Chapter 3 & 4: Glass Evidence

1. Chapter 3 & 4: Glass Evidence

2. What Is Glass? • Glass is considered a very slow moving liquid • Glass is a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides. • Amorphous solids have their atoms arranged randomly, unlike crystals.

3. Amorphous vs. Crystalline Structure Amorphous Structure: Glass Crystalline Structure: Salt (NaCl)

4. Glass Manufacturing • It is an automated process • Led to mass production •  Mass production means greater uniformity and less ability to discriminate • Most glass today is called float glass • Made by pouring the molten material onto a bath of melted tin, resulting in a very smooth surface

5. How Float Glass is Made

6. Glass Manufacturing • Tempered glass • Created by subjecting glass to extreme heat then cold, repeated over and over, during the manufacturing process •  Glass breaks into smaller squares rather than into sharp shards • Laminated Glass • Windshields are specialty glasses made by a having a plastic middle layer that is sandwiched between two layers of glass. • The plastic layer helps hold the glass layers together.

7. Tempered Glass: Shattering Examples A telephone booth with smashed tempered glass in Holloway, London.

8. Laminated Glass: Car Front Windshields

9. Glass In Forensics • Used in solving automobile accidents, hit-and-runs, burglaries, and assaults • Glass is a type of transfer evidence • Can be individualized and linked to a common source •  Only can individualize a glass fragment, if can fit it like puzzle piece to its source

10. Glass In Forensics • For the forensic scientist, the problem of glass comparison is one that depends on the need to find and measure those properties that will associate one glass fragment with another. • To compare glass fragments, a forensic scientist evaluates these important physical properties: • Appearance – shape, color, etc • Fracture patterns • Density • Refractive Index (and Becke lines) • Chemical analysis

11. GlassFractures

12. Glass Fractures • Because of a lack of order and pattern, glass breaks in random patterns • An impact in glass produces two types of fractures • Radial (radiating out from the point of impact) • Concentric (forming circles around the impact)

13. Radial and Concentric Glass Fractures

14. concentric radial

15. Direction of Force • Small projectiles passing through glass at a high velocity will produce characteristic patterns • Exit hole is larger than entry hole • Lower velocity impacts may not penetrate the glass but leave only a pit or crater on one side of the glass

16. Direction of Force • Direction of force can be determined by looking at the cross section of the fractures • The lines created in the glass, calledconchoidal fractures, show distinctive patterns depending on whether the fractures from which they are collected are concentric or radial

17. Direction of Force • Most reliable determinations are from results closest to the original impact • Perpendicular marks are found on the edge opposite way force was pushing • Remember: •  Radial cracks form Right angles on the Reverse side of the force

18. Direction of Force Perpendicular marks (~90 degree angles) are found on the edge opposite the way that the force was pushing

20. Shattering Due to Heat • The shattering of glass by heat creates a distinctive and different fracture pattern characterized by wavy smooth cracks

21. Order of Impact • If there are many impacts on the same pane of glass, the sequence of events can be deduced from the fracture patterns •  Existing fractures from the 1st impact act as barriers to fractures created by the second impact • Look for abrupt terminations

22. Order of Impact Example This photo depicts two bullet holes in safety glass. Which hole was created first? How can you tell? 1st 2nd The hole on the right was created first. Cracks radiating out from the hole will stop when they encounter another crack. Stress placed on the glass (causing it to crack) will be transferred along the existing crack rather than across it.

23. Interruption of radiating fractures

24. Determining Common Source • Only way to link two fragments of glass to a common source is by the process of physical matching • Each breakage is unique • Creates pieces of a puzzle that the criminalist must place the pieces together

25. Determining Common Source • The criminalist can obtain other information on the glass characteristics • Color • Thickness • Shape • Texture

26. Individual Glass Characteristics •  The two most common physical parameters used to characterize glass samples are the measurements of density and the refractive index

27. Density • Mass refers to the amount of matter an object contains independent of gravity. • Volume is the amount of space an object takes up. • Densityis mass / volume • Units for density are usually in g/mL, g/cm3

28. Density • Densityis an intensive property of matter, meaning it remains the same regardless of sample size. • It is considered a characteristic property of a substance and can be used as an aid in identification.

29. Density • The density of glass is determined by using a floatation technique in which the composition of liquid is varied until the glass sample remains suspended in it

30. Density Values

31. Light • Light waves travel in air at a constant speed until they penetrate another medium, such as glass or water, at which point they are suddenly slowed, causing the rays to bend.

32. Refraction • The bending of light waves because of a change in speed is called refraction.

33. Refractive Index (n) •  A quantity that measures the bending of light as it travels from one medium into another • n= speed of light in a vacuum speed of light in medium • Always will be greater than 1.00 • Water has a refractive index of 1.33 (light travels 1.33 times faster in a vacuum than in water)

34. Refractive Index •  The RI depends on the wavelength of light being used and the temperature of the material

35. Refractive Index • Measured using a microscope equipped with a hot stage • Glass is immersed in an oily material with a known refractive index • Oil is slowly heated • This changes the refractive index • When glass is not seen in the oil, it has the same refractive index

36. The Becke Line •  The Becke Line is a line that appears as a halo if the refractive indexes of the glass and the material are different •  The Becke Line will disappear when the refractive indexes are the same

37. Becke Lines from Glass Becke line on outside Becke line on inside RI of glass (1.525 > RI of medium (1.34) RI of glass (1.525) < RI of medium (1.6)

38. GRIM II System -automated system for matching glass RI -uses a step-heating stage and a RI liquid that varies with temperature

39. GRIM II identifies the RI match by monitoring a video image of the glass fragment in the liquid -as it is heated/cooled the contrast is measured until a minimum is reached-the match point

40. Snell’s Law (n1)(sin first angle) = (n2)(sin second angle) • (n1) = refractive index of first medium • (n2) = refractive index of second medium • Angle 1 & 2 measured to normal

41. Snell’s Law Normal line Lower RI to Higher RI = Angle bends toward normal line Higher to Lower RI = Angle bends away from normal line In air n= 1.00 Laser In glass n= 1.50

42. Which medium is denser? Medium 2

43. Refractive Index

44. Glass analysis Pick a liquid with a refractive index that matches the crime scene glass, and the glass will disappear when submerged!

45. Collection of Glass • If even the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all the glass found. • When an individual fit is thought improbable, the evidence collector must submit all glass evidence found in the possession of the suspect along with a representative sample of broken glass remaining at the crime scene.

46. Glass Videos • 1. Glass Shattering Montage by Jaime Vendura (4:12) • 2. Bullet Proof Glass Shooting (0:45) • 3. Large Fracture Breaking (3:22) • 4. Glass and Its Importance in Early Science (1:35) • 5. Refraction of Light Causes Rod to Appear Bent (0:09)