Unit 5, Glass Evidence
Clark
PVMHS
By the end of this unit you should be
able to:
• Explain how glass is formed
• List some of the characteristics of glass
• Provide examples of different types of glass
• Calculate the density of glass
• Know how the refractive index to identify
different types of glass
• Describe how glass fractures
• Analyze glass fracture patterns
Vocabulary, pg 395 in text
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Amorphous
Becke line
Density
Leaded glass
Normal line
Obsidian
Refraction
Refractive index
Silicon dioxide
What type of evidence is glass?
• Class?
• Individual?
• Why?
Obsidian
History of glass
• Glass forms naturally when certain types of rock are
exposed to extremely high temperatures.
• Pre-historic humans used obsidian as a cutting tool.
• Egypt circa 2500 B.C.—The earliest known humanmade glass objects (beads).
• 1st Century B.C.—glass blowing begins.
• 13th Century—specialized glass production was an
art, a science, and a state secret in the republic of
Venice.
• 14th Century—glass-making spreads through
Europe.
What is glass?
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MIT Glass video
Crystalline solids have a regular atomic
structure
Glass is an amorphous solid and so has an
irregular atomic structure
Therefore, glass breaks in a variety of fracture
patterns
Silica sand
What is glass?
Silica (SiO2) an
amorphous solid
• A hard amorphous material made by melting
silica (silicon dioxide), lime (calcium oxide), and
sodium oxide.
▫ Calcium oxide prevents glass from being soluble in
water.
▫ Sodium oxide reduces the melting point.
▫ Melting point: The temperature at which a
substance changes state from solid to liquid.
Types of glass
• The most common type of glass, soda-lime glass, is
inexpensive, easy to melt and shape, and reasonably
strong.
• Fine glassware and decorative art glass, called crystal or
leaded glass, substitutes calcium oxide with lead oxide
(PbO).
• Ovenware and laboratory glassware contain compounds
that improve the ability of the glass to withstand a wide
range of temperatures needed for cooling or heating
glassware in a kitchen or lab.
• Different colors of glass are produced by adding certain
metal oxides to the glass mixture.
Properties of glass
• Altering the compounds used to make glass
changes the composition and produces different
types of glass.
• Because glass is made of a variety of compounds,
it is possible to distinguish one type of glass
from another by examining the different
physical properties, such as density, refractive
index, and shatter patterns, and chemical
properties.
Why is glass transparent? TED-Ed
• http://ed.ted.com/lessons/why-is-glasstransparent-mark-miodownik
London Tower’s
glass bridge
Density
• Each type of glass has
a density specific to
that glass.
• Density (D) is
calculated by dividing
the mass (m) of a
substance by its
volume (V).
• The formula for
calculating density is:
D=m/V
Type of Glass
Density (g/ml)
Bottle glass
2.50
Window glass
2.53
Lead crystal
2.98-3.01
Pyrex
2.27
Tempered (auto)
2.98
Flint
3.70
Crown
2.50
Calculating the Density of a piece of
glass
• The mass of a piece of glass (in grams) can be
found using a balance.
• In order to determine the volume (in milliliters)
of water displaced by a piece of glass, fill a
graduated cylinder partway with water. Record
the volume. Next add your marble and record
the new volume. Then find the difference
between the 2 values.
• Divide the mass (g) by the volume displaced
(mL) to find density.
Refraction
• The change in the
direction of light as it
speeds up or slows down
when moving from one
medium into another.
• The direction and angle
of change varies
depending on the
density of the two
mediums.
Refractive Index
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Refractive Index—a tool
used to study how light bends
as it passes from one substance
to another
When a beam of light moves
from less dense medium (air)
into a more dense medium
(glass):
▫ Its speed slows, and
▫ Bends light toward the
normal line
Normal line is perpendicular
to the glass surface
Refractive index
•
When a beam of light moves
from a more dense medium
(water) into a less dense
medium (air):
▫ Its speed increases
▫ And bends light away
from the normal line
Application of refractive index to
forensics
• Investigators can use the refractive index of glass
fragments to determine if glass evidence at a
crime scene matches glass evidence found
elsewhere.
• Two common methods used:
▫ Submersion method
▫ Becke lines
Submersion method
• used when glass fragments
found at the crime scene are
small.
• Place the glass fragment
into different liquids of
known refractive indexes
• The glass fragment will
seem to disappear when
placed in a liquid of the
same refractive index.
Becke line
• A halo-like effect that appears at the edges of a
glass fragment when the refractive index of the
glass and liquid are different.
• It appears because the refracted light becomes
concentrated around the edges of the glass
fragment.
▫ If the line is inside the glass perimeter, the glass
index is higher than the index of the liquid
▫ If the line is outside the glass perimeter, the glass
index is lower
Becke line
1.
2.
Place the glass on a slide surrounded by a liquid medium of
known refractive index.
Place the slide on the microscope stage and focus the lens on the
glass.
• If the glass seems to disappear when focused, the glass and the surrounding
medium have the same refractive index.
• If the glass did not disappear, the surrounding medium and the glass have
different refractive indexes. In the next step, you will use the position of Becke
lines to estimate which medium, the glass or the surrounding liquid, has the
higher refractive index.
3.
Increase the distance between the stage and the lens. Look for
the appearance of a Becke line.
• If a Becke line appears inside the perimeter of the glass, then the glass has a
higher refractive index than the surrounding liquid.
• If a Becke line appears outside the perimeter of the glass, then the surrounding
liquid has a higher refractive index than the glass.
Fracture patterns
• Being an amorphous solid, glass will not break
into regular pieces with straight line fractures
• Fracture patterns provide clues about the
direction, rate, and sequence of the impacts
• Radial fractures radiate from the center of
impact.
• Concentric fractures form rings around the
center of impact
Tension vs Compression
• Tension: To pull or stretch.
• Compression: To press together.
• Torsion: To twist one end of an object with
respect to another.
Tension and compression
in glass
• Glass will bend to accommodate stress to a
certain level and suddenly fail once its
threshold is met.
• Impacted glass is compressed on the side it
is hit.
• It will stretch on the opposite side of the
glass, and the tension there will radiate
breaks in the glass outward from the point
of impact.
• Then fractures form in the shape of
concentric circles on the same side of the
impact.
Bullet Fractures
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As a bullet passes through glass, it pushes a cone
shaped piece of glass out of the glass ahead of it
The exit side of the hole is larger than the entrance
side of the hole
Radiating fracture lines from a subsequent shot will
stop at the edge of the fracture lines already present
in the glass
Path of a Bullet Passing through
Window Glass
perpendicular
to the glass
shot from
the left
shot from
the right
• The angles at which bullets enter window glass
help locate the position of the shooter
• Bits of the glass can fly backward (backscatter),
creating trace evidence
Bullet proof glass
Analysis of fracture lines
• Direction of force: when a projectile strikes
glass, it creates ridges called conchoidal
lines.
▫ These lines curve out and away from the point
of impact, and form a right angle on the side
opposite from the impact.
▫ The lines occur along the inside edge of radial
fractures.
• Sequence of force: the radial fractures of the
first impact extend completely, however,
subsequent impacts will have radial fractures
that stop once they come into contact with a
prior fracture.
Which impact happened first A or B?
Velocity
Star-like cracks can reveal the speed of a bullet
by Nipuna » Mon May 06, 2013 4:53 am
PIECING together details of a car accident or crime scene could become a bit easier thanks to
star-like cracks in glassy substances. These characteristic patterns can reveal the speed of the
objects that made them.
It has long been known that different materials require different amounts of energy to crack. But
until now few studies have examined the patterns of cracks left behind to trace back details about
the impact.
Nicolas Vandenberghe and colleagues at Aix-Marseille University in France used an air gun to
fire small metal cylinders at glass plates at increasingly higher speeds, reaching 432 kilometres
per hour. A high-speed camera filmed each shot, and the team counted the radial cracks formed
by the impact. "As surprising as it might be, that had not been done before," says team member
Emmanuel Villermaux.
The team found a unique relationship between the number of cracks and the
projectile's speed. They were able to use this to develop a simple equation that can
tell how fast an object was travelling, based on the type of material it hit (Physical
Review Letters, doi.org/mdd).
Robert Ritchie, a materials scientist at Lawrence Berkeley National Laboratory in California, says
the work is a useful analysis. "For forensics, you could use this to find the energy of a bullet." The
work could also reveal how fast a car was moving just before an accident, by looking for cracks in
the windshield created by stones kicked up from the road.
Handling of Crime Scene Glass
Samples
1. Identify and photograph any glass samples before
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6.
moving them.
Collect the largest fragments that can be reasonably
collected.
Identify the outside and inside surface of any glass.
If multiple panes are involved, make a diagram.
Note trace evidence such as skin, hair, blood, or fibers.
Package all materials collected to maintain the chain of
custody.
In summary
• Glass is an amorphous solid.
• Glass can be analyzed for its density, refractive index, and
fracture patterns.
• Density of glass = Mass (grams) divided by Volume
(milliliters)
• Refractive index is a measurement of how light bends, or
refracts, as it travels through a material.
• Methods for measuring the refractive index include:
▫ Snell’s law
▫ Submersion method
▫ Becke line method
• Fracture patterns provide information about such things as
the direction, the rate, and the sequence of the impacts.