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Chapter 3 & 4: Glass Evidence

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Chapter 3 & 4: Glass Evidence. What Is Glass?. Glass is considered a very slow moving liquid Glass is a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides. Amorphous solids have their atoms arranged randomly, unlike crystals.

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what is glass
What Is Glass?
  • Glass is considered a very slow moving liquid
  • Glass is a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides.
  • Amorphous solids have their atoms arranged randomly, unlike crystals.
amorphous vs crystalline structure
Amorphous vs. Crystalline Structure

Amorphous Structure: Glass

Crystalline Structure: Salt (NaCl)

glass manufacturing
Glass Manufacturing
  • It is an automated process
    • Led to mass production
      •  Mass production means greater uniformity and less ability to discriminate
  • Most glass today is called float glass
    • Made by pouring the molten material onto a bath of melted tin, resulting in a very smooth surface
glass manufacturing1
Glass Manufacturing
  • Tempered glass
    • Created by subjecting glass to extreme heat then cold, repeated over and over, during the manufacturing process
    •  Glass breaks into smaller squares rather than into sharp shards
  • Laminated Glass
    • Windshields are specialty glasses made by a having a plastic middle layer that is sandwiched between two layers of glass.
    • The plastic layer helps hold the glass layers together.
tempered glass shattering examples
Tempered Glass: Shattering Examples

A telephone booth with smashed tempered glass in Holloway, London.

glass in forensics
Glass In Forensics
  • Used in solving automobile accidents, hit-and-runs, burglaries, and assaults
  • Glass is a type of transfer evidence
    • Can be individualized and linked to a common source
      •  Only can individualize a glass fragment, if can fit it like puzzle piece to its source
glass in forensics1
Glass In Forensics
  • For the forensic scientist, the problem of glass comparison is one that depends on the need to find and measure those properties that will associate one glass fragment with another.
  • To compare glass fragments, a forensic scientist evaluates these important physical properties:
    • Appearance – shape, color, etc
    • Fracture patterns
    • Density
    • Refractive Index (and Becke lines)
    • Chemical analysis
glass fractures
Glass Fractures
  • Because of a lack of order and pattern, glass breaks in random patterns
  • An impact in glass produces two types of fractures
    • Radial (radiating out from the point of impact)
    • Concentric (forming circles around the impact)
direction of force
Direction of Force
  • Small projectiles passing through glass at a high velocity will produce characteristic patterns
    • Exit hole is larger than entry hole
  • Lower velocity impacts may not penetrate the glass but leave only a pit or crater on one side of the glass
direction of force1
Direction of Force
  • Direction of force can be determined by looking at the cross section of the fractures
  • The lines created in the glass, calledconchoidal fractures, show distinctive patterns depending on whether the fractures from which they are collected are concentric or radial
direction of force2
Direction of Force
  • Most reliable determinations are from results closest to the original impact
  • Perpendicular marks are found on the edge opposite way force was pushing
  • Remember:
    •  Radial cracks form Right angles on the Reverse side of the force
direction of force3
Direction of Force

Perpendicular marks (~90 degree angles) are found on the edge opposite the way that the force was pushing

shattering due to heat
Shattering Due to Heat
  • The shattering of glass by heat creates a distinctive and different fracture pattern characterized by wavy smooth cracks
order of impact
Order of Impact
  • If there are many impacts on the same pane of glass, the sequence of events can be deduced from the fracture patterns
    •  Existing fractures from the 1st impact act as barriers to fractures created by the second impact
    • Look for abrupt


order of impact example
Order of Impact Example

This photo depicts two bullet holes in safety glass. Which hole was created first? How can you tell?



The hole on the right was created first. Cracks radiating out from the hole will stop when they encounter another crack. Stress placed on the glass (causing it to crack) will be transferred along the existing crack rather than across it.

determining common source
Determining Common Source
  • Only way to link two fragments of glass to a common source is by the process of physical matching
  • Each breakage is unique
    • Creates pieces of a puzzle that the criminalist must place the pieces together
determining common source1
Determining Common Source
  • The criminalist can obtain other information on the glass characteristics
    • Color
    • Thickness
    • Shape
    • Texture
individual glass characteristics
Individual Glass Characteristics
  •  The two most common physical parameters used to characterize glass samples are the measurements of density and the refractive index
  • Mass refers to the amount of matter an object contains independent of gravity.
  • Volume is the amount of space an object takes up.
  • Densityis mass / volume
    • Units for density are usually in

g/mL, g/cm3

  • Densityis an intensive property of matter, meaning it remains the same regardless of sample size.
  • It is considered a characteristic property of a substance and can be used as an aid in identification.
  • The density of glass is determined by using a floatation technique in which the composition of liquid is varied until the glass sample remains suspended in it
  • Light waves travel in air at a constant speed until they penetrate another medium, such as glass or water, at which point they are suddenly slowed, causing the rays to bend.
  • The bending of light waves because of a change in speed is called refraction.
refractive index n
Refractive Index (n)
  •  A quantity that measures the bending of light as it travels from one medium into another
  • n= speed of light in a vacuum

speed of light in medium

  • Always will be greater than 1.00
    • Water has a refractive index of 1.33 (light travels 1.33 times faster in a vacuum than in water)
refractive index
Refractive Index
  •  The RI depends on the wavelength of light being used and the temperature of the material
refractive index1
Refractive Index
  • Measured using a microscope equipped with a hot stage
    • Glass is immersed in an oily material with a known refractive index
    • Oil is slowly heated
      • This changes the refractive index
    • When glass is not seen in the oil, it has the same refractive index
the becke line
The Becke Line
  •  The Becke Line is a line that appears as a halo if the refractive indexes of the glass and the material are different
    •  The Becke Line will disappear when the refractive indexes are the same
becke lines from glass
Becke Lines from Glass

Becke line on outside

Becke line on inside

RI of glass (1.525 > RI of medium (1.34)

RI of glass (1.525) < RI of medium (1.6)

GRIM II System

-automated system for matching glass RI

-uses a step-heating stage and a RI liquid that varies

with temperature

GRIM II identifies the RI match by monitoring a

video image of the glass fragment in the liquid

-as it is heated/cooled the contrast is measured until

a minimum is reached-the match point

snell s law
Snell’s Law

(n1)(sin first angle) = (n2)(sin second angle)

  • (n1) = refractive index of first medium
  • (n2) = refractive index of second medium
  • Angle 1 & 2 measured to normal
snell s law1
Snell’s Law

Normal line

Lower RI to Higher RI = Angle bends toward normal line

Higher to Lower RI = Angle bends away from normal line

In air

n= 1.00


In glass

n= 1.50

glass analysis
Glass analysis

Pick a liquid with a refractive index that matches the crime scene glass, and the glass will disappear when submerged!

collection of glass
Collection of Glass
  • If even the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all the glass found.
  • When an individual fit is thought improbable, the evidence collector must submit all glass evidence found in the possession of the suspect along with a representative sample of broken glass remaining at the crime scene.
glass videos
Glass Videos
  • 1. Glass Shattering Montage by Jaime Vendura (4:12)
  • 2. Bullet Proof Glass Shooting (0:45)
  • 3. Large Fracture Breaking (3:22)
  • 4. Glass and Its Importance in Early Science (1:35)
  • 5. Refraction of Light Causes Rod to Appear Bent (0:09)