Ch 4 - Physical Properties: Glass and Soil

1. Ch 4 - Physical Properties:Glass and Soil Physical and chemical properties. Metric and British systems. Celsius (Centigrade) vs Fahrenheit. Mass vs weight. Density Refractive Index Crystalline vs amorphous solids. Double refraction and birefringes.

2. Dispersion of light through a prism. Flotation and immersion methods for comparing glass specimens. Examining glass fractures to determine the direction of impact from a projectile. Proper collection of glass evidence. Forensic properties of soil. Density-gradient tube technique. Proper collection of soil evidence. http://www.fbi.gov/hq/lab/handbook/examglas.htm http://www.fbi.gov/hq/lab/handbook/examsoil.htm

3. Physical property: describes the behavior of a substance without having to alter the substance’s composition through a chemical reaction Chemical property: describes the behavior of a substance when it reacts or combines with another substance  Fahrenheit scale: the temperature scale using the melting point of ice as 320 and the boiling point of water as 2120, with 180 equal divisions or degrees between them.

4. Celsius scale: the temperature scale using the melting point of ice as 00 and the boiling point of water as 1000, with 100 equal divisions or degrees between  Weight: a property of matter that depends on the mass of a substance and the effects of gravity on that mass  Mass: a constant property of matter that reflects the amount of material present Density: a physical property of matter that is equivalent to the mass-per-unit volume of a substance

5. Refraction: the bending of a light wave as it passes from one medium to another  Refractive index: the ratio of the speed of light in a vacuum to its speed in a given substance  Crystalline solid: a solid in which the constituent atoms have a regular arrangement  Atom: the smallest unit of an element; not divisible by ordinary chemical means. Atoms are made up of electrons, protons, and neutrons plus other subatomic particles

6. Amorphous solid: a solid in which the constituent atoms or molecules are arranged in random or disordered positions. There is no regular order in amorphous solids.  Birefringence: a difference in the two indices of refraction exhibited by most crystalline materials  Dispersion: the separation of light into its component wavelengths  Tempered glass: glass to which strength is added by introducing stress through the rapid heating and cooling of the glass surfaces

7. Becke line: a bright halo that is observed near the border of a particle immersed in a liquid of different refractive index  Radial fracture: a crack in a glass that extends outward like the spoke of a wheel from the point at which the glass was struck  Concentric fracture: a crack in a glass that forms a rough circle around the point of impact  Mineral: a naturally occurring crystalline solid

8. Density-gradient tube: a glass tube filled from bottom to top with liquids of successively lighter densities; used to determine the density destruction of soil

9. Ch. 4 – Glass and Soil Properties of Matter The distinguishing characteristics of a substance used in its identification & description characteristics by which people are recognized hair color, tone of voice, walk, shape of nose chemical substances are recognized by how they look & behave each chemical substance has a unique set of properties that distinguish it from all other substances

10. Properties of Matter Chemical Properties a characteristic of a substance that describes the way the substance undergoes or resists change to form a new substance Physical Properties a characteristic of a substance that can be observed without changing the substance into another substance

11. Physical Properties Extensive Properties depend on the amount of sample volume, mass Intensive Properties do not depend on the amount of sample melting point, density

12. Density The ratio of the mass of an object to the volume occupied by that object g/cm3 (solids); g/mL (liquids) d = m/V Densities of solids & liquids are often compared to the density of water sink or float Varies with temperature

13. Refraction The bending that occurs when a light wave passes at an angle from one medium to another (air to glass) bending occurs because the velocity of the wave decreases

14. Refractive Index (ND) The ratio of the velocity of light in a vacuum to the velocity of light in a given medium ND (water) = 1.333 light travels 1.333 time faster in vacuum than in water An intensive property Varies with temperature and the light frequency

15. Double Refraction Crystals refract a beam of light into two different light-ray components extraordinary ray refracted (bent) ordinary ray path unchanged Causes a double image to be seen No double refraction with isometric crystals

16. Birefringence The difference between the two indices of refraction for calcite: 1.486 & 1.658 birefringence for calcite is 0.172 Use in identifying crystals

17. Dispersion Occurs when an incident parallel beam of light to fans out according to the refractive index of the glass for each of the component wavelengths, or colors.

19. Glass The Basics

20. What is Glass? One of the oldest of all manufactured materials A simple fusion of sand, soda & lime (all opaque) produces a transparent “solid” when cooled

21. What is Glass? An extended, 3D network of atoms which lacks the repeated, orderly arrangement typical of crystalline materials The viscosity is such a high value that the amorphous material acts like a solid

22. What is Glass? glass is formed upon the cooling of a molten liquid in such a manner that the ordering of atoms into a crystalline formation is prevented materials which form glasses are relatively rare SiO2 (silica) is the most common example

23. Structure of Glass

24. Physical Properties At ordinary temp. internal structure resembles a fluid random molecular orientation external structure displays the hardness & rigidity of of a solid Does not show a distinct melting point on heating gradually softens on cooling gradually thickens

25. Physical Properties Common Properties hard perfectly elastic brittle non-conductors of electricity chemically stable

26. Types of Glass Oxide Glasses

27. Types of Glasses ~a thousand chemical formulations each has its own combination of properties more than 700 compositions in commercial use Most common type encountered by the forensic scientist is “flat” glass glass used in windows & doors

28. Components Formers forms the glassy, non-crystalline structure fluxes improve melting properties but impart low chemical resistance typically alkali or alkaline earth oxides modifiers (stabilizers or intermediates) a material that improves stability typically oxides of Ca, Al, or Zn

29. Common Glass Components

30. Soda-lime-glass Silica itself makes a glass (fused silica) high mp (3133 oF or 1723 oC) high viscosity in liquid state difficult to melt & work Na2O (soda) lowers melting temp (A flux) glass lacks durability (soluble in water) CaO (lime) increases stability

31. Borosilicate Glass Over 5% B2O3 added to the silica a heat resistant glass that expands only ~1/3 as much as silicate glass more resistant to breaking on rapid heating & cooling Pyrex Uses laboratory ware & thermometers household glassware sealed-beam headlights

32. Lead Glasses Incorporates up to 80% PbO Has high refractive index & high electrical resistivity Suitable for hand fabrication Uses “crystal” tableware costume jewelry fine chandeliers neon sign tubing

33. Colored Glasses Metallic oxides or sulfides added to soda-lime glass chromium oxide (green) cobalt oxide (blue) cadmium or selenium sulfide (red) Colloidal particles of iron & sulfur produces the “carbon” brown beer bottle

34. Decolorized Glass General term for the soda-lime-glass marketed as “clear” for windows Color caused by impurities present in the raw materials removed or masked destruction of carbonaceous matter oxidation of Fe(II) (blue) to Fe(III) (yellow) NaNO3, KNO3,, BaNO3 Most “clear” glass is not absolutely colorless observable by viewing on edge

35. Light Sensitive Eyeglass Lenses Contain colloidal particles of silver halide Identified by exposure to uv light

36. Glass Production Flat Glass until late 1950s produced by sheet * plate processes primarily produced by the float glass process today molten glass is “floated” over a bath of molten time produces a distortion-free sheet

37. Glass Fracture Examination Types of Fractures

38. Impact Fractures Impact causes a pane of glass to bulge Side opposite the impact will stretch more & rupture first Radial cracks are rapidly propagated in short segments from the point of impact

39. Impact Fractures Ridges will be seen as irregularities on the broken edge of a radial crack

40. Impact Fractures If the pane is held firmly on both sides a circular pattern of cracks (concentric) will form around point of impact

41. Forensic Examination Ridges on radial cracks can be used to determine on which side of pane impact occurred To perform examination identify one or more pieces which have cracks terminating at a point of impact fit these pieces onto one or more pieces marked “inside” or “outside”

42. Four R Rule Ridges on Radial cracks are at Right angles to the Rear (side opposite the impact)

43. Four R Rule Exceptions 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”

45. Heat Fractures typical heat crack is curved has a smooth edge (“mirror edge”) no indication of point of impact

46. Fractures Caused by Projectiles High-velocity projectiles crater-like hole surrounded by a nearly symmetrical pattern of radial and concentric cracks the size of the hole & diameter of crater are relatively independent of the size of the projectile

47. Fractures Caused by Projectiles

48. Glass Cutters score the surface of glass by forcing out tiny chips along a line small chips will be missing on one side of the pane along the break Cutter type can’t be accurately determined Association to a particular cutter possible only when glass chips are deposited on a cutter

49. Mechanical Fit Examiner can determine that two or more pieces of glass were broken from the same pane or object Because glass is amorphous, no two glass objects will break the same way

51. Comparing Glass Fragments Using Physical Properties

52. Glass as Forensic Evidence Films of breaking glass have shown that glass flies backward from all parts of the window where cracks appear not just from point of impact Results in a shower of minute glass particles onto the window breaker

53. Glass as Forensic Evidence Glass fragments recovered from clothing number & distribution are important a piece of glass embedded in a shoe has low probative value many small fragments from a shirt or sweater can be highly significant Glass must be classified window glass vs broken bottle glass Individualization may be possible

54. Classification Tests Physical properties can be used to place glass into a class Density Refractive Index (RI) Individualization can’t be determined from these properties alone Can be used to evaluate the significance of a glass comparison

55. Flotation Test Based on density Glass is not perfectly homogeneous test allows for small internal density variations The control glass chip (known density) immersed in mixture of bromoform & bromobenzene composition of mixture altered until the chip remains suspended

56. Flotation Test The crime object (glass of unknown density) immersed in liquid mixture remains suspended liquid, control & unknown have same density sinks unknown is more dense than control different origins Accurate density measurements can be obtained with a Density Meter

57. Refractive Index By Immersion Entails finding the temp at which a glass particle & a liquid, into which the chip has been immersed, have identical refractive indices refractive index of glass is relatively unaffected by changing temp refractive index of liquid varies with temp dN/dT=-4 x 10-4/oC

58. Refractive Index By Immersion Immerse glass in high boiling liquid Increase temp at rate of 0.2 oC/min At match point the refractive index of liquid & glass are the same Becke line disappears minimum contrast between glass & liquid If all glass fragments have similar match points, they have comparable RI

60. Refractive Index By Immersion Internal RI variations may vary by 0.0002 Differences in RI must be greater than 0.0002 between suspect & crime glass to be significant The measurement of RI alone can be of limited use for classification because RI distributions of flat glasses and container glasses overlap

62. Other Classification Methods Microscopy float glass is absolutely flat wine glasses are slightly curved bottles have microscopic defects from mould Fluorescence when excited by uv radiation, many glasses exhibit fluorescence caused by heavy metals (including tin)

63. Fluorescence Can differentiate between float and nonfloat window glass Can differentiate between different samples of float glass in some cases

64. Scanning Electron Microcopy Can detect Si Na Ca Mg K very small samples can be analyzed (50 micrograms)

65. X-ray Fluorescence Can detect major elements in glass samples sometimes detects minor & trace level components

66. Elemental Analysis Many of the trace elements enter the glass via trace impurities in the raw materials Comparison of elemental analysis of crime glass & reference glass if ranges of elements overlap for every element indistinguishable if ranges of one or more elements are different samples are distinguishable