Glass

Common Types

1. Fused silica2. Soda lime (soft)3. Borosilicate

• Main component is silicon dioxide (SiO2) 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 ofRefractive 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

Liquid RI Glass RIWater 1.333 Vitreous silica 1.458

Olive oil 1.467 Headlight 1.47-1.49

Glycerin 1.473 Window 1.51-1.52

Castor oil 1.82 Bottle 1.51-1.52

Clove oil 1.543 Optical 1.52-1.53

Bromobenzene 1.560 Quartz 1.544-1.553

Bromoform 1.597 Lead 1.56-1.61

Cinnamon oil 1.619 Diamond 2.419

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