Okobu!
• Do Now:
– Take out Notes and prepare for Quiz
• Agenda:
– Review of Density techniques
– Refractive Index
• HW:
– Ch 4 #3,5,7-10,19-22,28-35 due 11/1
Sink/Float method
Egg Demo
Why Measure Density?
• Can be used as a screening technique with large
numbers of fragments.
• Useful in identifying multiple sources present in
the known and/or questioned samples.
• It is nondestructive and an intensive property (not
dependant on sample mass).
• Need to measure very precisely in parts per
hundred or thousand or better.
Glass Density
• Density can be measured by:
• directly determining mass and volume
(usually by displacement)
• comparison by flotation
• comparison using a density gradient
column
• Density gradient column method:
• Fragments of different densities settle at
different levels in the column of liquid of
varying density.
Flotation Method
precise and rapid method for comparing glass
densities.
1. a glass particle is immersed in a liquid.
1. The density of the liquid is carefully adjusted
–
–
by the addition of small amounts of an appropriate
liquid
until the glass chip remains suspended in the liquid
medium.
1. At this point, the glass will have the same
density as the liquid medium
–
can be compared to other relevant pieces of glass
which will remain suspended, sink, or float.
Density by the Flotation Method
• A glass particle is immersed in a liquid. The density of
the liquid is adjusted by the addition of small amounts
of another liquid until the glass chip remains
suspended.
• At this point, the glass will have the same density as the
liquid medium and can be compared to other relevant
pieces of glass which will remain suspended, sink, or
float.
Important Physical Properties
• Light waves travel in air at a constant velocity
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 velocity is called refraction.
• Refractive index is the ratio of the velocity of
light in a vacuum to that in the medium under
examination.
Important Physical Properties
• For example, at 25oC the refractive index of water
is 1.333.
• This means that light travels 1.333 times faster in
a vacuum than it does in water.
• Like density, refractive index is an intensive
property and will serve to characterize a
substance.
• Refractive index of glass varies with small
changes in composition or by how it is
manufactured.
• refractive index = velocity of light in vacuum
velocity of light in medium
Snell’s Law
N=1.52
N=1.33
The higher the n, the more the light bends
Refractive Index By Immersion
• Immersing a glass particle in a liquid medium
(silicone oil) whose refractive index can varied with
temperature until it is equal to that of the glass
particle.
• At this point, known as the match point, the Becke
line disappears and minimum contrast between
liquid and particle is observed: RI oil = RI glass.
• The Becke line is a bright halo near the boarder of a
particle that is immersed in a liquid of a different
refractive index.
Refractive Index By Immersion
When 2 substances have different RI, light passing through them produces
a Becke Line
A Becke line is a band or rim of light visible along a grain/crystal boundary
in plane-polarized light.
Becke Lines
Glass has higher refractive index-note white line inside
•
Glass has lower refractive index-note white line outside
Problems with Refractive Index
1964-1979
1980-1997
A rough statistical
estimate of the
likelihood of finding
glass of that refractive
before 1970
Manufacturing
changed in late
1970’s making glass
more uniform
FBI Refractive Index vs Density Data
•The FBI has compiled density and refractive index data for glass from
around the world.
•The FBI has identified a relationship between their refractive indices and
densities for 1400 glass specimens that is better at classification.
• Do Now:
• Agenda:
• HW
Putting it Back Together Again?
• Examiners can fit together two or more pieces of
glass that were broken from the same object.
• Because glass is amorphous, no two glass objects
will break the same way.
Glass Transfer Evidence
• When glass objects are broken, glass
flies backward from all parts of the
object where cracks appear not just
from point of impact.
• This creates a shower of minute glass
particles and a transfer of evidence.
• Glass fragment comparison depends
finding and measuring properties that
will associate one glass fragment with
another while eliminating other
sources.
Collection of Glass Samples
• The glass fragments should be packaged in boxes
to avoid further breakage.
• If evidence is to be examined for glass fragments, it
should be taken whole and each item individually
wrapped in paper and boxed.
• If even the remotest possibility exists that glass
fragments may be pieced together, every effort
must be made to collect all glass fragments.
• Submit glass evidence along with a representative
sample of each type of glass from the crime scene.
Optical Properties of Glass
Make side-by-side comparisons using similarsized fragments.
Place samples on a white surface using natural
light.
Use both fluorescent and incandescent light to
determine the glass’s color.
Visual color analysis is very subjective.
Dyes and pigments can be almost impossible to
extract.
Nonoptical Physical Properties of Glass
• Surface striations and markings
– Rollers leave parallel ream marks on sheet glass
– Markings may indicate the glass’s orientation
when pieces are missing
– Surface scratches, etchings, and other markings
may also be used to individualize evidence
• Other Properties
– Hardness=5-6 on Mohs scale; use a scratch test.
– Determinations of curvature can distinguish flat
glass from container, decorative, or ophthalmic.
Forensic Analysis of Glass
Quantitative Properties of Glass
Chemical Analysis of Glass
• Fluorescence
– Under UV radiation, many glasses exhibit
fluorescence (glow)
– Caused by heavy metals (including tin) from
“float” process or organic coatings
• Scanning Electron Microscopy Energy Dispersive
X-ray Analysis
– Can determine many elements simultaneously
– Surfaces of samples (>50 mg) can be analyzed
• Atomic Absorption Spectroscopy
– You must first know which elements are present
– Can analyze ppm levels of elements present
Learning Check
1. Which unique chemical component would be
found in each type of glass shown below?
(beaker)
(windshield)
(crystal)
2. Use “Table 2.3” to determine if lead borosilicate
glass can be distinguished from borosilicate glass
by density, refractive index, or both.
Quantitative Properties of Glass
The Wave of the Future: Laser Ablation
• Laser Ablation Inductively Coupled Plasma Mass
Spectrometry (LA-ICP-MS)
– Laser burns off a microscopic sample
– Elements are ionized by plasma
– Detects 46 trace elements and their isotopes
simultaneously in glass at < 1 ppb
• LA-ICP-MS (Laser Ablation Inductively Coupled
Plasma Mass Spectrometry) is a powerful
analytical technology that enables highly sensitive
elemental and isotopic analysis to be performed
directly on solid samples.
• powerful analytical technology that enables highly
sensitive elemental and isotopic analysis to be
performed directly on solid samples.