Lecture 17- Glass Flashcards
what was the first type of glass made from
melted sand
why can glass be a good source of trace evidence
it is fragile so likely to break and transfer
it is also able to persist for a long time enough to be recoverable
how is the flatness of glass controlled in the manufacturing of it
production of float glass - molten glass floats on a molten tin bath to become flat
can be controlled thickness, flat, smooth surfaces
gives identifiable features through surface analysis
how is float glass normally identified
the side that was in contact with the tin shows a luminescence at 254nm and can have a anisotropic gradient in the refractive index
but in the manufacturing this luminescence can be removed
how does the composition of glass vary
depending on the manufacturer the mixture and ratio of components changes
this addition of things for colour and strengthening
the order and amount of things added is likely to be different
presence of coatings
what is added to glass to improve heat durability and where is this often seen
boron oxide (B2O3)
seen in cookware, glasses and car headlamps
what is added glass to absorb radiation and where is this often seen
silver (Ag) in sunglasses
strontium in TV screens
what are the 3 properties of glass associated with the possibility of it breaking
flexibility
strength
nature of the impacting object (force and shape)
how can a side impact on glass be determined
by hackle or rib marks
what shape is often seen when a projectile impacts glass
a percussive cone
when is glass transfer often seen in crime scenes
during hit and runs and ram-raids
do the smaller or larger fragments of glass tend to be transferred to clothing and found in investigations
smaller ones
more likely to see if a big chunk of glass is on your clothing
what are the three ways normally used to recover glass shards
scraping
taping
forceps
what is the analytical workflow for glass evidence
- gross examination recovery and collection
- prelim evaluation of physical characteristics
- physical fit assessment
- microscopic analysis - refractive index
- density measurements (often not included now)
- elemental analysis with SEM, XRF (non destructive)
- mass spec elemental analysis (destructive)
why is physical fit assessment matches very unlikely with glass evidence
therefore what type of information is heavily relied on from glass evidence
harder with smaller pieces of glass
glass can chage shape slightly - upon heat
rely on elemental analysis
what defined the analytical scheme employed in the analysis of glass
the size of the recovered fragment
large vs small
what is analysed for larger glass fragments
thickness - take an average and standard deviation
colour - can be subjective though
matching edges - physical fit
density
RI measurement
what is analysed for smaller glass fragments
confirm it’s glass
examine surface for distinguishing features
how can the analysis of colour be made less subjective
by using MSP - microspectrophotometry
what can be looked at to see if the sample is glass or a type of quartz/mineral
the birefringence
what can be looked at to see if the sample is glass or a type of plastic
see if it compresses under pressure if breaks = glass, if compresses = plastic
how is the density of a piece of glass measured
using a density gradient column - vary the density of the liquid and see when the glass fragment remains even - doesn’t sink or float
not most accurate but get a rough idea
combine with RI measurement to be more discriminative
when measuring the RI of glass what method is normally used and why
the Single or double variation method as it is more precise than the Becke Line test
how is the single variation method carried out
how does the double variation method differ from this
- mount sample in a medium with higher RI
- fix light at a certain wavelength e.g 589nm
- slowly heat sample on a hot stage
- the medium’s RI will change quicker than the samples when heating
- temp of lowest contrast between sample and media is recorded computationally
- compare to a table of known RI values
double variation method varies temp and wavelength of light = even more precise
