GLASS ANALYSIS

Glass A hard substance made when heat is

applied to sand, lime and metal oxides (usually sodium, calcium, magnesium, and aluminum)

Silicon dioxide (SiO2), also called silica, is the primary ingredient in glass

Sodium oxide (Na2O) reduces the melting point of silica

Calcium oxide (CaO) prevents the glass from being soluble in water

Types of Glass Leaded Glass: also called crystal,

substitutes lead oxide (PbO) for calcium oxide and is used to make fine glassware and decorative art glass

Colored Glass: created by adding certain metal oxides• NiO: produces yellow and purple glass• CoO: produces purple-blue glass• SeO2: produces red glass

Types of Glass (cont.) Tempered Glass: made by rapid

heating and cooling of the glass; designed to break into small pieces that do not have sharp edges; it is commonly used in side and rear windows in cars

Safety Glass: made by sandwiching a layer of plastic between two pieces of window glass; this type of glass is break resistant and is used in car windshields

Properties of Glass Density: each type of glass has a

density that is specific to that glass Color Refractive Index

Refractive Index Refraction is the change in the

direction of light as it speeds up or slows down when moving from one medium into another; the direction and amount the light bends varies with the densities of the two mediums

Refractive index: calculated by dividing the speed of light in a vacuum (300,000 km/s) by the speed of light through that particular substance

Refractive Index (cont.) When light travels through a vacuum,

it does not slow When light travels through any other

medium, the particles in that medium slow the light down

As the density of the medium increases, the speed of light passing through that material decreases

Refractive Index (cont.) Normal: a line

perpendicular to the surface where the two different mediums meet

Incident Ray: the incoming beam of light passing through the first medium

Refracted Ray: the beam of light passing through the second medium

Refractive Index (cont.) Angle of Incidence: the

angle the incident ray in medium 1 forms with the normal

Angle of Refraction: the angle the refracted ray in medium 2 forms with the normal

Snell’s Law Describes the behavior of light as it

travels from one medium into a different medium

Written as:n1 (sine angle 1) = n2 (sine angle 2)

Where n1 is the refractive index of medium 1

n2 is the refractive index of medium 2

angle 1 is the angle of incidence angle 2 is the angle of refraction

Application of Refractive Index to Forensics

Submersion method: involves placing the glass fragment into different liquids of known refractive indexes; the glass fragment will seem to disappear when placed in the liquid with the same refractive index

Blowback Caused when a bullet or other object

goes through glass As the bullet passes through the

glass, the glass bends and then snaps back

The glass can blow back up to 18 feet

Bullet Holes Through Glass Round Hole: bullet entered the

glass at a right angle Elongated Hole: as the angle at

which the bullet enters the glass increases, the hole becomes more elongated

The exit hole will be larger than the entrance hole; this helps determine from which direction the bullet entered the glass

Glass Cracking By Bullet When a bullet goes

through glass, it usually will not break, but it will crack

The first cracks are in a straight line that extend from the point of impact and form on the opposite side of the point of impact

Glass Cracking By Bullet (cont.)

Concentric or circular cracks form next around the point of impact and form on the same side of impact

When more than one bullet goes through the glass, the first bullet causes long, thin, uninterrupted cracks

Glass Cracking By Bullet (cont.)

Subsequent bullets will cause cracks, but these cracks will end when they meet the cracks caused by the previous bullets

By looking at the pattern of the cracks, it is possible to determine the order in which the bullets entered the glass

Glass Broken By Heat High heat produces wavy fracture

lines The glass breaks toward the region of

higher temperature There should be no radial or

concentric circle fracture patterns