Lecture 18 - Glass Flashcards
1
Q
History of glass
A
- First human-made glass originates from 3500+ years ago!! Melted sand!!
- One of the earliest reports of glass usage as trace evidence in 1933 - linking splinters of glass in an brief case to that of a broken shop window by comparing RIs with 70 other samples
- Glass is fragile, likely to break and transfer in a controlled manner and persists long enough to be useful and recoverable
- ~10% of case work in the UK is glass evidence (greater in other countries)
2
Q
Glass Manufacturing
A
- Crystal Palace in 1851 became one of the first buildings to use glass as the main material for construction
- Glass produced by blowing cylinders, slicing them lengthwise and then flattening in an oven!!
- ~50 years later Pilkington developed the first semi-mechanical process to make flat glass by drawing between rollers from a molten source
- Inconsistent thickness, but could be strengthened by introducing wire etc. into the process
- Float glass developed in 1950s by Pilkington - Used today for the manufacture of the vast majority of flat glass
- The molten glass is delivered onto a bed of liquid tin where the glass “floats” over the metal
- Produces smooth, flat surface at large scales which can then be processed to customer needs including surface coatings.
3
Q
Glass coating
A
- Coating is a vital process in modern glass making for bespoke applications
- e.g. self-cleaning, photo-reactive, toughened etc.
- This gives it identifiable features through surface analysis
4
Q
Float glass
A
- Float glass is also generally identifiable due to side in contact with tin showing luminescence at 254nm and also a anisotropic gradient in RI in some cases.
- A unique thing for float glass is the side of the glass that touches the tin has changed ans tranferred the surface layer and it will flurese a little.
- Some glass with have post-modifications to remove/reduce this fluesecence but cheaper glasses will still most likely have this property.
5
Q
Molten tin in glass making
A
- Taking some kind of silica sand which is heated up to a very high temp to melt and mix it together and then in the tank there is a massive bath of molten tin metal.
- Molten tin is more dense than the glass material, so when the glass is poured it floats on top of it so you then end up with perfectly flat level glass
- As you flow the molten silica you get this nice flat level glass.
- It is then cooled down slowly to minimise defects like cracking. It is then divided into the shape desired.
6
Q
Glass composition
A
- Elemental composition of glass can vary based on manufacturing site and even within a single plant - complex variation!!
- Other components can be added to give specific properties other than colour:
- Boron oxide (B2O3) is added to improve heat durability in cookware, glassware and automobile headlamps
- Silver (Ag) added in sunglasses and Strontium in TV screens to absorb radiation
- Glass screens on phones are even more complex as they have had stuff added to them to make them stronger.
- Phone screens are deisnged to have defects and then it gets filled in.
- Don’t forget coatings!
- Chemistry of glass is inconsistent which makes things difficult. The orders of materials as they appear from the glass arent always consistent.
- A lot of glass evidence comes from hit and runs.
7
Q
Why is boron added to glass?
A
In order to toughen and strengthen it.
8
Q
Glass breakage and transfer
A
- Key elements in examining breakage are flexibility vs. strength of the glass and the nature of the impacting object
- An object cannot start a percussion flaw if it is too soft to do so!!
- Determination of side impact by comparison of hackle marks or rib marks
- Percussive cone more likely for projectiles impacting glass
- Transfer from a crime scene most likely in hit-and-run and ram-raids
- These produce large shards but it is usually only the small fragments that transfer on clothing.
9
Q
Recovery of glass evidence
A
- You should use oblique lighting particular on soles of shoes
- Look at transfer from river onto a seat
- Look at car for small bits of glass
- You can recover glass from taping or scraping
- Taping is good for car seats or when there’s small amounts but there will be a lot of noise.
10
Q
Analytical workflow for glass
A
- Gross examination, recovery and collection
- Preliminary evaluation of physical characteristics
- Physical fit assessment - incredibly unlikely as you’ll have lots of shards and missing pieces but sometimes it’s possible when you have larger pieces
- Microscopic Analysis - Refractive Index
- Density Measurements - In a lot of modern processes density isn’t included. It is offten done hand in hand with RI
- Elemental Analysis - SEM & XRF
- Elemetnal analysis will always be in the workflow for glass!
- We have to do these in order to get anything useful and be able to differentiate.
- Elemental Analysis - Mass Spectrometry
11
Q
Physical and microscopic examination
Glass
A
- Thickness, colour, edge comparison, fracture features
12
Q
Elemental analysis
Techniques for glass
A
- SEM-EDS
- uXRF
- ICP-MS
- LA-ICP-MS
13
Q
Physical examination of glass
Large fragments
A
- Size of the recovered fragments defines the analytical scheme employed
- Large Fragments:
- Comparison of thickness - Careful as need to take standard deviation into account
- Float glass still has variation in the thickness of glass so you need to acknowledge this.
- Comparison of colour - Remember this can be subjective!!
- Could put it into a MSP but this is unlikely as there are other methods that are more useful. Colour is good for an initial assessment.
- Matching edges - potential for physical fit assessment
- Density Comparison
- RI Measurement
14
Q
Physical examination of glass
Small fragments
A
- Confirmation it is glass!!
- Quartz & Minerals - Birefringent
- Plastic - Compresses under pressure
- Could be a small polymeric matieral
- Microscopic examination of surface fragments for distinguishing features and fluorescence etc. to identify float glass
- See if theres an outer layer.
- If we have fluorescence we will have an outer layer which will also have coatings on it.
- RI Measurement
15
Q
Density measurement
Glass
A
- Measure using a Density Gradient Column (graduated cylinder) where you put two different density materials and you add the fragment and keep adding liquid of different density until the fragment it sits in the middle and doesn’t float or sink to the top or bottom.
- Variation of the density of the liquid until the glass fragment ‘floats’
- Measuring both Density & RI gives more discrimination but many forensic labs moved to only measuring RI as standard
- In most cases there is a correlation between RI and density. However, there are some outliers so using both methods is better.
