Glass Evidence. Automobile Accidents Automobile Accidents – Windshield, head lamps Store Break-in...
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Transcript of Glass Evidence. Automobile Accidents Automobile Accidents – Windshield, head lamps Store Break-in...
![Page 1: Glass Evidence. Automobile Accidents Automobile Accidents – Windshield, head lamps Store Break-in Store Break-in – Window glass with trace evidence Suspect.](https://reader035.fdocuments.in/reader035/viewer/2022062304/56649ef35503460f94c05988/html5/thumbnails/1.jpg)
Glass Evidence
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Glass Evidence
• Automobile Accidents– Windshield, head lamps
• Store Break-in– Window glass with trace evidence
• Suspect clothing and shoes– Minute particles link suspect to
crime
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How is Glass Made?
• Melt at high temperature:– sand (SiO2) – lime (CaO) – makes glass insoluble– soda(Na2CO3) – lowers melting pt
• hard, brittle, amorphous solid
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Major Types of Glass
1. Soda-lime glass2. Borosilicate glass3. Leaded glass4. Tempered glass5. Laminated glass6. Bulletproof glass
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1. Soda-lime Glass
• Window glass (aka float glass)– Cooled on molten tin
• Bottle glass
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2. Borosilicate Glass• Soda lime + B2O3
– Low coefficient of expansion– resists thermal shock
• Uses– Automobile headlamps– Pyrex, – cookware and – lab glassware
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3. Leaded Glass• PbO for CaO– denser– higher refractive index– lower melting temperature
• Uses– decorative
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4. Tempered Glass• Thermally stressed• Dices when broken• Side and rear
windows
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5. Laminated Glass
• Glass-plastic-glass sandwich
• Car Windshields
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6. Bulletproof Glass
• Layers -close refractive indices• ¾” to 3” thick
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Key Glass Properties
• Fragments• Color• Thickness• Fluorescence• Markings – striations, dimples, etc.• Density• Refractive Index
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Glass as Forensic Evidence - Strategy
Use glass properties to associate a glass fragment with another while minimizing or eliminating the existence of other sources
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Flowchart for Glass Analysis
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Density of Glass
• Usually a class characteristic – not an individual characteristic
• Can be used to exclude glass from other sources
• Each type of glass has a characteristic density• D = M/V
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Glass Densities
Type of Glass Density (g/mL)Soda lime glass (bottles) 2.50Soda lime glass (window) 2.53Borosilicate (pyrex) 2.27Leaded Glass 2.98-3.01Tempered Glass (auto glass) 2.98
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What and Why - Refractive Index
• Refraction = bending of light as it passes across the boundary between two media.
• caused by the change in wave speed when it changes medium
• Snell’s Law: – ni sini = nrsinr
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Determining Refractive Indices
• Immersion Method– Place glass in media with known “n’s”
• Becke Line – microscopic investigation
• Match Point– Place glass in liquid and vary temperature– Becke Line• halo that disappears at match point
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Immersion MethodType of Glass Refractive IndexWater 1.33Olive Oil 1.467Borosilicate (pyrex) 1.47Glycerin 1.473Castor Oil 1.482Headlamp Glass 1.47-1.49Soda lime glass (bottles) 1.51-1.52Soda lime glass (window) 1.51-1.52Tempered Glass (auto glass) 1.52Clove Oil 1.543Leaded Glass 1.56-1.61Cinnamon Oil 1.619
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Estimating the Refractive Index
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Becke Lines
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Match Point and Becke Line
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Fracture Patterns • When glass breaks – “3R’s”– Primary fracture patterns
• Form on opposite (reverse) side from point of impact
• Radiate from point of impact • Forming right angles
– Secondary fracture patterns• Form on same side as point
of impact• Form concentric circles
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An example
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Sequencing Patterns
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Glass as Evidence
• Class Evidence• physical properties• chemical properties• chemical composition• Individual Evidence• Fragments fit together and are unique in their fit
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Collecting Glass Evidence• Are the fragments from same or different sources?
• Obtain reference samples if possible
• Collect in pill bottles, bindles or large cardboard evidence boxes.
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When Collecting – Remember!•Collect each object separately• Label inside and outside• Include a reference sample • Sample various parts and note their locations• Other trace evidence may be present (blood, fiber, hair, etc)!