FRSC 302 Lecture Notes - Lecture 3: Atomic Number, Refractive Index, Scanning Electron Microscope

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FRSC 302
WEEK 3 GLASS NOTES
Glass Evidence
Common types of physical evidence
o Glass
o Paint chip
o Documents
Important to know:
o Its manufacture
o Properties
o Methods of analysis
Structure
o Glass is a rigid liquid
o Its viscosity is temperature dependent
At high temperatures glass is in a molten liquid state
When cooled the glass stops flowing and takes the appearance of a solid
but still amorphous
o The internal structure is not regularly crystalline but randomly disordered as a
liquid even though it appears as a solid
Mechanical Properties
o Glass behaves like an elastic solid
In bulk it appears solid
In thin sheets or fibers it is malleable elastic
When glass is cracked, the side where the force is applied is strengthened, but the
opposite side is weakened
Tempering of Glass
Process for producing safety glass by controlled thermal or chemical treatments to
increase strength compared to normal glass
Thermal toughening heat glass above transformation temperature and then the
surface is cooled rapidly
Chemical toughening white hot glass is placed in contact with salt, ion exchange
with smaller ions from glass
When tempered glass breaks it does not shatter, but breaks into smaller fragments,
it is used in side and rear windows in automobiles and buildings
Annealing
o Glass is a poor heat conductor and therefore cools irregularly, this can lead
to internal stresses within the glass, which can lead to spontaneous
shattering
o All commercial glassware is re-heated where the internal stresses relax and is
then cooled very slowly
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Glass Manufacture
Soda Glass SiO2 (70%)/Na2O (15%)/CaO (10%)
o Na2O reduces the melting temperature making it easier to work with
o CaO makes the glass more chemically resistant
o Uses most prevalent type of glass, to shore chemicals, beverages, windows
Silica Glass SiO2 (greater than or equal to 99.5%)
o Its manufacture requires high temperatures
o Mainly specialist uses because of its transparency to a wide range of
wavelengths
Borosilicate glass SiO2 (60-80%)/B2O (10-25%)/Al2)3 (1-4%)
o Boron addition enables glass to withstand high temperatures without fracturing
o Uses lighting, electronics, cookware
Manufacture of float glass
1. Raw materials
2. Melting furnace
3. Float chamber with molten tin N0O2
4. Annealing
5. Cutting and packing
Glass in Forensic Science
Most glass submitted to forensic laboratories originates from?
o Cars
Ordinary commercial glasses vary little in composition and therefore common chemical
analysis methods are unhelpful
Most glass is identified by physical properties (i.e. impact part of glass, rib marks,
Polarized Light Microscope (PLM))
Annealing of Glass
Breaking glass
o Glass will bend in response to force
Analysis of Glass
The first fracture to form will be radial fractures on the surface opposite to the blow
The continued motion of the force will result in formation of concentric fractures
Fractures
Primary lines of fracture run out radially from the point of impact
Secondary lines run between the primary ones in concentric circles
If glass fragments are recovered rib marks might be visible
Rib Marks
It is important to determine the direction of impact
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