass Made?Following the mixing of the raw

materials, they are transported to the furnace and heated to over 1200oC or 2200oF and changed into a molten mixture. Video

There are different formulas and assembly for different glass applications. Ex: car wind shields are 2 layers with plastic in between.

Types of GlassTypes of Glass

Types of Glass:Types of Glass:Soda-lime glass:

Mostly sand, sodium carbonate and calcium oxide:◦Used for

manufacturing most window and bottle glass

Making Glass Bottles

Rolling Glass mfg

Making Window Panes

Float GlassFloat GlassFlat glass

typically used for windows.

Soda-lime glass that has been cooled on top of a bath of molten tin. video

BorosilicatesBorosilicatesThe common metal-oxides

found in soda-lime glass are sodium, calcium, magnesium and aluminum.

In addition, a wide variety of special glasses can be made by substituting in whole or in part other metal oxides for the silica, sodium and calcium oxides.

Automobile headlights, heat-resistant glass such as Pyrex are manufactured by adding Boron oxide to the oxide mix for Lab glassware, thermometers, cookware.Auto glass video

Leaded GlassLeaded GlassFine glassware and

decorative art glass, called crystal or leaded glass substitutes lead oxide for calcium oxide (lime).

The addition of lead oxide makes the glass denser. As light passes through the more-dense glass, the light waves are bent, giving the glass a sparkling effect.

Tempered GlassTempered Glass This glass is made

stronger than ordinary window glass by introducing stress through rapid heating and cooling of the glass surfaces.

When tempered glass breaks, it does not shatter but rather fragments or “dices” into small squares with little splintering.

Used for side and rear windows of automobiles sold in the United States.

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Glass Breakage

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AnnealingAnnealingWhen trying to make a distinction between

tempered glass and nontempered glass particles a process known as annealing is used.

Annealing- slowly heating and then cooling the glass. A heat treatment that alters the microstructure of a material causing changes in properties such as strength and hardness. It is much stronger than normal glass.

The change in the refractive index value for tempered glass upon annealing is significantly greater when compared to nontempered glass and thus serves as a point of distinction.

Bending Glass

Laminated GlassLaminated GlassThis glass derives its

strength by sandwiching one layer of plastic between two pieces of ordinary window glass.

The windshields of all cars manufactured in the United States are constructed from laminated glass.

Laminated Glass in Laminated Glass in WindshieldsWindshields

USA law requires that all auto windshields are made from laminated glass. This is a safety consideration because it does not break into shards of glass that can injure. The combination of tempered glass and the sandwiched plastic layer try to keep the glass together.

Bulletproof GlassBulletproof GlassBulletproof glass is a combination of two or

more types of glass, one hard and one soft.The softer layer makes the glass more

elastic so it can flex instead of shatter.The index of refraction for both of the

glasses used in the bulletproof layers must be almost the same to keep the glass transparent and allow a clear view through the glass.

Bulletproof glass varies in thickness from three-quarter inch to three inches.

Properties of Glass and Comparing Properties of Glass and Comparing Glass FragmentsGlass Fragments

For the forensic scientist, comparing glass consists of finding and measuring the properties that will associate one glass fragment with another while minimizing or eliminating the possible existence of other sources.

Considering the prevalence of glass in our society, it is easy to appreciate the magnitude of this analytical problem.

Obviously, glass possesses its greatest evidential value when it can be individualized to one source.

Glass can flex to a point and then it breaks. Sound waves can transmit enough energy to vibrate a glass crystal fast enough to break. video

Glass LABORATORY Glass LABORATORY PropertiesPropertiesForensic Science uses physical and

chemical properties for analyzing glass in the lab.

These include refraction, reflection, dispersion, spectroscopy, density, and others.

Some physical analysis is as simple as fitting the broken pieces together.

Optical Properties of GlassOptical Properties of Glass►Glass is in widespread use largely due to

most glass composition are transparent to visible wavelengths of light.

►When glass is shaped differently and polished, it can make passing light behave differently. It can allow it to pass unchanged or cause it to disperse into a rainbow.

►Glass has the ability to refract, reflect, and transmit light following geometrical optics, without scattering it. This is how glasses and contacts work, except we use plastics.

Density of Glass Density of Glass DeterminationDetermination

►To determine the density of glass, it is best to use the immersion method.

►Density = mass/volume►Mass is measured on a

balance / scale.►Volume is determined by

immersing parts of the glass and seeing the level change in a measurement glassware.

Comparing Densities: Comparing Densities: FlotationFlotation A solid particle will either float, sink, or remain

suspended in a liquid, depending upon its density relative to the liquid medium.

Flotation = a standard / reference glass particle is immersed in a liquid; a mixture of bromoform or bromobenzene may be used. The composition of the liquid is carefully adjusted by adding small amounts of bromoform or bromobenzene until the glass chip remains suspended in the liquid medium. At this point, the standard / reference glass sample and the liquid each have the same density. Glass chips (same size and shape as reference sample) are added to the liquid for comparison. If both the unknown and standard / reference samples remain suspended, they have the same density.

FlotationFlotation

Refraction of LightRefraction of Light►Refraction of light is the bending of the

light as it passes the boundary between two different optically dense mediums. It changes directions because it changes speed

Snell’s LawSnell’s Law

Refractive IndexRefractive Index►When light changes from space to air to

water, it slows down. This changes the path it takes. That’s refraction.

►The refractive index is the ratio of the velocity of light in space to the velocity inside a different material.

►Refractive index = Vel in space ► Vel in medium►Think of it as the optical density of a

material. The thicker it is the slower you go

Examples of Refractive IndexExamples of Refractive IndexMaterial Refractive index Speed of light

Space 1.0000 3 x 108 m/s2

Air 1.0005 2.99 x 108 m/s2

Water 1.3333 2.25 x 108 m/s2

Glass, flint 1.6600 1.81 x 108 m/s2

Glass, crown 1.5200 1.97 x 108 m/s2

NaCL 1.5440 1.94 x 108 m/s2

Quartz (SiO2) 1.4580 2.07x 108 m/s2

Diamond 2.4190 1.24 x 108 m/s2

Snell’s Law of refraction is the formula that tells us how much the light will

change directions. That is covered in physics.

Lab Determination of Lab Determination of Refractive IndexRefractive Index►An automated approach for

measuring the refractive index of glass is by the immersion method into oil of different temperatures.

►The Grim 3 instrument is used and the FBI has a database of refractive indexes from different manufacturers glass.

Refractive IndexRefractive Index

Dispersion of LightDispersion of Light►Dispersion of light is when

white light passes through a prism and is separated into the different colored wavelengths by the refraction of the prism.

BirefringenceBirefringence►Asymmetric crystals will refract the

light in two directions because of their shape and the way the light goes through them. You will see a double image or rainbow.

Becke lineBecke lineThe Becke line is a bright line observed close to the boundary between

two grains in plane polarized light. If the microscope focus is raised, increasing the distance between the objective lens and the sample, the Becke line will move into the material with the higher refractive index.

Forensics of Broken GlassForensics of Broken Glass►When broken glass is found at a crime

scene it is gathered and checked for several things:◦ Puzzle pieces fitting together.◦ Fracture pattern. ◦ Blood or any DNA source◦ Fingerprints◦ Composition or type of glass◦ Density – mass per unit of volume.◦ Determining refractive index.◦ Any unique characteristics: ex. Paint, scratches

Proper Collection of Glass EvidenceProper Collection of Glass Evidence

Standard reference glass should be taken from the crime scene (1 in2)

Package in solid containers to prevent breakage

Preserve garment (shoe, pants, shirt) with glass on it

All broken glass must be recovered and submitted for analysis when direction of impact is desired.

Whenever possible, the exterior and interior surfaces of the glass must be indicated. The presence of dirt, paint, grease or putty may indicate the exterior surface of the glass.

Jigsaw Effect – Most Jigsaw Effect – Most BeneficialBeneficialWhen the suspect and crime-scene fragments

are assembled and physically fitted together. Comparisons of this type require piecing

together irregular edges of broken glass as well as matching all irregularities and striations on the broken surfaces. The possibility that two pieces of glass originating from different sources will fit together exactly is so unlikely as to exclude all other sources from practical consideration.

Unfortunately, most glass evidence is either too fragmentary or too minute to permit a comparison of this type

Glass Fracture PatternsGlass Fracture Patterns►When a force is applied to the

amorphous glass structure it spreads it out as much as possible over the surface.

►When the limit of its elasticity is reached, the glass fractures.

►The observed patterns of fracture tells the forensic investigator many things about the force magnitude, direction, and glass type.

Projectile Breaking GlassProjectile Breaking Glass

Analyzing Glass Fracture Analyzing Glass Fracture PatternsPatterns

Radial Fractures of GlassRadial Fractures of Glass►Projectiles (like bullets) will often

create such a high impulse force that the glass reaches its elastic limit and breaks into a “Radial” pattern and is called a Primary Radial Fracture. It looks like the spokes of a bicycle radiating out from the center of the projectile hole.

►The cracks will form on the opposite side of the glass from the force application side.

Secondary FracturesSecondary FracturesSecondary fractures may also form.These fractures take the form of

concentric circles around the point of impact.

Concentric circles are circles that have the same center.

Concentric circles form on the same side of the glass as the point of impact.

Concentric Fractures of Concentric Fractures of GlassGlass

Concentric FracturesConcentric Fractures►For concentric fractures, the

perpendicular end of the stress marks happen on the surface on which the force originated. Note that this is the opposite of radial fractures !

3 R Rule- Determining Side of 3 R Rule- Determining Side of ImpactImpact

Radial Cracks form a Right Angle on the Reverse side of the force.

3R Rule3R RuleRadial cracks are at Right

angles to the Rear (side opposite the impact)

Exceptions–tempered glass“dices” without forming ridges–very small windows held tightly in framecan’t bend or bulge appreciably – windows broken by heat or explosion: no “point of impact”

Multiple Hole Fracture Multiple Hole Fracture PatternsPatterns►If there have been multiple

penetrations of glass, it is possible to determine the sequence of events by tracing the migration patterns out to points of termination where the crack ends in another crack.

Successive Successive PenetrationsPenetrationsA fracture always terminates

at an existing line of fracture bsapp.com

Breakage of Glass from a Breakage of Glass from a FireFire During a fire, glass may break as a result of heat

fracturing. Heat fracturing produces breakage patterns on glass

that are different from breakage patterns caused by impact.

Wavy fracture lines develop in glass that has been exposed to high heat.

Glass will tend to break toward the region of higher temperature.

If the glass was not broken before the fire, there will be no radial or concentric circle fracture patterns in glass that is broken by high heat.

Exit vs Entry Holes in Exit vs Entry Holes in GlassGlass►The exit hole of a projectile going

through a glass is always larger than the entry hole because of the shock waves traveling out in a concentric pattern from the impact point.

DirectionDirection of of Penetration Penetration

A projectile hole is inevitably wider at

the exit side

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Glass Fracture ExamplesGlass Fracture Examples

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Glass Fracture ExamplesGlass Fracture Examples

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