Glass Flashcards
Glass definition
The inorganic product of fusion that has cooled to a rigid condition without crystallization
Different inorganic components and their relative concentrations
Glass additives
Responsible for the different physical properties
Some added to dictate the glass structure
Some decrease manufacturing costs or provide properties such as colour, viscosity, heat resistance, and safety (colligative properties)
Glass as trace evidence
Common evidence found at many crime scenes Ability to transfer as fragments Break and enter Hit and run Assault Shootings and bombings
Chemical classes of glass
Soda-lime glass
Borosilicate glass
Lead silicate glass
Soda-lime glass
Silica, Soda Ash, limestone, and other modifying agents
Building windows, automotive windows, glass containers
Borosilicate glass
Silica and boric oxide
Thermal and shock resistance
Headlamps, cookware
Lead silicate glass
Silica and lead oxide
Pb increases refractory properties
Optical glass, crystal glass, electrical glass, ornamental
Glass manufacturing. 5 general steps
- Storage, weighing, and mixing raw materials
- Melting of the raw materials
- Forming the melt into the required shape
- Annealing of the glass (kiln)
- Warehousing and/or secondary processing
Common types of manufactured glass in forensics
Flat glass (cars, buildings)
Containers (blowing/flowing)
Fibreglass (insulation)
Specialty glass (fibre optics/semiconductors)
Tempered glass (outside cools faster, differential stress, dices)
Laminated glass (polyvinyl butyryl between layers of flat glass)
Two main methods of producing flat glass
Rolling - textures/patterns, water cooled metallic rollers
Float - Chamber with a pool of liquid tin, smooth/flat/polished
What does float glass offer chemists?
During manufacturing elements present in the furnace bricks may leech into the glass - increases with age of furnace
Contamination creates elemental signature - discrimination
Aid in reconstruction - under certain types of light the tin will fluoresce - shows top and bottom
Glass fractures
Compression on impact side, tension on the other side -> radial fractures
Opposite, secondary -> concentric fractures
4 R Rule: ridges on radial cracks are at right angles to the reverse
Sequencing can be determined
High velocity = cone shaped fracture
Glass as trace evidence - analysis
Class characteristics
Individualizations = physical match
Why is glass an ideal form of trace evidence?
- Commonly found - fragile, wide use
- Easily transferred
- Easy to recover
- Fragments usually large enough for analysis
- Chemical composition does not vary with time
- Homogeneity across pane
- Standard methods of analysis exist
- Numerical measurements - stats and probabilities
- Source variation can be detected
Basic scheme for analyzing glass evidence
Physical properties -> yes/no -> Optical properties -> yes/no -> Elemental analysis -> yes /no -> report
Any “no” - end analysis
Physical properties of glass
Colour, thickness, curvature, manufacturer markings, fluorescence, density
Why has density been replaced with RI?
Toxic liquids, time intensive, requires at least 5 mg, sample and control must be similar size and shape
Density and RI are highly correlated, can use density if can’t measure RI
Optical properties of glass
Most important for glass analysis
RI
Isotropism: discriminate between glass and not glass
ASTM E1967 Glass RI measurements
GRIM method: glass refractive index measurement
Oil immersion
1. Fragments cleaned and crushed
2. Sample place on slide in microdrop of oil, covered
3. Hot sample stage, phase contrast microscope
4. ~x160, CCD camera
5. Temperature changed until glass disappears
6. Average temperature on heating and cooling cycles, determine RI
-> 0.1 C precision, min 3 standards, separate verification standards
Glass manufacturing and forensics
Contaminants in raw materials
Manufacturing leeching
Elemental leeching
-> possible points of discrimination
Chemical composition of glass
Physical and optical can’t discriminate ICP-MS/AES LA-ICP-MS u-XRF SEM-EDS LIBS
ICP-MS/AES advantages
- Good detection limits
- MS: isotopic
- multi element
- large linear dynamic ranges
- reduced matrix (XRF)
- accepted in court
- most common
ICP-MS/AES disadvantages
- acid digestion
- contamination (prep)
- expensive (argon)
- time consuming
- challenging to operate
- destructive
- more sample required
LA-ICP-MS advantages
- reduced prep time/complexity
- no acids
- minimum sample needed
- minimizes interferences
- lower background signal
- no contamination
LA-ICP-MS disadvantages
- costly
- rare
- orientation factor
- still destructive
u-XRF advantages
- Non destructive
- low cost
- easy to operate
- operated unattended
- multi element
- more sensitive (EDS)
- check performance ahead of time
- no predetermined element list
- good for high Z elements
- fast
u-XRF disadvantages
- semiquantitative
- small irregular fragments tough to analyze
- results and repeatability -> instrument conditions and sample size/orientation
- higher detection limit
- limited # of elements
- matrix matched standards
- flat, polished embedding of sample
SEM-EDS advantages
- non destructive
- very small fragments or pulverized/embedded samples
SEM-EDS disadvantages
- limited sensitivity
- only major and minor components, not trace
- less precision
