FORENSIC CHEMISTRY 3
Name: Date:
Section:
Activity 4.1
Directions: Enumerate the steps of the testing procedure for gunshot based on gunshot residue.
Activity 4.2 What I Know Chart
What I know Questions What I learned ( Activity 4)
1. What is a Glass
Fracture?
2. How can glass fracture
aid in a criminal
investigation?
Directions. In this activity, you need to read and underline or highlight keywords for you to keep in mind
some essential terms/words used in the content notes; this will help you remember facts or theories and
make the learning session more meaningful. You may also do note-taking for your easy access to the
information.
GLASS
Glass is technically defined as “the inorganic product of fusion which has cooled to a rigid condition
without crystallizing. Glass is reasonably hard, transparent, or translucent material composed of
fused inorganic materials, primarily of silicon dioxide (sand). The wide variety of chemical
compositions can alter its characteristics. Glass is very useful evidence because it is often encountered in
criminal investigations. For example, when a burglar breaks a windowpane, small fragments of glass are
often showered onto their hair, clothing, or shoes, and these fragments can later be found on the suspect
as transfer evidence.
GLASS FRACTURE
The glass bends in response to any force that is exerted on anyone of its surfaces; when the limit of its
elasticity is reached, the glass fractures. Frequently, fractured window glass will reveal information that
can be related to the force and direction of an impact; such knowledge may be useful for reconstructing
events at a crime-scene investigation. The penetration of ordinary window glass by a projectile,
whether it is a bullet or a stone, produces a familiar fracture pattern in which cracks both radiate outward
and encircle the hole. The radiating lines are appropriately known as radial fractures, and the circular
lines are termed concentric fracture.
Radial fracture – a crack in the glass that extends outwardlike the spoke of a wheel from the point at
which the glass wasstruck.
Concentric fracture – a crack in the glass from a rough circle around the point of impact.
Conchoidal fracture - is asmoothly curving fracturesurface of fine-grainedmaterials with
no planarsurfaces of internal weaknessor planes of separation (no
THEORIES RELATED TO GLASS FRACTURE
The direction of force determination
This method determines which direction a projectile went through the glass by evaluating radial fractures
in the glass fracture’s first concentric ring. The determination of force direction is a process quickly
done by a crime scene technician. The purpose of this determination is to establish which
direction the projectile went through the glass. The method used to determine this isthe 4R Rule:
Ridgelines on Radial fractures are at Right angles to the Rear. o The first step in this method is to find
radial fractures that are within the first concentric fracture. Radial fractures are similar to the spokes of a
wheel. Concentric fractures connect the radial fractures in a pattern the same to a spider web.o The next
step is to figure out which side of the fragment was facing in and which side was facing out.
Contaminants or residues from the inside surface will feel different than the outside surface and help
determine the sides. Once the technician finds a radial fracture and determines which side of the glass-
faced where they must look at the broken edge of the glass. When a projectile strikes glass, it creates
ridges called conchoidal fractures along the edge that are visible in profile. These conchoidal fractures
are nearly parallel to the side in which force was applied (the direction which the projectile came from).
The side of the glass opposite of the force is the rear of the glass; this is the side of the glass in which
theconchoidal fractures lie at right angles. The sequence of force determination An examiner can
establish the sequence of the shots by considering the radial fracture’s termination points. The first shot’s
radial fractures will extend completely while the subsequent shots’ radial fractures will be stopped or cut
off as they come into contact with prior fractures.
FACTORS THAT CONTRIBUTE TO THE RESULTING CHARACTERISTICS OF THE FRACTURE
The strength of a glass object has a profound effect on the resulting fracture features. The level of
thefracture stress directly affects the crack patterns which develop when propagating cracks
intersect thesurface (traces) of a glass object. This is because it is the strength, the
fracture stress that determines the level of stored elastic strain energy in the glass.
The trajectory of the Projectile or path of the object thrown.
Force exerted during the collision.
COLLECTING AND HANDLING GLASS AND GLASS FRAGMENTS
The gathering of glass evidence at the crime scene and from thesuspect must be thorough if the examiner
is to have any chance toindividualize, the fragments to a common source. If even the remotestpossibility
exists that fragments may be pieced together, every effortmust be made to collect all the glass found.
For example, the collection of evidence at hit-and-run scenes must include all the broken parts of the
headlight and reflector lenses. This evidence may ultimately prove to be an invaluable means of placing a
suspect vehicle at the accident scene by actually matching the fragments with glass remaining in the
headlight or reflector shell of the suspect vehicle. Also, an examination of the headlight's filaments
may resolve any questions regarding whether or not an automobile's head lights were on or off
before the impact. When an individual fit is thought improbable, the evidence collector must submit all-
glass evidence found in possession of the suspect along with a representative sample of broken glass
remaining at the crime scene. This standard/reference glass should always be taken from any remaining
glass in the window or door as close as possible to the point of breakage. About one square inch of
sample is usually adequate for this purpose The glass fragments should be packaged in solid containers
to avoid further breakage. If the suspect's shoes or clothing are to be examined for the presence of glass
fragments, they should be individually wrapped in paper and transmitted to the laboratory. The field
investigator should avoid removing such evidence from garments unless it is thought necessary for its
preservation. When a determination of the direction of impact is desired, all broken glass must be
recovered and submitted for analysis. Wherever possible, the exterior and interior surfaces of the glass
must be indicated. In cases in which this is not immediately apparent, the presence of dirt, paint, grease,
or putty may indicate as to the exterior surface of the glass.
GLASS ANALYSIS
Forensic analysis of glass consists of the examination of two ormore fragments to determine
whether they have a common origin.Identifying broken glass fragments having a common source
to theexclusion of all other sources may only be done if they can bephysically matched
together.
Samples from a crime scene must first be confirmed as glass.Differentiating glass from
plastic is done by examining relative hardness, solubility, and observation with apolarized light
microscope. Once confirmed as glass, the examiner will perform an analysis of
physicalproperties (colour, fluorescence, thickness, curvature, and density), optical properties
(Refractive Index)and chemical composition. If a difference between questioned and known
samples is found at any pointin this process, no further comparison is needed.
Directions: Read each question carefully. Write your answer on the space provided before the
number.
________________1. Is a smoothly curving fracture surface of fine-grained materials which
have no planar surfaces of internal weakness or planes of separation?
________________2. In the non-tempered glass, a typical heat crack is curved, has a smooth edge,
and has no indication of the point of origin of the crack.
_______________ 3. A crack in the glass that extends outward like the spoke of a wheel from the
point at which the glass was struck.
________________ 4. Is technically defined as “the inorganic product of fusion which has cooled to
a rigid conditionwithout crystallizing?
________________ 5. A crack in the glass from a rough circle around the point of impact.
