Chapter 4Chapter 4GlassGlass

Glass FragmentsGlass Fragments

Glass that is broken and shattered into Glass that is broken and shattered into fragments and minute particles during a crime fragments and minute particles during a crime can be used to place a suspect at the scenecan be used to place a suspect at the scene

For example, For example, Chips of broken glass from a window may be lodged Chips of broken glass from a window may be lodged

in a suspect’s shoes or garmentsin a suspect’s shoes or garments Particles of headlight glass found at the scene of a Particles of headlight glass found at the scene of a

hit-and-run accident may offer clues that can hit-and-run accident may offer clues that can confirm the identity of a suspect vehicleconfirm the identity of a suspect vehicle

Glass FragmentsGlass Fragments

Glass is considered trace evidenceGlass is considered trace evidence Because breaking of glass can occur at a variety of Because breaking of glass can occur at a variety of

crimes, forensic scientists should be able to distinguish crimes, forensic scientists should be able to distinguish between various types of glass such as automobile, between various types of glass such as automobile, window, and householdwindow, and household

When collecting glass samples from a scene, it is When collecting glass samples from a scene, it is important to take samples from the intact source for important to take samples from the intact source for comparisoncomparison

Investigators label the outside surface and insideInvestigators label the outside surface and inside When looking at glass fragments, investigators will try When looking at glass fragments, investigators will try

to determine the orientation of the glass before it was to determine the orientation of the glass before it was broken; dirt, paint, or putty can indicate orientationbroken; dirt, paint, or putty can indicate orientation

Glass PropertiesGlass Properties

Glass is a hard, brittle, amorphous substance that is composed of Glass is a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxidessilicon oxides mixed with various metal oxides Amorphous – a solid in which the molecules are arranged in random or Amorphous – a solid in which the molecules are arranged in random or

disordered position; there is no orderdisordered position; there is no order When sand is mixed with metal oxides, melted at high temperatures, When sand is mixed with metal oxides, melted at high temperatures,

and then cooled to a rigid condition without crystallization, the and then cooled to a rigid condition without crystallization, the product is glassproduct is glass

The main ingredient in ordinary glass is sandThe main ingredient in ordinary glass is sand By adding soda (NaBy adding soda (Na22COCO33) to the sand, its melting point and viscosity ) to the sand, its melting point and viscosity

are both lowered, making it much easier to work with are both lowered, making it much easier to work with Lime (CaO) is added to the sand and soda mixture so that the Lime (CaO) is added to the sand and soda mixture so that the

“soda-lime” glass will not dissolve in water“soda-lime” glass will not dissolve in water The soda-lime glass is used for manufacturing most window and The soda-lime glass is used for manufacturing most window and

bottle glassbottle glass The common metal oxides found in this type of glass are sodium, The common metal oxides found in this type of glass are sodium,

calcium, magnesium, and aluminumcalcium, magnesium, and aluminum

Glass PropertiesGlass Properties

Tempered glass – Tempered glass – Glass that is made stronger than ordinary Glass that is made stronger than ordinary

window glass by introducing stress through window glass by introducing stress through rapid heating and cooling of the glass surfaces. rapid heating and cooling of the glass surfaces.

When tempered glass breaks, it does not When tempered glass breaks, it does not shatter but rather fragments or “dices” into shatter but rather fragments or “dices” into small squares with little splintering. Used in small squares with little splintering. Used in side and rear windows of automobilesside and rear windows of automobiles

Laminated glass – Laminated glass – Used in windshields of all cars in the US.Used in windshields of all cars in the US. Derives its strength by sandwiching one later Derives its strength by sandwiching one later

of plastic between two pieces of ordinary of plastic between two pieces of ordinary window glass.window glass.

Glass FragmentsGlass Fragments

Forensic scientists need to find and measure those Forensic scientists need to find and measure those properties that will associate one glass fragment properties that will associate one glass fragment with another while minimizing or eliminating the with another while minimizing or eliminating the possible existence of other sourcespossible existence of other sources

Glass will possess the greatest evidential value Glass will possess the greatest evidential value when it can be individualized to one source. Such when it can be individualized to one source. Such a determination can only be made when the a determination can only be made when the suspect and crime scene fragments are assembled suspect and crime scene fragments are assembled and physically fitted together. Comparisons of this and physically fitted together. Comparisons of this type will require piecing together irregular edges of type will require piecing together irregular edges of broken glass as well as matching all irregularities broken glass as well as matching all irregularities and striations of the broken surfacesand striations of the broken surfaces

The majority of glass evidence presented to the The majority of glass evidence presented to the forensic scientist is either too fragmentary or too forensic scientist is either too fragmentary or too minute to permit a comparison of this typeminute to permit a comparison of this type

Glass FragmentsGlass Fragments

Physical properties of density and refractive Physical properties of density and refractive index and used to characterize glass particlesindex and used to characterize glass particles Refractive index – the ratio of the speed of light in a Refractive index – the ratio of the speed of light in a

vacuum to its speed in a given substancevacuum to its speed in a given substance

These are class characteristics and cannot be These are class characteristics and cannot be used to individualize glass to a common sourceused to individualize glass to a common source

Glass FracturesGlass Fractures

Glass FragmentsGlass Fragments