Glass as physical evidence

1. D R . S H A Y A N I G H O S H
A S S I S T A N T P R O F E S S O R
K R I S T U J A Y A N T I C O L L E G E
Glass as Physical evidence

2. Contents
 Introduction
 Physical Properties
 Measurement of Glass density
 Measurement of Refractive Index
 Glass as Evidence
 Glass Fractures
 Types of Glass Fractures
 Collection and Preservation of Glass
 References
 Acknowledgements

3. Glass
 Glass is a hard, brittle, amorphous transparent solid,
an inorganic substance made from silicon oxides,
mixed with various metal oxides.
 Glass is made by mixing sand(silica) with other
metal oxides like aluminum, sodium and calcium,
melting the mixture and cooling it without
crystallization.

4.  Special glasses are made by changing their chemical
composition by adding or removing a particular
metal oxide. Eg: Pyrex
 Tempered glass, used in automobile windshields, is
made by rapid heating and cooling of ordinary glass,
to prevent shattering.

5. Physical Properties
A. Density: The density of a substance is defined as its
mass per unit volume. It is an intensive property, i.e.
it does not change with a change in the size of the
object. The density of glass at 20 degrees Celsius is
said to be approximately 2.5 g/cm3.

6. Measurement of glass density in Forensic
Laboratories
A characteristic feature of glass is its density.
Different types of glass have different densities,
hence, a comparison of the densities of the glass
found at the crime scene, on the victim and on the
suspect may provide an important link,
establishing the presence of the suspect, at the
crime scene. The density of glass, in the laboratory,
is measured by the floatation method.

7. Physical Properties
B. Refractive Index: Light travels
in air (or vacuum) at the speed
of 3x108 metres per second.
However, when light enters
another medium, its velocity
changes. This change in velocity
results in the bending of the
light wave, this phenomenon is
known as refraction. The ratio
of the velocity of light in air and
in the medium is known as the
refractive index of the
substance and is denoted by µ.

8. Measurement of Refractive Index in Forensic
Laboratories
Refractive index, along with glass
density, is used for the
identification and comparison of
glass. The refractive index of
normal glass is 1.52, but the
presence or absence of various
metal oxides can change the value
of the refractive indices. In the
Laboratory, the refractive index of
glass is measured using the
immersion method. It is mainly
used to compare the refractive
indices of two samples of glass
rather than to measure the value.

9. Glass As Evidence
 Glass may be present at the crime
scene in a variety of cases, such
as burglary, murder, hit-and-run
etc. In order to establish the
presence of a suspect at a crime
scene, glass from his person must
be successfully compared and
matched to the glass present at
the crime scene. For this to be
accomplished it is necessary to
collect all pieces of glass found at
or away from the crime scene.

10. Glass Fractures
In case a projectile passes through
glass, it leaves a particular pattern
on the glass, known as fractures.
Glass fractures when any force
applied to it crosses its limit of
elasticity.
Fractured window glass often
yields important information such
as direction of impact, force, etc,
which are useful in criminal
investigations.

11. When a glass fracture is caused due to a projectile,
two types of patterns are seen, radial fractures and
concentric fractures.
Radial fractures are formed on the reverse side
of the penetrating force while concentric
fractures are formed on the same side.

12. Types of Glass Fractures
 Pressure fracture
 Compression fracture
 Impact fracture
 Electrostatic discharge
fracture
 Thermal fracture

13. Collection and Preservation of Glass
 For a proper analysis of glass evidence, it is essential to
collect and preserve the evidence according to protocol.
 A representative sample of glass is also to be collected as
a control.
 The glass fragments must be in solid containers to
prevent further breakage.
 If any glass fragments are present on the clothing or
shoes of the suspect, the entire article must be wrapped
in paper.

14.  Glass evidence must not be removed from the
article of clothing unless absolutely necessary.
 Wherever possible, the exterior and interior
surfaces of the glass must be indicated.
 In case of broken windows, the entire window must
be transported to the laboratory.
 Broken glass must be transported with adequate
cushioning to prevent breakage.

15. Thank You