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.

What Types of Glass Are There? 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? 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.

Rolled Glass Float Glass Float Glass Process

Bullet Proof Glass constructed using polycarbonate, thermoplastic, and layers of laminated glass. The aim is to make a material with the appearance and clarity of standard glass but with effective protection from small arms. usually made from 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.

How Do Glass Windows Break? 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 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? 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 Rule Tempered glass “dices” without forming ridges Very small windows held tightly in frame can’t bend or bulge appreciably Windows broken by heat or explosion no “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.

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.

Kendall/Hunt Publishing Company 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 Kendall/Hunt Publishing Company

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 Method precise and rapid method for comparing glass densities. a glass particle is immersed in a liquid. 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. 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?  

Determination of Refractive 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.

Kendall/Hunt Publishing Company 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. Kendall/Hunt Publishing Company

Refractive Index By Immersion

Becke Lines Glass has higher refractive index-note white line inside Glass has lower refractive index-note white line outside

Kendall/Hunt Publishing Company Refractive Index Liquid RI Glass Water 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 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 Kendall/Hunt Publishing Company

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

Snell’s Law N=1.52 N=1.33 The higher the n, the more the light bends

Kendall/Hunt Publishing Company 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. Kendall/Hunt Publishing Company

Kendall/Hunt Publishing Company Chapter 14 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 Kendall/Hunt Publishing Company Kendall/Hunt

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

Problems with Refractive Index A rough statistical estimate of the likelihood of finding glass of that refractive before 1970 1964-1979 Manufacturing changed in late 1970’s making glass more uniform 1980-1997

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.

Learning Check Which unique chemical component would be found in each type of glass shown below? (beaker) (windshield) (crystal)

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