Understanding Glass Evidence in Forensic Science
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Trace Evidence Continued . . .
I. Glass Introduction
A.
= a common type of trace evidence
B.
Characteristics of glass
1.
Common material in our environment
2.
Found in many shapes, sizes, colors, and types
3.
Composed of fused inorganic material
Mixture of:
-
Sand
-
Soda
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Lime
-
Other trace elements
4.
Variation in elemental formulas can alter
significantly its characteristic uses
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5.
Additives’ responsibilities
a.
Alumina (Al
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Aluminum oxide
Improves chemical durability and viscosity
b.
Boron Oxide (B
2
O
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Addition used in borosilicate &
aluminoborosilicate glasses.
Very heat resistant
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5.
Additives’ responsibilities (cont.)
c.
Lime (CaCO
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Added to improve hardness & chemical durability
d.
Lead oxide (PbO)
High lead content lowers melting pt.= decreased
hardness, but increases refractive index
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II. Types of glass
A.
Aluminosilicate & borosilicate
Can withstand high temps.
B.
Laminated glass
Glass w/ plastic layer - used in car windshields
C.
Lead glass
Fine crystal
D.
Soda lime glass
Plate & window glass, glass containers, electric light
bulbs, art objects
E.
Tempered (stressed) glass
Side & back windows of cars; breaks into tiny pieces
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III. Glass as Evidence
A.
Physical—Fracture
Can be used to place people at a crime scene
and reconstruct sequence of events
○
Does this make glass
CLASS or INDIVIDUAL evidence???
Can be used to reconstruct events
Fingerprints
Blood
B.
Density determination
C.
Refractive index (R.I.) determination
D.
Chemical testing
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A.
Physical—Fracture
Glass is slightly flexible
When it reaches its elastic limit = break
○
this leads to blowback
2 Distinct types of fractures
1. Radial
2. Concentric
Fractures
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bsapp.com
Slide from: http://www.bsapp.com/forensics_illustrated/pppresentations.htm
A.
Physical—Fracture (cont.)
Image from: http://www.bsapp.com/forensics_illustrated/pppresentations.htm
The radial crack from impact
point B stops when it hits the
radial crack from point A. Thus
A must have occurred first.
A.
Physical—Fracture (cont.)
Direction
of
Penetration
Concentric
Concentric
Radial
Radial
bsapp.com
Slide from: http://www.bsapp.com/forensics_illustrated/pppresentations.htm
A.
Physical—Fracture (cont.)
Direction of Impact
Found from stress marks on the edges
of broken glass.
At the point of impact, glass will break
with a conchoidal pattern (shell-like)
Direction of
force
High velocity projectiles always
leave a hole wider at the exit
surface.
Image from: http://www.hsa.gov.sg/publish/etc/medialib/hsa_library/applied_sciences/forensic_science_files/images_-
_criminalistic_2.Par.28737.Image.gif
A
.
Physical—Fracture (cont.)
Direction
of
Penetration
A projectile hole is
inevitably wider at the
exit side
bsapp.com
Slide from: http://www.bsapp.com/forensics_illustrated/pppresentations.htm
A.
Physical—Fracture (cont.)
B.
Density Determination = m
V (water displacement)
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Common
Glass
Densities
1. Background:
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Images from: http://en.wikipedia.org/wiki/Refraction
1. Background:
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1
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○
Thus RI of water
3.00 x 10
10
=
________________ =
1.33
2.25 x 10
10
2. Determining the RI from samples in an investigation:
a.
Different types of glass will have different RI
b.
To analyze this property
Glass fragments can be immersed in a liquid with a
known refractive index to help approximate the RI
of the evidence
C.
RI (continued)
i.
Analysis method 1
If the liquid has the same RI as the glass sample, the
glass will disappear under a microscope.
If the liquid has a higher or lower RI, the glass is visible
and a “halo” appears around perimeter (we call this halo
the “Becke Line.”)
. . . when RIs are the same, the Becke line
disappears
. . . if Becke line appears on the outside perimeter
= Glass has lower refractive index
. . . if Becke line appears on inside perimeter =
Glass has higher refractive index
C.
RI (continued)
Becke Line
Inside
Disappears
Outside
*
*
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*
C.
RI (continued)
Becke line-- outside
Glass has lower refractive index
C.
RI (continued)
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Becke line-- inside
Glass has higher refractive index
C.
RI (continued)
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Common liquid refractive indices
C.
RI (continued)
ii.
Analysis method 2
RI is dependant on:
The wavelength of light
The temperature of the medium
○
When the temp. of a liquid is changed, the RI changes
rapidly, but the RI of an immersed solid will not
C.
RI (continued)
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Silicon oil usually used
Oil is calibrated so RI can be determined
from its temp.
Sample glass is immersed in oil
Oil is heated/cooled to determine match
temp.
Glass “disappears”
Oil RI = Glass RI
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C.
RI (continued)
ii.
Analysis method 2
D.
Chemical testing
Destructive
Test for silicates, metal oxides,
trace evidence
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Glass evidence plays a crucial role in forensic investigations. This article delves into the characteristics of glass, additives used, types of glass, and how glass can be utilized as evidence in reconstructing events at crime scenes. Key aspects covered include physical fracture analysis, density determination, refractive index testing, and chemical examinations.
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Presentation Transcript
Trace Evidence Continued . . . GLASS EVIDENCE GLASS EVIDENCE
I. Glass Introduction A. = a common type of trace evidence B. Characteristics of glass 1. Common material in our environment 2. Found in many shapes, sizes, colors, and types 3. Composed of fused inorganic material Mixture of: - Sand - Soda - Lime - Other trace elements 4. Variation in elemental formulas can alter significantly its characteristic uses Components of glass content from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
5. Additives responsibilities a. Alumina (Al2O3) Aluminum oxide Improves chemical durability and viscosity b. Boron Oxide (B2O3) Addition used in borosilicate & aluminoborosilicate glasses. Very heat resistant Slide from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
5. Additives responsibilities (cont.) c. Lime (CaCO3) Added to improve hardness & chemical durability d. Lead oxide (PbO) High lead content lowers melting pt.= decreased hardness, but increases refractive index Slide from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
II. Types of glass A. Aluminosilicate & borosilicate Can withstand high temps. B. Laminated glass Glass w/ plastic layer - used in car windshields C. Lead glass Fine crystal D. Soda lime glass Plate & window glass, glass containers, electric light bulbs, art objects E. Tempered (stressed) glass Side & back windows of cars; breaks into tiny pieces Slide from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
III. Glass as Evidence A. Physical Fracture Can be used to place people at a crime scene and reconstruct sequence of events Does this make glass CLASS or INDIVIDUAL evidence??? Can be used to reconstruct events Fingerprints Blood B. Density determination C. Refractive index (R.I.) determination D. Chemical testing Last 3 bullet points content from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
A. PhysicalFracture Glass is slightly flexible When it reaches its elastic limit = break this leads to blowback 2 Distinct types of fractures 1. Radial 2. Concentric
A. PhysicalFracture (cont.) Fractures Concentric Radial bsapp.com Slide from: http://www.bsapp.com/forensics_illustrated/pppresentations.htm
A. PhysicalFracture (cont.) A. B. The radial crack from impact point B stops when it hits the radial crack from point A. Thus A must have occurred first. Image from: http://www.bsapp.com/forensics_illustrated/pppresentations.htm
A. PhysicalFracture (cont.) Direction of Penetration Radial Concentric bsapp.comSlide from: http://www.bsapp.com/forensics_illustrated/pppresentations.htm
A. PhysicalFracture (cont.) Direction of Impact Found from stress marks on the edges of broken glass. At the point of impact, glass will break with a conchoidal pattern (shell-like) Image from: http://www.hsa.gov.sg/publish/etc/medialib/hsa_library/applied_sciences/forensic_science_files/images_- _criminalistic_2.Par.28737.Image.gif Direction of force
A. PhysicalFracture (cont.) Direction of Penetration A projectile hole is inevitably wider at the exit side bsapp.com Slide from: http://www.bsapp.com/forensics_illustrated/pppresentations.htm
B. Density Determination = m V (water displacement) Common Glass Densities Density (g/cm3) 2.4880 Glass Type Blue ornamental Double glazing 2.5125 Lead crystal 2.9614 Pyrex 2.2300 Quartz 2.2106 Red ornamental 2.5539 Soda 2.4862 Window glass 2.5092 Zinc titania 2.5310 Content of slide from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
1. Background: a. Refraction = the bending of a light wave as it passes from one medium to another http://upload.wikimedia.org/wikipedia/commons/thumb/b/b9/Refraction-with-soda-straw.jpg/220px-Refraction-with-soda-straw.jpg http://upload.wikimedia.org/wikipedia/commons/6/62/Pencil_in_a_bowl_of_water.png Images from: http://en.wikipedia.org/wiki/Refraction
1. Background: a. Refraction = the bending of a light wave as it passes from one medium to another b. Refractive Index (RI)= a comparison of the speed of light in a vacuum to the speed of light in another substance Sample calculation: speed of light in vacuum is 3.00 x 1010 and the speed of light in water is 2.25 x 1010 Thus RI of water 3.00 x 1010 =________________ = 1.33 2.25 x 1010
C. RI (continued) 2. Determining the RI from samples in an investigation: a. Different types of glass will have different RI b. To analyze this property Glass fragments can be immersed in a liquid with a known refractive index to help approximate the RI of the evidence
C. RI (continued) i. Analysis method 1 If the liquid has the same RI as the glass sample, the glass will disappear under a microscope. If the liquid has a higher or lower RI, the glass is visible and a halo appears around perimeter (we call this halo the Becke Line. ) . . . when RIs are the same, the Becke line disappears . . . if Becke line appears on the outside perimeter = Glass has lower refractive index . . . if Becke line appears on inside perimeter = Glass has higher refractive index
C. RI (continued) Becke Line Disappears **GOAL** Outside Inside
C. RI (continued) Becke line-- outside Glass has lower refractive index Content of slide from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
C. RI (continued) Becke line-- inside Glass has higher refractive index Content of slide from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
C. RI (continued) Common liquid refractive indices Liquid Refractive index 1.373 1.402 1.467 1.473 1.482 1.522 1.543 1.465-1.467 Ethyl acetate n-butyl alcohol Olive oil Corn oil Castor oil Methyl salicylate Clove oil Canola oil
C. RI (continued) ii. Analysis method 2 RI is dependant on: The wavelength of light The temperature of the medium When the temp. of a liquid is changed, the RI changes rapidly, but the RI of an immersed solid will not Content of slide from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
C. RI (continued) ii. Analysis method 2 Silicon oil usually used Oil is calibrated so RI can be determined from its temp. Sample glass is immersed in oil Oil is heated/cooled to determine match temp. Glass disappears Oil RI = Glass RI Content of slide from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
D. Chemical testing Destructive Test for silicates, metal oxides, trace evidence Content of slide from: www.kaukauna.k12.wi.us/teachers/mader/glass.ppt
