Glass

Common Types 1.Fused silica 2.Soda lime (soft) 3.Borosilicate Main component is silicon dioxide (SiO 2 ) which is more commonly referred to as sand Must be heated to over 3000°F

Fused Silica A.k.a. quartz Strongest and most thermally stable form Windows for the space shuttle are made of fused silica

Soda Lime A.k.a. soft glass Mixture of soda and lime lowers the melting point of the mixture to about 1300°F Relatively cheap to make Used as windows, bottles, jars, and glass items that do not have to be heated Not very stable thermally and tends to shatter when heated

Borosilicate Soda mixed with boron oxide Mixture melts at 1650°F More thermally stable than soda lime, but not as stable as fused silica Can be heated and will not crack unless plunged into cold water

Physical Properties of Glass  Hard, amorphous solid  Usually transparent  Primarily composed of silica with various amounts of elemental oxides  Brittle  Exhibits conchoidal fracture

Glass Characteristics 1.Color 2.Thickness 3.Fluorescence 4.Density 5.Refractive index

Determination of Refractive Index  Immersion method —lower fragments into liquids whose refractive index is different.  Match point —when the refractive index of the glass is equal to that of the liquid  Becke line —a halo-like shadow that appears around an object immersed in a liquid. It disappears when the refractive index of the liquid matches the refractive index of the glass fragment (the match point)

Refractive Index LiquidRIGlassRI Water1.333Vitreous silica1.458 Olive oil1.467Headlight Glycerin1.473Window Castor oil1.82Bottle Clove oil1.543Optical Bromobenzene1.560Quartz Bromoform1.597Lead Cinnamon oil1.619Diamond2.419

Density Mass of object divided by its volume Varies according to the chemical type of the glass Determined by the flotation method Glass is put in a vial filled with bromoform which has a higher density than glass, therefore the glass floats Next alcohol is added until the glass is suspended within the solution, neither sinking or floating Mass and volume of solution are the measured using a graduated cylinder and an electronic balance

Glass Processing 1.Flat glass Heated and slowly cooled by a process known as annealing, which removes any thermal stress in the glass and makes it stronger Made by rolling glass between metal drums to produce sheet or plate glass Made from soda lime glass Most common, used for windows

2.Float glass Produced by using a bath of molten tin to support glass as it is being manufactured Made mostly from soda-lime glass Produces distortion- free glass used for display purposes

3.Tempered glass Soda lime glass heated, softens, and then air is blown across the surface The surface solidifies before the inside, creating tension, and this makes the glass very strong If the surface of tempered glass is compromised, the glass will disintegrate into many of tiny particles that are not likely to cut anyone Used for side and rear windows of cars Frameless doors Cell phones, iPods

4.Safety glass Has two layers of soda lime glass with a thin film of plastic sandwiched between Thin film of plastic keeps glass intact even if the glass is broken Front windshield of all cars sold in the U.S. must be made of safety glass A.k.a. laminate glass Bulletproof

Types of Fractures High-speed projectile punctures glass rather than causing the pane to shatter Entrance side shows a smaller, more regular hole Exit side shows a larger, more irregular hole Small concentric circles form around the hole on the exit side Radial fractures begin at the hole and radiate out like the spikes on a wheel, always stopping on preexisting fracture lines (used to determine order)

Matching Glass Samples Fragments can be large enough that the randomly generate edges can be matched together like pieces of a jigsaw puzzle Window reconstruction is aided by ultraviolet light and polarized light –UV light is absorbed by the side of the glass that was in contact with the liquid tin used in manufacturing –Polarized light is then used to make the stress lines visible

Collecting the Sample  The glass sample should consist of the largest amount that can be practically collected from each broken object and packaged separately. The sample should be removed from the structure (e.g., window frame, light assembly). The inside and outside surfaces of the known sample should be labeled if a determination of direction of breakage or reconstruction of the pane is desired.  When multiple broken glass sources are identified, it is necessary to sample all sources.  A sample should be collected from various locations throughout the broken portion of the object in order to be as representative as possible.  The sample should be collected with consideration being given to the presence of other types of evidence on that sample (e.g., fibers, blood). —Forensic Science Communications