L6 - Glass evidence

Evidential value, discrimination between diff glass types

Where does glass appear at a crime scene?

• Window/vehicle glass at point of entry​

• Float: Fragmentary, backscattered​ (sharp/angular)

• Toughened: Cuboid​

• Laminated: Fragmentary; impact cracks

• Container/ornament or mirror glass broken during the crime​

• Glass picked out by an intruder and discarded at or removed from the scene​

What happens when a window is broken?

• large pieces of glass travel in the direction of the blow​.

• The glass flexes as it breaks and tiny fragments are thrown backwards towards the breaker of the window​

• The fragments may lodge in the hair, clothing and shoes of the person​

Where does glass recovered from mostly a suspects upper parts of the body place them?

In vicinity of breaking glass at the time it was broken

= mainly upper body parts/hair combings

Where does glass from a suspects lower body parts place them?

At the scene of broken glass after it was broken.

lower body parts = walking through scene after

How does proximity affect the evidential value of glass?

The closer to the window the breaker stands, the greater the chance of finding glass on clothing.

How does persistence affect the evidential value of glass?

he shorter the time delay between breaking and recovery of suspect’s clothing the better​.

Which types of garments retain glass better?

Coarser garments retain glass better than smooth fabrics.

e.g. knitted pullover has greater retention than a woven polyester shirt

Which size of glass fragments are retained more readily?

Smaller glass fragments are retained more readily than larger glass fragments

What size are the glass fragments recovered from hair and clothing?

generally 0.25 - 1 millimetre

When is glass lost fairly rapidly dependent one?

• activity of the wearer​

• texture of clothing.

Where can glass fragments be trapped in?

• Pockets​

• Crevices on shoe uppers​

• Embedded in shoe soles ​

How many pieces of glass fragments are found in general everyday wear?

0 -2 fragments

Finding 10,30 or 100 indicates close proximity to breaking glass

Which types of glass provides a more stronger evidential link?

Imported old glass, patterned, coloured, toughened may be more uncommon and provide a stronger link.

How does location of glass fragments impact the evidential value?

• Glass in hair combings is stronger evidence than in soles of footwear​

Which aspects of glass are not as important?

• The physical or chemical aspects of the actual glass are not as important as the location of the glass on the suspect and the circumstances of the case​

What are the outcomes of glass evidence evidentially?

• Negative​

• Corroborative​

• The value can vary from a very weak link to a strong indication that the person was close to the pane when it was smashed​

• The shorter the time delay between breaking and recovery from suspect’s clothing, the better​

What other types of evidence can be recovered from glass?

• Glass fragments themselves​

• Fingerprints​

• Footwear marks​

• Fingerprints​

• Hairs and fibres​

• Blood​

• Glove Prints​

What physical aspect of glass should be found?

• Physical match of broken fragments​

• Fracture analysis (flat glass) ​

• Nature of force​

• Point of contact ​

• Location on pane​

• Direction of force​

• Radial and concentric breaks​

• Wallner or concentric lines​

What are macroscopic examinations of glass?

• When fragments sufficiently large​

• Gross features ​

• Colour​

• Thickness​

• Flat or curvature?​

• Evidence of manufacture?​ (bottles)

How would you discriminate between different glasses?

• freshly broken glass​

• Has fresh breaks (clean and sharp-edge) ​

• recovered from hair combings ​

• or the surfaces of garments, ​

• compared with reference samples of broken glass from the scene​

• Refractive index (RI) measurements ​

• Compositional Analysis ​

What is the refractive index (RI)?

a measure of how much light bends, or refracts, when it passes from a vacuum into a specific material

What can the refractive index of glass tell you?

• can distinguish between a large number of different glasses. ​

• If the recovered fragments differ in refractive index from a reference glass sample, then they could not have originated from the same piece of glass.

• RIs occur more commonly in some types of glass than in others​

• Can be used to assess the rarity value and therefore the evidential significance of the glass in this particular case

What is the refractive index of glass?

1.5

Glass fragments - clove oil. Heating it changes RI glass edges should disappear

What is the refractive index of water?

1.33

What is the refractive index of air?

1.0

What are the microscopic techniques developed to determine RI of small glass fragments?

• Based on RI of liquids change as heated and cooled, while solids less temperature sensitive​

• Based on examination of fragment in oil (e.g. clove) on heated stage​

• clove oil. Heating it changes RI glass edges should disappear

• Measured the disappearance temperature of glass fragment​

What is foster & freeman GRIM?

Automated approach to measurement of glass RI

What is GRIM3?

• Based on oil immersion/temperature variation method​

• operates through a standard laboratory microscope ​

• phase contrast optics ​

• Mettler hotstage, for temperature control.​

• By varying temperature to alter the refractive​

• index of a calibrated oil, the RI of an immersed​

• fragment of glass can be determined ​

• Point of null refraction (point at which the refractive indices of glass and immersion oil match) - measure critical points.

• Better for excluding matches

What is compositional analysis?

• Derived from glass raw materials​

• can be used to distinguish between glass samples which have the same refractive index but different chemical composition​

• Measurement of major, minor and trace elements

What does elemental analysis of glass put categories in?

• Sheet (or float) ​

• Container ​

• Vehicle window​

• Vehicle headlamp (borosilicate glass)​

• Table glass (including lead crystal) ​

What are the non destructive options for analysis of glass?

• SEM/EDX or WDX​

• LA ICP-MS​

What is the issue when using SEM/EDX?

Wont pick up sodium (light elements)

What are the destructive methods for analysis of glass?

• AA​

• ICP-AES​

• = requires putting glass in solution

What is the gold standard for elemental analysis of glass?

= ICP-MS

• Inductively Coupled plasma-mass spectrometry​

• Combination of ICP technique with a quadrapole mass spectrometer​

• Separates ions according to mass-to-charge ratio​

• Laser ablation (LA) ability to analyse direct from solid

What is the most used technique for elemental analysis of glass?

• Laser Ablation Inductively Coupled Plasma Mass Spectrometry LA-ICP-MS

• Minimal sample handling​

• Sample in µg range​

• Non-destructive​

• Multielement ​

• Significantly lower detection limits compared to EDX/XRF​ = low trace elements - better to differentiate

What are the X-ray methods for elemental analysis of glass?

• Includes XRF, SEM/EDX, SEM/WDX​

• Provides elemental analysis​

• Can be used on small fragments​

• Non destructive​

• Limitations​

• Light elements​

• Variable detection limits​

How would you measure density of glass?

• Mass volume calculation​

• Traditional method uses liquid displacement to calculate volume​

• Large errors on small fragments​

• Flotation method​

• Reference glass particle​

• Immersed in liquid (bromoform and bromobenzene)​

• Composition of liquid adjusted until glass particle remains suspended​

• Another glass particle of same density will also be suspended in liquid​

• If different it will either sink or float​

• Obsolete methodology​

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