Glass: Forensic Analysis

GLASS

• Glass is technically defined as: “The inorganic product of fusion which has cooled to a rigid condition without crystallizing”.

• Glass is a hard, amorphous material made by melting sand, lime (CaO) and sodium oxide (Na2O) at very high temperatures and then cooling it so quickly that there is no time for crystals to form in the glass. Its primary ingredient is silicon dioxide (SiO2), also called silica.

• Na2O reduces the melting point of silica / sand, and CaO is prevents the glass from being soluble in water.

• In contrast to crystalline solids, which have an ordered internal arrangement of atoms, the internal structure of glass consists of a 3D network of atoms lacking long-range symmetry or orderly arrangement. This condition is referred to as the vitreous, or glassy, state.

• Does not have a specific M.P.• Softens over a temperature range

GLASS COMPONENTS

• Formers - forms the glassy, non-crystalline structure

Examples: SiO2, B2O3, P2O5, GeO2, V2O5, As2O3

• Fluxes - improve melting properties but impart low chemical resistance -

typically alkali or alkaline earth oxides

Examples: Na2O, K2O, LiO, Al2O3, B2O3, Cs2O

• Modifiers (stabilizers or intermediates) - a material that improves stability.

Typically oxides of Ca, Al, or Zn

• Stabilizers – Chemical/Corrosion Resistance:

Examples: CaO2, MgO2, Al2O3, PbO2, SrO, BaO, ZnO2, ZrO

TYPES OF GLASS

A) On the basis of manufacturing process:

– Ordinary sheet glass– Float glass(plate)

B) On the basis of composition:– Oxide glass– Non oxide glass

C) On the basis of market application:– Commercial /soda lime glass– Lead glass– Borosilicate glass– Laminated glass– Tempered glass

Other Types of Glass• Glass fibre• Vitreous silica• Alumino-silicate glass• Alkali-barium silicate glass• Glass ceramics• Technical glass• Phosphate glass• Optical glass• Sealing glass

ROLLED AND FLOAT GLASS

TEMPERED AND LAMINATED GLASS

GLASS ANALYSIS

• Visual Inspection of Known/ Questioned for Fracture Matches• Comparison of Glass:

– Physical Properties

– Optical Properties

– Chemical Properties

– Classification of Glass into End Use Category - The ability to use some measured

characteristics of a questioned object to place it into a product use class.

– Discrimination between glass samples - The ability to distinguish between two or

more objects within the same product use class.

– Interpretation and Value of Results

First of all physical properties are assessed.Optical properties of the specimens are measured next.

Chemical composition of the glass is typically measured last.

SCOPE OF ANALYSIS

Altering the compounds used to make glass changes the composition and produces different types of glass. The composition of a particular piece of glass may be unique and therefore identifiable. Because glass is made of a variety of compounds, it is possible to distinguish one type of glass from another by examining the different physical and chemical properties. We will examine some of the properties of glass, such as density, refractive index, and fracture patterns that are used in the forensic examination.

• Glass, as a physical clue, is frequently encountered in various crimes; such as burglary, road accidents, murder, sexual assaults, shooting incidents, arson, and vandalism.

• Thus during investigations, glass forms one of the evidentiary materials in many criminal

SCOPE OF ANALYSIS

• The chips of broken glass window may be lodged in suspect’s shoes or garments during the act of burglary/crime; particles of headlight glass found at the crime scene may offer clues that confirm the identity of a suspected vehicle.

• Whenever there is violence, bottles, window pane glass, mirrors, eye glasses and other glass objects can be accidently scattered and fragments of these can also adhere to the criminal’s clothing or shoes.

• Broken glass fragments ranging in size from large pieces to tiny shards may be transferred to and retained by nearby persons or objects or alleged assailantwhere a bottle is used as weapon.

• The mere presence of fragments of glass on the clothing of an alleged burglar in a case involving entry through a broken window may be significant evidence if fragments are found. The significance of such evidence will be enhanced if the fragments are determined to be indistinguishable in all measured properties from the broken window.

MAJOR FORENSIC GLASS SOURCES

Flat Glass – Soda lime silicate - Drawing, Rolling, or FloatingCoated: Surface modification - MirrorsLaminated: Sandwiched around plastic - Automotive windshieldsHeadlights: Sometimes borosilicateLight bulbs: Soda lime glassHeat absorbing/ UV filtering - TintedPhotochromic (Light Sensitive) - Eyeglasses

DENSITY = MASS / VOLUME

• The ratio of the mass of an object to the volume occupied by that object. (g/cm3 (solids); g/mL (liquids))

• One method of matching glass fragments is by a density comparison. Each type of glass has a density that is specific to that glass. If two samples of glass can be differentiated by density, they could not have originated from the same source.

• Densities of solids and liquids are often compared to the density of water.• Glass density can be measured by Displacement, Flotation, and Density

Gradient Column methods.• Density varies with temperature.

DENSITY MEASUREMENT

• Density Gradient Method - The method involves placing, in a vertical glass tube, a liquid containing a gradient of density. The gradient is such that the density at any level is less than that at any level lower in the tube and greater than that of any level higher in the tube. When glass fragments are introduced to the column, each will become suspended in the liquid at the level that is the same density as that glass fragment. Fragments of different density will settle to different levels in the column.

• Flotation Method - Is a precise and rapid method for comparing glass densities. A glass particle is immersed in a liquid. The density of the liquid is adjusted by addition of small amounts of appropriate liquid until the glass chips remains suspended in the liquid. At this point glass will have same density as a liquid medium . Ohter pieces of glass will sink or float.

• Displacement Method - The mass of a fragment of glass and the volume of water it displaces, can help calculate the density

WHY IT IS LESS FREQUENTLY MEASURED

Density measurements are performed less frequently than refractive index determinations because:

The glass fragment must be scrupulously clean and free of inclusions.

Accurate density measurements require a sample that is two to three

millimeters in diameter.

Density measurements required the use of hazardous liquids, such as

bromoform.

REFRACTIVE INDEX

Refraction is the change in the direction of light as it speeds up or slows down when moving from one medium into another. The direction and amount the light bends varies with the densities of the two mediums.

The refractive index is a tool used to study how light bends as it passes through one substance and into another. Any substance through which light can pass has its own characteristic refractive index. The refractive index of a substance is calculated by dividing the speed of light in a vacuum a space empty of all matter by the speed of light through that particular substance.

It IS THE MOST COMMONLY MEASURED PROPERTY IN THE FORENSIC EXAMINATION OF GLASS FRAGMENTS. It can aid in the

characterization of glass. It provides good discrimination potential.

REFRACTIVE INDEX

• If a colourless piece of glass is put into water, you can still see it because the water and glass have different refractive indices.

• The refractive index of glass does not vary significantly with temperature, but those of liquids do.

• If a piece of glass is placed in a liquid which is then heated, at some point the refractive indices will be identical and you will no longer be able to see the piece of glass.

• If there are two pieces of glass – one the suspect and one from the scene of crime, have identical refractive indices, then they are from the same source.

Automobile head light Glass - 1.47 – 1.49Bottles - 1.51 – 1.52Window Glass - 1.51 – 1.52Opthalmic Glass - 1.51 – 1.52

• Example 1: A beam of light travels in air (medium 1) and then passes through a piece of glass (medium 2). As the light passes from the air into the piece of glass, the light ray is bent . What is the angle of refraction measured from the normal?

• refractive index of air (medium 1) = 1.00, refractive index of glass (medium 2) = 1.50, angle 1 = 45°, angle 2 = ?

Solution:• n1 (sin of angle 1) = n2 (sin of angle 2)• Substituting what we know into Snell’s law:• 1.00 (sin of 45°) = 1.50 (sin of angle 2)• The sine of 45° is 0.7071.• 1.00 (0.7071) = 1.50 (sin of angle 2)• (0.7071)/1.50 = sin of angle 2• 0.4714 = sin of angle 2• angle 2 = 28.1° ≈ 28°• This answer makes sense because the light is moving from a less-dense substance

in medium 1 (air) to a denser substance in medium 2 (glass). The light will slow down and bend toward the normal. Angle 1 is 45° and angle 2 is smaller at 28°, indicating that the light did bend toward the normal.

Becke Line

• If the refractive index (n) of the liquid medium is different from the refractive index of the piece of glass, a halo-like ring appears around the edge of the glass. This halo-like effect is called a Becke line. It appears because the refracted light becomes concentrated around the edges of the glass fragment.

• If the Becke line is located inside the perimeter of the glass fragment, then the refractive index of the glass is higher than the refractive index of the surrounding liquid.

• If the Becke line is located on the outside of the perimeter of the glass fragment, then the refractive index of the surrounding medium is higher than the refractive index of the glass.

COLOR

Materials can be added to the batch to produce glass in practically any color.

Impurities present in the raw materials used to produce glass can impart

unintentional color.

Differences in color represent a change in glass chemistry and can be used to

differentiate specimens.

Typically not possible to reliably perform colorimetry on glass fragments in

forensic casework due to too small size and too low color density of samples.

Color assessment is performed visually against a white background in natural

light with the particle on edge.

Side-by-side comparison should be used with similarly sized particles.

DETECTION OF CURVATURE

An Interferometer can be used to detect the most minimal curvature on the glass surface. A Spherometer is used to measure the radius of curvature of the glass fragments having curved surface.

Curvature indicates possible sources:– windshield– containers– other non-flat glass source

FLUORESCENCE

• Fluorescence can be used as a basis to differentiate glass specimens.• The glass surface that was in contact with the tin bath during the

manufacturing procedure will fluoresce when exposed to short-wave (~254 nm) ultraviolet light.

• Fluorescence examinations can also be performed using fluorescence spectroscopy on specimens as small as 0.05 mm2.

• Fluorescence on a glass surface will be detected only if the surface that will fluoresce is preserved, collected, and analyzed.

Thickness Considerations– Tempered glass is greater than 3.0 mm thick– Vehicle side windows are typically 3.3-3.6 mm thick

ELEMENTAL ANALYSIS

Glass composition analysis can be used to differentiate between:

• glasses made by different manufacturers,• glasses from different production lines of the same manufacturer,• glasses made over a period of time in a single production line.

Glass composition analysis is performed infrequently because:

Most methods of glass composition analysis are destructive.

Most methods require glass samples larger than those routinely encountered in

forensic casework.

Most of the instrumentation used to measure glass composition is expensive to

purchase and maintain, and much of the instrumentation has few other

applications.

Because of the complexity of the calculations, Bayesian statistical analysis including

compositional data is extremely difficult to apply.

TECHNIQUES USED FOR ELEMENTAL ANALYSIS

Semi-quantitative techniques

• Scanning electron microscopy-energy dispersive spectrometry

• X-ray fluorescence

Quantitative techniques • neutron activation analysis • flameless atomic absorption

spectrometry • spark-source mass spectrometry • inductively coupled plasma-optical

emission spectrometry • inductively coupled plasma-mass

spectrometry• laser ablation-inductively coupled

plasma-mass spectrometry

THICKNESS OF GLASS

Not all glass is the same thickness, and this difference provides another clue for identifying glass. Picture frame glass is 1/8 inch thick, while window glass must be 3/32 inch to 1/8 inch thick to resist wind gusts without breaking. Door glass will vary in thickness from 3/16 inch to inch thickness and can also be reinforced with wire threads running through it.

The trained examiner will be able to determine the composition, type, and perhaps the manufacturer from a sample of glass found on a victim, suspect, or at a crime scene. By determining the thickness, refractive index, and density of the glass collected, glass fragments can be matched assuming a large enough piece can be recovered.

ANALYZING GLASS FRACTURES

• Glass breaks in a characteristic manner which indicates the direction of travel of the impacting object. Conchoidial striations are ripples seen through the cross section of broken glass. They are always at right angles to the impacted surface.

• Radial cracks are formed first, commencing on the side of the glass opposite to the destructive force.

• Concentric cracks occur afterward, starting on the same side as the force• As the velocity of the penetrating projectile decrease, the irregularity of the shape

of the hole and of its surrounding cracks increase• Fracture always terminates at the existing line fracture. • Stress marks occur on the edge of a radial glass fracture. Stress marks run

perpendicular to one edge and parallel to the other edge of glass.• Stress’ perpendicular edge is always located opposite from which the force of

impact occurred.• Concentric fractures, the perpendicular end always faces the surface on which the

force originated.• Radial cracks form a Right angle on the Reverse side of the force (4 R rule).

GLASS FRACTURE

• When force is applied on any surface of glass it bends but since the elasticity of glass is limited ultimately, it gets fractured after the threshold force application.

• An investigator often has to decide whether a pane of glass was broken from outside or from inside, whether it was broken with a bullet or with a blunt object.

Impact of force on glass• Impact causes a pane of glass to

bulge• Side opposite the impact will stretch

more & rupture first• Radial cracks are rapidly

propagated in short segments from the point of impact

RADIAL AND CONCENTRIC CRACKS

Elasticity permits bending until radial cracks form on the opposite side of the force. Continued force places tension on the front surface (force side), forming the concentric cracks.

Radial Cracks:• When an object has been thrown through a glass pane, a fracture forming a

pattern somewhat like a spider-web will be seen.• The cracks will appear radiating outwards from the point of impact making a

star shaped fracture known as radial fracture. • The radial fracture originates on the surface opposite to that on which force

was applied. This type of fracture is always the first to appear on glass.Concentric Cracks:• A series of broken circles originate on the surface, on which force is being

applied around the point of impact. These are the secondary fractures as they always appear after radial fractures. Concentric circles have same centre.

RADIAL AND CONCENTRIC CRACKS

CONE FRACTURES

• The direction of a single bullet fired

through glass can be easily determined.

• As the bullet passes through the glass, it

pushes some glass ahead of it, causing a

cone-shaped piece of glass to exit along

with the bullet.

• This cone of glass makes the exit hole

larger than the entrance hole of the

bullet.When a bullet is travelling at high velocity the opening on the reverse side of impact

will be larger

CONE FRACTURES: BY HIGH SPEED PROJECTILES

An object moving at a high rate of speed at impact, such as a bullet striking the glass, produces fewer concentric circles. An object moving at a slower rate of speed at impact, such as a rock thrown at a window, produces a greater number of circles.

SIGNIFICANCE OF STUDY OF GLASS FRACTURES

• If several shots are fired through the glass, the order in which the shots were fired can be determined if enough of the glass is available or can be reconstructed. The first shot produces the first set of fracture lines. These lines set the boundaries for further fracturing by following shots. Radiating fracture lines from a second shot stop at the edge of fracture lines already present in the glass.

• Fracture patterns are unique; Pieces from the broken glass pane or hole often show marks that are characteristics of the type of injury and direction of force.

• If correctly interpreted, these findings gives useful information about the object used for breaking and velocity of breaking object.

• Fracture examinations can provide information as to the direction of the breaking force and the sequence of multiple impacts.

PROPAGATION OF FRACTURES DUE TO MULTIPLE IMPACTS

PATH OF A BULLET PASSING THROUGH WINDOW GLASS

• The angle at which a bullet enters a piece of window glass can help locate the position of the shooter.

• If the bullet was fired perpendicular to the windowpane, the entry hole of the bullet will be round. If the bullet was fired into the window at an angle, fracture patterns in the glass left by the bullet can be used to help locate the shooter’s position.

• If the shooter was firing at an angle coming from the left, glass pieces will be forced out to the right. The bullet’s exit hole will form an irregular oval as it exits to the right. If the shot originated at an angle coming from the right, glass pieces will be forced out to the left, leaving an irregular oval hole to the left.

MISCELLANEOUS

• Ammunition type may be determined from the size and characteristics of the bullet hole. The distance from the shooter to the window can be estimated based on knowledge of the type of ammunition and its effect on the window. However, a high-speed bullet fired from a great distance will often exhibit characteristics of a slower-speed bullet fired from a closer range.

• Bulletproof 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.

• Patterns or scratches other than those already mentioned may be found on evidence glass. For example, windshield wipers may leave marks on the windshield or scratches on side glass etc.

NUMBER OF GLASS FRAGMENTS THAT CAN BE TRANSFERRED IS CONTROLLED BY A NUMBER OF FACTORS:

The closer something is to the breaking glass, the more likely it is to have glass fragments transferred to it. The number of fragments transferred decreases with distance from the break (Pounds and Smalldon 1978).

The person breaking a window will have more glass on him or her than a bystander, and the more blows required to break out the glass, the more glass that will be transferred (Allen et al. 1998b).

The number of glass fragments generated by a break is independent of the size and thickness of the window but increases with greater damage to the glass (Locke and Unikowski 1992).

Less glass is retained on slick clothing, such as nylon jackets, than on rough clothing, such as wool sweaters. Wet clothing retains more glass than dry clothing. Glass fragments fall off clothing over time, and larger pieces fall off before smaller pieces.