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?