Glass Flashcards
Why is glass a good form of trace evidence?
- It is fragile
- Likely to break and transfer in a controlled manner
- Persists long enough to be useful and recoverable
- Elemental composition can vary based on manufacturing site and even within a single plant (complex variation)
Is the process used to make most of flat glass
How is float glass made?
- The motlen 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
Why is float glass identifiable?
- Due to side in contact with tin showing luminescence at 254nm
- A anisotropic gradient in RI in some cases
- Can have post processing to get rid of the luminescence but cheap glass wont have had this done
What gives identifiable features in glass and through what type of analysis?
- The coating
- Through surface analysis
What other components can be added to give different properties in glass other than colour?
- Boron oxide (B2O3) - improve heat durability in cookware, glassware and automobile headlamps
- Silver (Ag) - added in sunglasses to absorb radiation
- Strongtium - in TV screens to absorb radiation
What are the key elements in examining breakage?
- Flexibility vs strength of the glass
- The nature of the impacting object
What is the analytical workflow when analysing glass?
- Gross examination, recovery, collection
- Preliminary evaluation of physical characteristics
- Physical fit assessment
- Microscopic analysis - refractive index
- Density measurements
- Elemental analysis - SEM & XRF
- Elemental analysis - Mass spectrometry
What is the double variation method?
The single variation but instead of fixing at a single wavelength we do it at different wavelengths
* Vary both the temperature and the wavelength in controlled manner
* Even more precise
Large and small fragments
What things will you look for in the physical examination of glass?
Large fragments
* Comparision of thickness (need to take SD into account)
* Comparision of colour (can be subjective)
* Matching edges - potental for physical fit assessment
* Density comparision
* RI measurment
Small Fragments
* Confirmation it is glass - Quartz & minerals (birefringment), plastic (compresses under pressure)
* Microscopic examination of surface fragments for distinguishing features and fluorescence (identify float glass)
* RI measurment
What is the single varition method?
- Mount in a special high RI medium above that of sample
- Fix light at a single wavelength (typically 589nm - sodium line)
- Slowly heat the sample on a hot stage
- The medium RI changes on heating much faster than the sample
- The temperature of lowest contrast is noted
- Compare to table of RI value corresponding to temperature
Advantages and disadvantages of SEM-EDX for glass analysis?
- Minimally destructive
- Can analyse tiny fragments
- Sample prep is easy
- Poor precision - variation in fragment orientation, shape, and thickness affecting the measurments and make quantitative analysis challenging
- Not a very low detection limit (1000ppm)
Advantages and disadvantages of uXRF for glass analysis?
- Uses same detection but excites using an X-Ray source - penetrates deeper
- Bulk analysis technique
- Less affected by fragment shape with a detection limit improved to 10-50 ppm
- Can measure small fragments - 100-300 um
Advantages of ICP spectroscopy analysis of glass
- More complete atomisation
- Background emission is low
- Ionisation can be high
- No oxide formation
- Minimal chemical interference
- Low self-absorption due to the high proportion of excited atoms
- Good detection limits
- Multi-element determination
- Reproducible
- Combine with mass spec - more sensitive than AAS-AES
Disadvantage of ICP spectroscopy analysis of glass
- Glass sample must be made into a solution which can take a long time
