Chapter 4
Glass
Properties of Matter
Review
 Forensic scientists determine the
properties that impart distinguishing
characteristics:
 Physical Properties – weight, volume, color,
boiling/melting point
 Chemical Properties – how a substance
reacts/combines with other substances
The Nature of Matter
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Elements & Compounds
States of Matter
Theory of Light
Temperature
Weight & Mass
Density
Refractive Index
Glass Fragments
 Glass that is broken and shattered into
fragments and minute particles during a crime
can be used to place a suspect at the scene
 For example,
 Chips of broken glass from a window may be lodged
in a suspect’s shoes or garments
 Particles of headlight glass found at the scene of a
hit-and-run accident may offer clues that can
confirm the identity of a suspect vehicle
Collection & Preservation
of Glass Fragments
 Forensic scientists need to find and measure those
properties that will associate one glass fragment
with another while minimizing or eliminating the
possible existence of other sources
 Glass will possess the greatest evidential value
when it can be individualized to one source. Such
a determination can only be made when the
suspect and crime scene fragments are assembled
and physically fitted together. Comparisons of this
type will require piecing together irregular edges of
broken glass as well as matching all irregularities
and striations of the broken surfaces
 The majority of glass evidence presented to the
forensic scientist is either too fragmentary or too
minute to permit a comparison of this type
Glass Composition
 Glass is a hard, brittle, amorphous substance that is
composed of silicon oxides (sand) mixed with various
metal oxides
 Amorphous – a solid in which the molecules are arranged in
random or disordered position; there is no order
 When sand is mixed with metal oxides, melted at high
temperatures, and then cooled to a rigid condition
without crystallization, the product is glass
 By adding soda (Na2CO3) to the sand, its melting point
and viscosity are both lowered, making it much easier to
work with
 Lime (CaO) is added to the sand and soda mixture so
that the “soda-lime” glass will not dissolve in water
 The soda-lime glass is used for manufacturing most window and
bottle glass
 The common metal oxides found in soda-lime glass are
sodium, calcium, magnesium, and aluminum
Glass Composition
 Glass is considered trace evidence
 Because breaking of glass can occur at a
variety of crimes, forensic scientists should be
able to distinguish between various types of
glass such as automobile, window, and
household
 When collecting glass samples from a scene, it
is important to take samples from the intact
source for comparison
 Investigators label the outside surface and
inside
 When looking at glass fragments, investigators
will try to determine the orientation of the glass
before it was broken; dirt, paint, or putty can
indicate orientation
Glass Properties
 Tempered glass –
 Glass that is made stronger than ordinary
window glass by introducing stress through
rapid heating and cooling of the glass
surfaces.
 When tempered glass breaks, it does not
shatter but rather fragments or “dices” into
small squares with little splintering. Used in
side and rear windows of automobiles
 Laminated glass –
 Used in windshields of all cars in the US.
 Derives its strength by sandwiching one later
of plastic between two pieces of ordinary
window glass.
Glass Properties
 Automobile headlights /
Heat-resistant glass /
Pyrex
 Types of specialized glass made by
substituting in whole or in part other
metal oxides for the silica, sodium, and
calcium oxides.
 These glass types are manufactured
by adding boron oxides to the oxide
mix.
 Therefore they are called borosilicates.
Glass Properties
Tempered glass
Glass Properties
Laminated Glass
Comparing
Glass Fragments
 Physical properties of density and refractive
index and used to characterize glass particles
 Refractive index – the ratio of the speed of light in a
vacuum to its speed in a given substance
 These are class characteristics and cannot be
used to individualize glass to a common source
Comparing Glass Fragments
 Once you know the refractive index of liquids, you
can immerse glass fragments in the liquids
 If you can distinctly see the glass in the liquid, then it has
a different refractive index and bends the light making it
visible
 If the glass disappears in the liquid, then it has a
refractive index close to that of the liquid and it not
reflected by the glass
 “Becke” lines are the visible outlines of the glass
sample in the liquid due to difference in refractive
values
Becke Lines
 http://www.youtube.com/watch?v=O-KALJ3p26Q
Density
 A solid particle with either float, sink or
remain suspended in a liquid, depending
on its density relative to the liquid
medium.
 This allows the criminalist a rather
precise and rapid method for comparing
densities of glass!
Density Flotation
 This method uses a standard/reference
glass particle in an immersed liquid (often
a mixture of bromoform and
bromobenzene).
 The composition of the liquid is adjusted
by adding some amount of the liquid until
the glass chip remains suspended in the
liquid.
Density Flotation
 When the chip is suspended then
standard reference glass and the liquid
have the same density. Glass chips of
the same size and shape as the standard
or reference are then added to the liquid
for additional comparison.
Glass Fractures
Glass Fragments