warm-up after attending a gallery opening slylock has his assistant speak to the gallery and tell...
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Forensic Science: Fundamentals & Investigations, Chapter 14 3 History of Glass The earliest human-made glass objects (beads) date back to about 2500 B.C. Egypt. Specialized glass production was an art, a science, and a state secret in the republic of Venice over a span of hundreds of years. By the fourteenth century, the knowledge of glass production spread throughout Europe. The industrial revolution brought the mass production of many kinds of glass.TRANSCRIPT
Warm-Up• After attending a gallery opening Slylock has his assistant
speak to the gallery and tell them one painting is a fake. Why does Slylock Fox suspect this painting is a fake?
Glass Evidence
Forensic Science: Fundamentals & Investigations, Chapter 14
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History of Glass The earliest human-made glass objects (beads) date back
to about 2500 B.C. Egypt. Specialized glass production was an art, a science, and a
state secret in the republic of Venice over a span of hundreds of years.
By the fourteenth century, the knowledge of glass production spread throughout Europe.
The industrial revolution brought the mass production of many kinds of glass.
Characteristics of Glass
Hard, amorphous solid
Usually transparent
Primarily composed of silica, with various amounts of elemental oxides
Brittle
Exhibits conchoidal fracture
What’s in Glass? Formers (bulk of glass, form glassy, non-
crystalline structure):SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5
Fluxes (lowers melting point):Na2O, K2O, LiO, Al2O3, B2O3, Cs2O
Stabilizers (Strengthen the glass and make it resistant to water and corrosion):CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO
How is glass made?
• http://science.howstuffworks.com/innovation/28024-some-assembly-required-how-glass-is-made-video.htm
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Composition of Glass (continued)
• The raw materials for making glass are all oxides– The composition of any sample can be given in terms
of the percent of each oxide used to make it– Example: the approximate composition of window or
bottle glass is• Silica (SiO2) – 73.6 % • Soda (Na2O) – 16.0 %• Lime (CaO) – 5.2 %• Potash (K2O) – 0.6 %• Magnesia (MgO) – 3.6 %• Alumina (Al2O3) – 1.0
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Types of Glass
• Obsidian is a natural form of glass that is created by volcanoes
• Soda-lime glass– The most basic, common, inexpensive glass – also
the easiest to make– Used for manufacturing windows and bottle glass
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Types of Glass (continued)
• Leaded glass– Contains lead oxide which makes it denser– Sparkles as light passes through it (light waves
are bent) – Used for manufacturing fine glassware and art
glass– Is commonly called crystal
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Types of Glass (continued)
• Tempered glass– Stronger than ordinary glass – Strengthened by introducing stress through rapid
heating and cooling of its surface– When broken, this glass does not shatter, but
fragments or breaks into small squares– Used in the side and rear windows of automobiles
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Types of Glass (continued)
• Laminated glass– Constructed by bonding two ordinary sheets of
glass together with a plastic film– Also used by automobile manufactures
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Glass as Evidence
Class characteristics: physical and chemical properties
Individual characteristics: if the fragments can fit together like pieces of a puzzle, the source can be considered unique
Considerations for CollectionThe collector must consider that fragments within a questioned
sample may have multiple origins. If possible, the collector should attempt an initial separation based on physical properties.
The collector must consider the possibility that there may be a physical match to a known sample (e.g., a piece of glass to a fractured vehicle headlamp). When an attempt to make a physical match is made at the site of collection, the collector should take precautions to avoid mixing of the known and questioned samples.
Any glass samples collected should be documented, marked (if necessary), packaged, and labeled.
—Forensic Science Communications
Density—mass divided by volume
Refractive index (RI)—the measure of light bending due to a change in velocity when traveling from one medium to another
Fractures
Color
Thickness
Fluorescence
Markings—striations, dimples, etc.
Physical Characteristics
Forensic Science: Fundamentals & Investigations, Chapter 14
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Density The formula for calculating density is:
D = m / V The mass (m) of a fragment of glass can be found
using a scale. Place the fragment of glass into a beaker filled with
water and measure the volume (V) of overflow. Divide the mass (in grams) by the volume (in
milliliters) to find the density (D) of the glass fragment.
Density Practice
Find the density for the following pieces of glass:1.Mass: 4g, Volume Displaced: 2mL• 2 g/mL2. Mass: 1g, Volume Displaced: 4mL• .25 g/mL
Common Densities
Type of Glass Density
window 2.46–2.49
headlight 2.47–2.63
Pyrex 2.23–2.36
lead glass 2.9–5.9
porcelain 2.3–2.5
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Refractive Index
When a beam of light moves from one medium into another, its speed changes. That change causes the beam to change direction, bend.
Measure of how much the speed of light changes (n>1).(Air’s RI: n=1)
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Refractive Index
The direction of the light forms two angles with the normal . If the light passes into a denser medium (the gray area), its
direction will bend toward the normal.
Determination of Refractive Index
Immersion method—lower fragments into liquids whose refractive index is different
Match point—when the refractive index of the glass is equal to that of the liquid
Becke line—a halo-like glow that appears around an object immersed in a liquid. It disappears when the refractive index of the liquid matches the refractive index of the object (the match point).
The Becke Line
The Becke line is a “halo” used to determine refractive index.It can be seen on the inside of the glass on the left, indicating that the
glass has a higher refractive index than the liquid medium. The Becke line as seen on the right is on the outside of the glass, indicating just the opposite.
Common Refractive Index
Liquid RI Glass RIWater 1.333 Vitreous silica 1.458
Olive oil 1.467 Headlight 1.47–1.49
Glycerin 1.473 Window 1.51–1.52
Castor oil 1.482 Bottle 1.51–1.52
Clove oil 1.543 Optical 1.52–1.53
Bromobenzene 1.560 Quartz 1.544–1.553
Bromoform 1.597 Lead 1.56–1.61
Cinnamon oil 1.619 Diamond 2.419
Snell’s Law of Refraction
• Can figure out the RI of materials based on the angle the light hits and bends.
n1sin(θ1) = n2sin(θ2)
n1 = RI of incident light
θ1= angle of incident light
n2 = RI of refracted light
θ2= angle of refracted light
Snell’s Law Example 1• Light traveling in air (n1=1) hits a piece
of glass at an angle of 30 degrees. The light refracts in the glass at an angle of 15 degrees. What is the index of refraction (RI) of the glass?
n1=1, θ1= 30, θ2= 15, n2=?
1sin(30)=n2sin(15)
0.5=0.2588n2
n2 = 1.93
Snell’s Law Example 2• Light traveling in air (n1=1) hits a piece
of glass at an angle of 45 degrees. If the glass has a refractive index of 1.5, what angle should the light bend at?
n1=1, θ1= 45, θ2= ?, n2=1.5
1sin(45)=1.5sin(θ2)
0.7071=1.5sin(θ2)
.4714= sin(θ2)
θ2= 27 degrees
Snell’s Law Practice 1. Light traveling in air (n1=1) hits a piece of glass at
an angle of 37 degrees. The light refracts in the glass at an angle of 30 degrees. What is the index of refraction (RI) of the glass?