Glass Flashcards
What are some properties of glass that make it good as evidence?
- glass is fragile, likely to break and transfer in a controlled manner and persists long enough to be useful and recoverable
How has the manufacturing of glass developed over the years?
- 1851 crystal palace
- this glass produced by blowing cylinders, slicing them lengthwise and then flattening in an oven – over and over again for whole building – impractical and hard work
- 50 years later Pilkington glass developed the first semi-mechanical process to make flat glass by drawing between rollers from a molten source
- this glass had 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 (around 90 %)
How is float glass manufactured?
- limestone soda ash or silicate sand heated to very high temperature to melt things together and homogenise them
- the molten glass is delivered onto a bed of liquid tin (denser than glass) where the glass “floats” over the metal
- it produces smooth, flat surface at large scales
- cooled down slowly in order to reduce number of defects often by air to minimise impact e.g. cracking
- after scanned for imperfections, divided into shape that is wanted and specific dimensions
- taken out and then processed to customer needs including surface coatings
- coating is vital process in modern glass making for bespoke applications e.g. self cleaning, photo-reactive, toughened
In what two ways is float glass generally identifiable?
- coatings give it identifiable features through surface analysis which allows us to differentiate
- float glass is also generally identifiable due to side in contact with tin showing luminescence at 254nm and also an anisotropic gradient in RI in some cases (unless been removed in post processing)
What is the elemental composition of glass?
What are other components that can be added to give specific properties?
- basic glass: silica, calcium and sodium
- within manufacturing process, tried to make consistent but won’t always be consistent - a lot of complexity and variations
- can vary based on manufacturing site and even within a single plant
- boron oxide (B2O3) - improve heat durability in cookware, glassware and automobile headlamps
- silver (Ag) - added in sunglasses
- strontium - in TV screens or mobile phones to absorb radiation
What are the key elements in examining breakage of glass?
- flexibility vs strength of the glass
- nature of impacting object
When is a percussive cone more likely?
- more likely for projectiles impacting glass
When is transfer of glass from a crime scene most likely?
- hit-and-run and ram-raids
- usually only small fragments that transfer on clothing
What is the analytical workflow for glass evidence?
1 - gross examination, recovery, and collection
- investigate using oblique lighting
- recover through scraping/taping/forceps
2 - preliminary evaluation of physical characteristics
(physical fit assessment is incredibly unlikely as really difficult to put jigsaw back together – will have lots of shards of glass)
3 - microscopic analysis – refractive index
4 - density measurements
5 - elemental analysis – SEM & XRF
6 - elemental analysis – mass spectrometry
- elemental analysis must be done in glass analysis in order to get anything useful from glass to be able to use to differentiate
What is the process in the physical examination of glass?
- size of recovered fragments defines the analytical scheme employed:
- large fragments:
1 - comparison o thickness (careful as need to take standard deviation into account)
2 - comparison of colour - subjective
3 - matching edges for physical fit assessment - more likely for larger fragments
4 - density comparison
5 - RI measurement - small fragments
1 - confirmation it is glass: might be quartz and minerals (check birefringence) or plastic (compresses under pressure)
2 - microscopic examination of surface fragments for distinguishing features (float glass) and fluorescence (if has fluorescence then has coatings = distinguishable)
3 - RI measurement
How is density measurement done?
- density gradient column
- vary the density of the liquid until glass fragment floats
- gives most distinguishing information and best repeatability
What can be said about measuring both density and RI?
- it gives more discrimination but many forensic labs moved to only measuring RI as standard
- the two are correlated but RI can be more precise so can get more discrimination
- if going to do one just do RI
What method is used to measure RI of glass?
- Becke line or single variation methods are not enough so use double variation methods (vary both the temperature and the wavelength in a controlled manner)
1 - temperature is fixed and wavelength varied until a match is found
2 - temperature is increased by 5 degrees and process repeated to find new matching wavelength
3 - data plotted on Hartmann net - wavelength at match temperature are plotted on net and RI read off
4 - equation of straight line established and converted to RI based on calibration data of immersion liquid (more precise)
What can be said about surface and bulk glass?
- RI of float glass surface will be different to bulk
- bulk is usually lower due to enrichment with tin oxide unless it has been manipulated or coated or strange material
- this offers an additional aspect of discrimination if have surface and bulk
What is the amount of discrimination like if you are comparing RI of a known and unknown?
- unless can be absolutely certain got BOTH bulk and surface glass then unlikely will get enough discriminatory information
- to be able to discriminate between glass samples is very low - 10 % mark
- to do with complexities of glass
