chapter 14 glass evidence - ms. mouillesseaux -...

Forensic Science: Fundamentals & Investigations, Chapter 14 1

Chapter 14 Glass Evidence By the end of this chapter you will be able to:

o Explain how glass is formed o List some of the characteristics of glass o Provide examples of different types of glass o Calculate the density of glass o Use the refractive index to identify different types of

glass o Describe how glass fractures o Analyze glass fracture patterns

All Rights Reserved South-Western / Cengage Learning © 2012, 2009


Introduction and History of Glass

o Humans have used obsidian (volcanic glass) since prehistoric times – cutting tools, arrowheads

o Egypt circa 2500 B.C.—The earliest known human-made glass objects (beads)

o 1st Century B.C.—glass blowing begins o 13th Century—specialized glass production was

an art, a science, and a state secret in the republic of Venice


Introduction and History of Glass

o 14th Century—glass-making spreads through Europe

o The industrial revolution applies mass production to many types of glass

o Analysis of glass found at a crime scene can yield trace evidence

• Fibers or blood on glass shards • Sequence and direction of bullets • Glass particles may be transferred to suspect’s shoes

or clothing


What Is Glass?

o Materials used to create (soda-lime) glass: • Silicon dioxide (SiO2) – silica found in sand • Sodium oxide (Na20) – soda Reduces melting point of silica

• Calcium oxide (CaO) – lime Prevents glass from being soluble in water

o Once cooled, glass can be polished, ground,

or cut


What Is Glass?

o Crystalline solids have a regular atomic structure

o Glass is an amorphous solid and so has an irregular atomic structure

o Therefore, glass breaks in a variety of fracture patterns


Types of Glass

o Many types of glass o Soda-lime glass most common o Leaded glass (crystal) uses lead oxide instead of

calcium oxide • Denser glass, sparkles as light waves bend

o Pyrex contains compounds that allows it to withstand a wide range of temperatures

o Adding metal oxides yields different colors o Different densities o Refractive indexes o These characteristics allow comparisons


Density

The formula for calculating density is: D = m / V o m = mass, measured by a balance beam

device o V = volume, place the glass fragment into a

beaker filled with water and measure the overflow

o D = density, divide the mass (in grams) by the volume (in milliliters)


Density—Common Examples

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


Refractive Index

o When a beam of light moves from one medium into another:

• The speed changes • The direction bends

o Refractive Index—a tool used to study how light bends as it passes from one substance to another

o Normal line is perpendicular to the glass surface


Refractive Index

When a beam of light moves from less dense medium (air) into a more dense medium (water):

• Its speed slows, and • Bends light toward

the normal line


Refractive Index

When a beam of light moves from a more dense medium (glass) into a less denser medium (air):

• Its speed increases • And bends light away

from the normal line


Application of Refractive Index to Forensics

Submersion method—used when glass fragments found at the crime scene are small


Application of Refractive Index to Forensics

o Place the glass fragment into different liquids of known refractive indexes

o The glass fragment will seem to disappear when placed in a liquid of the same refractive index

Using refractive index as evidence

o Car dealers and glass manufacturers have databases containing the refractive index of their products.



Becke Line

o halo-like effect appearing at the edges of a glass fragment when the refractive index of the glass and liquid are different • Put glass fragment on a slide in a drop of liquid with

known RI • Lower microscope stage • 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 video

https://youtu.be/B6Gis-wloSI


Fracture Patterns in Broken Glass

o Being an amorphous solid, glass will not break into regular pieces with straight line fractures

o Fracture patterns provide clues about the direction, rate, and sequence of the impacts


Why Radial and Concentric Fractures Form

o Impacted glass is compressed on the side it is hit. o 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.

o Then fractures form in the shape of concentric circles on the same side of the impact.

o Slower objects will make more concentric circles than faster objects



Glass after an impact shows radial fractures (red) and concentric circle fractures (blue)


Bullet Fractures

o As a bullet passes through glass, it pushes a cone shaped piece of glass out of the glass ahead of it

o The exit side of the hole is larger than the entrance side of the hole

o 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

o The angles at which bullets enter window glass help locate the position of the shooter

o Bits of the glass can fly backward (backscatter), creating trace evidence

perpendicular to the glass

shot from the left

shot from the right

Path of a Bullet

o If shot is perpendicular to window, entry hole will be round

o If shot is from the left, glass will be forced to the right, exit hole will be an irregular oval to the right

o If shot is from right, glass will be forced to the left exit hole will be an irregular oval to the left.

o Ammunition type and distance can be determined by pattern • But, high speed bullet from a distance will make a

similar pattern to slower speed bullet up close. Forensic Science: Fundamentals & Investigations, Chapter 14 24

Bulletproof glass

o Combo of two or more types of glass o Softer layer makes glass more elastic

• Won’t shatter as easily, absorbs energy of bullet o Index of refraction must be similar for both

pieces of glass • transparent

o Varies in thickness • ¾ inch – 3 inches


Tempered glass

o Safety glass used in windshields o Two layers of glass bonded with plastic in middle o Breaks into small, cubic pieces that don’t cut o Makes fewer concentric circle patterns o Hard to tell point of impact


Backscatter

o When a window is broken, most glass flies forward

o Some will be projected backwards o Can land on person who broke window o Trace evidence


Heat fractures

o During fire glass may fracture o Wavy lines o Will break toward region of high heat o No radial or concentric fracture patterns



Handling of Crime Scene Glass Samples

1. Identify and photograph any glass samples before moving them.

2. Collect the largest fragments that can be reasonably collected.

3. Identify the outside and inside surface of any glass. 4. If multiple panes are involved, make a diagram. 5. Note trace evidence such as skin, hair, blood, or

fibers. 6. Package all materials collected to maintain the chain

of custody.

Analyzing Glass fragments in lab

o Separated by physical properties • Size, color, texture

o Trace evidence should be separated from glass fragments

o Clothing related to crime scene and tools used to break glass should be inspected for glass

o Glass needs to be cleaned before it is analyzed for density and RI Forensic Science: Fundamentals & Investigations, Chapter 14 31


. . . . . . . . . . . . . Summary . . . .

o Glass is an amorphous solid o Glass can be analyzed for its density,

refractiveness, and fracture patterns o Density of glass = Mass (grams) divided by

Volume (milliliters) o Refractive index is a measurement of how light

bends, or refracts, as it travels through a material


. . . . . . . . . . . . . . . . . Summary

o Methods for measuring the refractive index include: • Snell’s law • Submersion method • Becke line method

o Fracture patterns provide information about such things as the direction, the rate, and the sequence of the impacts

chapter 14 glass evidence - ms. mouillesseaux -...

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