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 birefringence.

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. Bullet Analysis • If a window is broken by a bullet, it is possible to determine the bullet's direction by noting the side of the cone-shaped hole left by the bullet. The small opening is on the entrance side and the large opening is on the exit side. • A determination of the sequence of bullet holes can be made by noting the radial fractures. Radial fractures caused by the passage of a bullet will stop at any pre-existing fracture.

49. 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

50. 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