Glass Evidence Chapter 14 Unit 4

Objectives Explain how glass is formed. List characteristics of glass. Provide examples of different types of glass. Calculate the density of glass. Use the refractive index to identify different types of glass. Describe how glass fractures. Analyze glass fracture patterns to determine how glass was broken. Explain how glass is used as evidence.

History of Glass Before humans made it, glass formed naturally. When certain types of rock are exposed to high temperatures (lightening or lava) – glass can form. Obsidian- used by prehistoric humans as a cutting tool Pliny, a Roman historian, described how glass was accidently made in ancient Syria Blocks of rock made of compounds of nitrogen & oxygen were used as cooking surfaces. Sand was placed on top and heated. They melted together and formed glass. 2500B.C. – earliest man-made glass objects were found in Egypt (glass beads) 1B.C. – glass blowing began 1300s – knowledge of glass making spread throughout Europe. Industrial Revolution brought about mass production of glass.

What is Glass? Amorphous material – does not have a fixed internal, crystal structure. Produces a variety of fracture patterns when broken Made by melting sand (silicon dioxide or silica), lime, calcium oxide, and sodium oxide at high temperatures Sodium oxide reduces the melting point of the sand Calcium oxide prevents glass from being soluble (dissolvable) in water. Called soda-lime glass

Types of Glass Soda-lime glass Many uses, most glass containers inexpensive Easy to melt & shape Leaded glass- substitutes lead oxide for calcium oxide Denser than soda-lime glass Sparkles Decorative Ovenware or Lab Glassware – called Pyrex or Kimax Contain compounds that improve its ability to withstand large temperature shifts

Gorilla Glass Sapphire Glass Scratch resistant & durable glass made by Corning Made by dipping the glass sheet in a molten salt bath where potassium ions are infused in the glass Sapphire Glass Man-made sapphire crystal, apply high heat & pressure to aluminum oxide powder & processed into thin sheets Nearly as hard as diamond, almost impossible to scratch Used in armored vehicles, bullet-proof glass, screens/visors in military body armor suits, barcode readers

Gorilla Glass vs. Sapphire Glass Cheaper to produce (10 x’s cheaper) Weighs 67% less Harder Perfectly transparent

Properties of Glass Altering compounds used t make glass changes the composition and produces different types of glass than can be identified. Density Refractive Index

Density Density (D) = mass (m) ÷ volume (V) D = m/V Each type of glass has a specific density

Calculating the Density of Glass – Water Displacement Method Find the mass of the piece of glass in grams (g) using a balance. Fill a beaker completely with water so that adding a single drop would cause spillage. Position a graduated cylinder under the spout of the beaker. Gently lower the glass into the beaker and collect the spillage in the graduated cylinder. Measure the volume of water in the graduated cylinder. It’s the volume of the glass. Divide the mass of the glass by its volume.

Refractive Index Refraction- the bending of the direction of light when it moves from one medium to another Refractive Index – tool used to study how light bends as it passes through one substance into another. Calculated by dividing the speed of light in a vacuum by the speed of light through that particular substance.

Application of Refractive Index to Forensics 1 method of determining if the evidence glass matches the crime scene glass is to compare the refractive indexes of the two pieces to each other. Another method is the submersion method. Used when piece of glass is too small to use refractive index. Placing the glass fragment into different liquids of known refractive index. When the piece of glass seems to disappear, it has the same refractive index as the liquid.

3rd method- Becke Lines Submerge the glass in liquid and then view it under a low power using a compound microscope. If the refractive index of the liquid is different from the glass, then a halo-like ring appears around the edge of the glass – called a BECKE LINE. If the Becke line is visible inside the perimeter of the glass, the refractive index of the glass is higher than the liquid. If the Becke line is located outside of the glass, the liquid has a higher refractive index than the glass

Thickness of Glass Not all glass has the same thickness Picture frame glass – 1/8 inch Window glass – 3/32 inch to 1/8 inch Door glass – 3/16 inch to ¼ inch

Fracture Patterns in Broken Glass Glass has some flexibility. When stretched too far, fracture lines appear and the glass may break. Fracture patterns can provide clues about the direction and rate of impact.

Fractures Primary Radial Fractures Secondary Concentric Fractures Start at the point of impact and radiate, move outward, from there. Form on the side opposite the point of impact Secondary Concentric Fractures Concentric circles that have the same center. Form on the same side of the glass as the point of impact.

Why Radial & Concentric Fractures Form On the side of impact, the glass surface is compressed (squeezed together) The Opposite side stretches and is under tension. Glass is weaker under tension than under compression so the opposite side fractures 1st. Then the concentric fractures occur on the side of impact.

Bullet Fractures The direction of the bullet thru the glass is easily determined. The bullet pushes some glass ahead of it as it passes thru, causing a cone shaped piece of glass to exit along with the bullet. This makes the exit hole is larger than the entrance hole. If several shots are fired thru the same pane of glass, order of the shots can be determined from the fracture lines The 1st set of fractures set the boundary for further fracturing by following shots

Bullet Fractures Single Shot Multiple Shots

Path of Bullet Passing Through Window Glass The angle at which the bullet enters the glass can help locate the location of the shooter If bullet is fired perpendicular to the window, a round entry hole is seen. Fired from an angle coming from the left? Glass pieces are forced out to the right, creating an irregular oval as it exits right. Fired from the right? Irregular oval to the left. Ammunition type can also be determined from the size and characteristics of the bullet hole. Although, high speed bullets fired from a great distance can mimic the characteristics of a slower speed bullet at close range.

Bulletproof Glass Combination of 2 or more types of glass, one hard and one soft. Softer layer makes glass more elastic, so it flexes instead of shattering. The refractive indexes of both glasses must be the same to keep the glass transparent. Varies in thickness from ¾ inch to 3 inches

Tempered Glass Point of impact is more difficult to determine on windshields made of tempered glass (made of 2 layers of glass with a layer of plastic in the middle) When hit, tempered glass is supposed to crack but not break apart. Produces a pattern of large pieces with fewer concentric circle fractures than other types of glass.

Backscatter When a piece if glass break, most of the pieces fly forward BUT Some can be projected backward, known as backscatter. Occurs as glass pieces collide and tumble in various directions Form of trace evidence

Heat Fractures During a fire, glass can break as a result of heat fracturing Produces breakage patterns on glass that are different from breakage patterns caused by impacts Wavy fracture lines develop.

Other Scratch Patterns Windshield wipers can leave marks on glass Dirt or other particles can scratch glass as a window is rolled up or down repeatedly