Glass
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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