Glass Flashcards
What is the elemental composition of glass?
The elemental composition of glass can vary based on the manufacturing site and even within a single plant - complex variation.
Boron oxide - added to improve heat durability in cookware
Silver - added in sunglasses and strontium in TV screens to absorb radiation.
Glass breakage and transfer
Key elements in examining breakage are flexibility vs strength of the glass and the nature impacting object
Transfer from a crime scene is most likely in hit and run and ram raids. These will produce large shards however it is the small fragments that transfer onto clothing.
Recovery of glass
Use an oblique torch
check soles of shoes and drivers seat in a hit and run
taping and scrapings of glass
use forceps
collect in vial and then bag it
What is the analytical workflow for glass?
Gross examination, recovery and collection
Preliminary evaluation of physical characteristics
Physical fir assessment
microscopic analysis - refractive index
Density measurements
Elemental analysis - SEM & XRF
Elemental analysis - mass spectrometry
What analytical scheme is used for large glass fragments?
Comparison of thickness - careful as you need to take standard deviation into account
comparison of colour - this can be subjective
matching edges can be used for physical fit assessment
density comparison
RI measurement
What analytical scheme is used for small glass fragments?
confirm it is glass at first
(quartz and minerals - birefringent)
(plastic - compresses under pressure)
microscopic examination of surface fragments for distinguishing features and fluorescence e.g to identify if it is float glass
RI measurement
what is a density measurement?
Variation of the density of the liquid until the glass fragment “floats”
measuring both density and the RI gives more discrimination but many forensic labs only measure RI
