Chapter 5 Forensic Analysis of Glass Objectives Students

Chapter 5 Forensic Analysis of Glass

Objectives • Students should gain an understanding of: – The composition of different types of glass – The optical and nonoptical properties of glass – Techniques to determine the way in which glass has fractured – Techniques to match glass fragments – The use of scanning electron microscopy and X-ray fluorescence to determine the elemental composition of glass

Introduction • Glass is often encountered in criminal investigations. • Fragments may be found on a suspect as transfer evidence. • Key considerations for investigators: – Which types of glass are found at the scene – How glass fragments can be classified – How to individualize glass fragments

Types of Glass (1 of 3) • Glass is a solid with an amorphous structure; its atoms have a random, disordered arrangement. • Glass softens over a wide temperature range. • Soda-lime glass is common in windows and bottles • The addition of metal oxides can give a special appearance to glass.

Types of Glass (2 of 3) • Tempered glass is four times stronger than window glass. • Manufacture of tempered glass: – Hot glass is rolled into sheets. – The upper and lower surfaces are cooled rapidly with jets of air. • Tempered glass does not shatter; it breaks into “dices. ”

Types of Glass (3 of 3) • Windshield glass is a kind of laminated glass. • It consists of two layers of glass with highstrength vinyl plastic film in between the layers. • The film holds the glass in place when it breaks.

Forensic Examination of Glass Evidence: An Overview (1 of 2) • Goals in examining glass evidence: – Determine the class to which the glass belongs – Individualize the glass to one source

Forensic Examination of Glass Evidence: An Overview (2 of 2) • Two samples are needed forensic analysis: – Glass from the crime scene – Glass on items belonging to the suspect • Compare characteristics of the samples: – Color, fracture pattern, scratches, striations from manufacturing, unevenness of thickness, surface wear, surface film or dirt, weathering patterns

Nonoptical Physical Properties of Glass (1 of 5) • Nonoptical physical properties include the glass’s surface curvature, texture, and any special treatments. • Such properties are useful in proving two pieces of glass are not associated.

Nonoptical Physical Properties of Glass (2 of 5) • Surface striations and markings – Rollers leave parallel ream marks on sheet glass – Markings may indicate the glass’s orientation when pieces are missing – Relative spacing of marks may be used for individualization – Surface scratches, etchings, and other markings may also be used to individualize evidence

Nonoptical Physical Properties of Glass (3 of 5) • Surface contaminants – Contaminants show the presence of impurities. – Patterns of adhering materials might suggest how pieces fit together. – Chemical analysis of adhering materials might further individualize the pieces.

Nonoptical Physical Properties of Glass (4 of 5) • Thickness – The thickness of glass must be measured with a high degree of accuracy. – The investigator cannot assume the glass’s thickness is constant. – Determinations of curvature can distinguish flat glass from container, decorative, or ophthalmic glass.

Nonoptical Physical Properties of Glass (5 of 5) • Hardness: measured on the Mohs scale, which indicates a substance’s hardness relative to other substances – Talc (softest): assigned a value of 1 – Diamond (hardest): assigned a value of 10 – Glass: tends to fall in the 5– 6 range • For unknown minerals, relative hardness is determined by using the sample to try to scratch benchmark minerals

Glass Fractures (1 of 8) • Elasticity: the ability of a material to return to its previous shape after a force is exerted on it • Use in forensic investigation: may be able to analyze fractured window panes at a crime scene and determine what happened

Glass Fractures (2 of 8) • Three types of forces are distinguished: – Compressive: squeezes the material – Tensile: expands the material – Shear: slides one part of material in one direction and another part in another direction • Each force causes a deformation.

Glass Fractures (3 of 8) • Glass breaks when a tensile strain is applied that overcomes the material’s natural tensile stress limit. – Cracks grow from the unloaded side to the loaded side. – Radial cracks radiate outward, away from the load point. – Tangential (concentric) cracks form if a load persists, leading to a spider web appearance.

Glass Fractures (4 of 8) • Bullets are projectile loads through glass. – Load side is the entrance; unloaded side is the exit. – 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.

Glass Fractures (5 of 8) • 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 opposite is true for a tangential crack.

Glass Fractures (6 of 8) • 3 R rule: – Radial cracks give rib marks that make – Right angles on the – Reverse side from where the force was applied

Glass Fractures (7 of 8) • High-speed projectiles: exit hole will be wider than entrance hole • Low-speed projectiles: rib marks may indicate where breaking force was applied

Glass Fractures (8 of 8) • Contaminants such as paint or window putty might help identify the inside and outsides of glass. • Analysis can even determine the sequence of impacts for multiple bullet holes. • Glass fragments from suspects will be very small and lost fairly rapidly.

Glass Density Tests • Density: a class characteristic • Density gradient column method: used to determine glass density – 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 tests can exclude fragments that do not match the known specimen

Optical Physical Properties of Glass (1 of 11) • Color – Make side-by-side comparisons using similarsized fragments – Place the samples over a white surface using natural light – Use both fluorescent and incandescent light to determine the glass’s color

Optical Physical Properties of Glass (2 of 11) • Refractive index: measure of how much light is bent as it enters the glass – Velocity of light in the air velocity of light in the glass – The temperature of the sample affects its density; the density change affects the velocity of light as it passes through the sample – Single sheets of plate glass do not usually have a uniform refractive index value – The index of refraction can vary as much as 0. 0002 from one side of the glass to another

Optical Physical Properties of Glass (3 of 11) • Oil immersion method – The forensic examiner places the questioned glass fragments in specialized silicone oils with known refractive indices. – The immersion oil is heated to change its refractive index. – The refractive index of the oil decreases as temperature increases until the Becke line disappears. – At the match point, the refractive indices of the oil and the glass fragment are the same.

Optical Physical Properties of Glass (4 of 11) • Emmons procedure: uses a hot stage microscope plus different source lamps – Measures the index of refraction for the sample at multiple wavelengths – Correlates the refractive index and the wavelength at fixed temperatures for the silicone oil into which the glass sample is placed

Optical Physical Properties of Glass (5 of 11) • Emmons procedure – Step 1: crush the glass and place it in silicone oil on the hot stage – Step 2: measure the glass’s index of refraction with a sodium lamp and with a hydrogen lamp – Step 3: as the temperature of the hot stage increases, take measurements at three different wavelengths – Step 4: record a line for the refractive index of the glass at each wavelength for each temperature – Step 5: superimpose the lines to create a Hartmann net

Optical Physical Properties of Glass (6 of 11) • Annealing is used to distinguish tempered glass from nontempered glass. • Glass fragments are heated in a furnace at a temperature > 600 °C. • The change in refractive index reveals whether the glass is tempered or nontempered.

Optical Physical Properties of Glass (7 of 11) • Properties of glass are more often used to exonerate suspects than to prove an association. • Glass evidence is not always individualized: Fragments from different sources may have similar indices of refraction and similar densities. • Once the refractive index is known, measurement of the glass’s density will improve discrimination capability twofold.

Optical Physical Properties of Glass (8 of 11) • The FBI has compiled density and refractive index data for glass from around the world. • The FBI has also identified the relationship between their refractive indices and densities for 1400 glass specimens.

Optical Physical Properties of Glass (9 of 11) • Elemental analysis of glass – Elemental analysis should be performed only after all nondestructive methods of examination are complete and when additional discrimination is necessary. – Scanning electron microscopy may be used to analyze glass, but precise quantitative determination of the element concentrations is not possible.

Optical Physical Properties of Glass (10 of 11) • Techniques for elemental analysis of glass – X-ray fluorescence: focuses a beam of X-rays on the glass’s surface and measures the energy of the Xrays emitted – Flameless atomic absorption spectrophotometry (FAAS) – Inductively coupled plasma (ICP): when used with an optical emissions spectrometer, can identify 10 elements in glass (aluminum, barium, calcium, iron, magnesium, manganese, sodium, strontium, titanium, zirconium)

Optical Physical Properties of Glass (11 of 11) • Disadvantages of the elemental analysis of glass: – The fragment must be dissolved in acid, so the original sample is destroyed – Techniques require the use of hazardous chemicals