Unit 5, Glass Evidence Clark PVMHS

By the end of this unit you should be able to: Explain how glass is formed List some of the characteristics of glass Provide examples of different types of glass Calculate the density of glass Know how the refractive index to identify different types of glass Describe how glass fractures Analyze glass fracture patterns

Vocabulary, pg 395 in text Amorphous Becke line Density Leaded glass Normal line Obsidian Refraction Refractive index Silicon dioxide

What type of evidence is glass? Class? Individual? Why?

History of glass Glass forms naturally when certain types of rock are exposed to extremely high temperatures. Pre-historic humans used obsidian as a cutting tool. Egypt circa 2500 B.C.—The earliest known human- made glass objects (beads). 1st Century B.C.—glass blowing begins. 13th Century—specialized glass production was an art, a science, and a state secret in the republic of Venice. 14th Century—glass-making spreads through Europe. Obsidian

What is glass? Crystalline solids have a regular atomic structure Glass is an amorphous solid and so has an irregular atomic structure Therefore, glass breaks in a variety of fracture patterns MIT Glass video

What is glass? A hard amorphous material made by melting silica (silicon dioxide), lime (calcium oxide), and sodium oxide. ▫Calcium oxide prevents glass from being soluble in water. ▫Sodium oxide reduces the melting point. ▫Melting point: The temperature at which a substance changes state from solid to liquid. Silica (SiO2) an amorphous solid Silica sand

Types of glass The most common type of glass, soda-lime glass, is inexpensive, easy to melt and shape, and reasonably strong. Fine glassware and decorative art glass, called crystal or leaded glass, substitutes calcium oxide with lead oxide (PbO). Ovenware and laboratory glassware contain compounds that improve the ability of the glass to withstand a wide range of temperatures needed for cooling or heating glassware in a kitchen or lab. Different colors of glass are produced by adding certain metal oxides to the glass mixture.

Properties of glass Altering the compounds used to make glass changes the composition and produces different types of glass. 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 properties, such as density, refractive index, and shatter patterns, and chemical properties.

Why is glass transparent? TED-Ed transparent-mark-miodownikhttp://ed.ted.com/lessons/why-is-glass- transparent-mark-miodownik London Tower’s glass bridge

Density Each type of glass has a density specific to that glass. Density (D) is calculated by dividing the mass (m) of a substance by its volume (V). The formula for calculating density is: D = m / V Type of GlassDensity (g/ml) Bottle glass2.50 Window glass2.53 Lead crystal Pyrex2.27 Tempered (auto)2.98 Flint3.70 Crown2.50

Calculating the Density of a piece of glass The mass of a piece of glass (in grams) can be found using a balance. In order to determine the volume (in milliliters) of water displaced by a piece of glass, fill a graduated cylinder partway with water. Record the volume. Next add your marble and record the new volume. Then find the difference between the 2 values. Divide the mass (g) by the volume displaced (mL) to find density.

Refraction The change in the direction of light as it speeds up or slows down when moving from one medium into another. The direction and angle of change varies depending on the density of the two mediums.

Refractive Index Refractive Index—a tool used to study how light bends as it passes from one substance to another When a beam of light moves from less dense medium (air) into a more dense medium (glass): ▫ Its speed slows, and ▫ Bends light toward the normal line Normal line is perpendicular to the glass surface

Refractive index When a beam of light moves from a more dense medium (water) into a less dense medium (air): ▫ Its speed increases ▫ And bends light away from the normal line

Application of refractive index to forensics Investigators can use the refractive index of glass fragments to determine if glass evidence at a crime scene matches glass evidence found elsewhere. Two common methods used: ▫Submersion method ▫Becke lines

Submersion method used when glass fragments found at the crime scene are small. Place the glass fragment into different liquids of known refractive indexes The glass fragment will seem to disappear when placed in a liquid of the same refractive index.

Becke line A halo-like effect that appears at the edges of a glass fragment when the refractive index of the glass and liquid are different. It appears because the refracted light becomes concentrated around the edges of the glass fragment. ▫ If the line is inside the glass perimeter, the glass index is higher than the index of the liquid ▫ If the line is outside the glass perimeter, the glass index is lower

Becke line 1.Place the glass on a slide surrounded by a liquid medium of known refractive index. 2.Place the slide on the microscope stage and focus the lens on the glass. If the glass seems to disappear when focused, the glass and the surrounding medium have the same refractive index. If the glass did not disappear, the surrounding medium and the glass have different refractive indexes. In the next step, you will use the position of Becke lines to estimate which medium, the glass or the surrounding liquid, has the higher refractive index. 3.Increase the distance between the stage and the lens. Look for the appearance of a Becke line. If a Becke line appears inside the perimeter of the glass, then the glass has a higher refractive index than the surrounding liquid. If a Becke line appears outside the perimeter of the glass, then the surrounding liquid has a higher refractive index than the glass.

Fracture patterns Being an amorphous solid, glass will not break into regular pieces with straight line fractures Fracture patterns provide clues about the direction, rate, and sequence of the impacts Radial fractures radiate from the center of impact. Concentric fractures form rings around the center of impact

Tension vs Compression Tension: To pull or stretch. Compression: To press together. Torsion: To twist one end of an object with respect to another.

Tension and compression in glass Glass will bend to accommodate stress to a certain level and suddenly fail once its threshold is met. Impacted glass is compressed on the side it is hit. It will stretch on the opposite side of the glass, and the tension there will radiate breaks in the glass outward from the point of impact. Then fractures form in the shape of concentric circles on the same side of the impact.

Bullet Fractures As a bullet passes through glass, it pushes a cone shaped piece of glass out of the glass ahead of it The exit side of the hole is larger than the entrance side of the hole Radiating fracture lines from a subsequent shot will stop at the edge of the fracture lines already present in the glass

Path of a Bullet Passing through Window Glass The angles at which bullets enter window glass help locate the position of the shooter Bits of the glass can fly backward (backscatter), creating trace evidence perpendicular to the glass shot from the left shot from the right

Bullet proof glass

Analysis of fracture lines Direction of force: when a projectile strikes glass, it creates ridges called conchoidal lines. ▫These lines curve out and away from the point of impact, and form a right angle on the side opposite from the impact. ▫The lines occur along the inside edge of radial fractures. Sequence of force: the radial fractures of the first impact extend completely, however, subsequent impacts will have radial fractures that stop once they come into contact with a prior fracture.

Which impact happened first A or B?

Velocity Star-like cracks can reveal the speed of a bullet by Nipuna » Mon May 06, :53 amNipuna PIECING together details of a car accident or crime scene could become a bit easier thanks to star-like cracks in glassy substances. These characteristic patterns can reveal the speed of the objects that made them. It has long been known that different materials require different amounts of energy to crack. But until now few studies have examined the patterns of cracks left behind to trace back details about the impact. Nicolas Vandenberghe and colleagues at Aix-Marseille University in France used an air gun to fire small metal cylinders at glass plates at increasingly higher speeds, reaching 432 kilometres per hour. A high-speed camera filmed each shot, and the team counted the radial cracks formed by the impact. "As surprising as it might be, that had not been done before," says team member Emmanuel Villermaux. The team found a unique relationship between the number of cracks and the projectile's speed. They were able to use this to develop a simple equation that can tell how fast an object was travelling, based on the type of material it hit (Physical Review Letters, doi.org/mdd). Robert Ritchie, a materials scientist at Lawrence Berkeley National Laboratory in California, says the work is a useful analysis. "For forensics, you could use this to find the energy of a bullet." The work could also reveal how fast a car was moving just before an accident, by looking for cracks in the windshield created by stones kicked up from the road.different materials require different amounts of energy to crack

Handling of Crime Scene Glass Samples 1. Identify and photograph any glass samples before moving them. 2. Collect the largest fragments that can be reasonably collected. 3. Identify the outside and inside surface of any glass. 4. If multiple panes are involved, make a diagram. 5. Note trace evidence such as skin, hair, blood, or fibers. 6. Package all materials collected to maintain the chain of custody.

In summary Glass is an amorphous solid. Glass can be analyzed for its density, refractive index, and fracture patterns. Density of glass = Mass (grams) divided by Volume (milliliters) Refractive index is a measurement of how light bends, or refracts, as it travels through a material. Methods for measuring the refractive index include: ▫ Snell ’ s law ▫ Submersion method ▫ Becke line method Fracture patterns provide information about such things as the direction, the rate, and the sequence of the impacts.