1. Glass Evidence

2. Automobile Accidents Automobile Accidents – Windshield, head lamps Store Break-in Store Break-in – Window glass with trace evidence Suspect clothing and shoes Suspect clothing and shoes – Minute particles link suspect to crime

3. How is Glass Made? Melt at high temperature: – sand (SiO 2 ) – lime (CaO) – makes glass insoluble – soda(Na 2 CO 3 ) – lowers melting pt hard, brittle, amorphous solid

4. Major Types of Glass 1.Soda-lime glass 2.Borosilicate glass 3.Leaded glass 4.Tempered glass 5.Laminated glass 6.Bulletproof glass

5. 1. Soda-lime Glass Window glass (aka float glass) – Cooled on molten tin Bottle glass

6. 2. Borosilicate Glass Soda lime + B 2 O 3 – Low coefficient of expansion – resists thermal shock Uses – Automobile headlamps – Pyrex, – cookware and – lab glassware

7. 3. Leaded Glass PbO for CaO – denser – higher refractive index – lower melting temperature Uses – decorative

8. 4. Tempered Glass Thermally stressed Dices when broken Side and rear windows

9. 5. Laminated Glass Glass-plastic-glass sandwich Car Windshields

10. 6. Bulletproof Glass Layers -close refractive indices ¾” to 3” thick

11. Key Glass Properties Fragments Color Thickness Fluorescence Markings – striations, dimples, etc. Density Refractive Index

12. Glass as Forensic Evidence - Strategy Use glass properties to associate a glass fragment with another while minimizing or eliminating the existence of other sources

13. Flowchart for Glass Analysis

14. 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

15. Glass Densities Type of GlassDensity (g/mL) Soda lime glass (bottles)2.50 Soda lime glass (window)2.53 Borosilicate (pyrex)2.27 Leaded Glass2.98-3.01 Tempered Glass (auto glass)2.98

16. 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: – n i  sin  i = n r  sin  r

17. 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

18. Immersion Method Type of GlassRefractive Index Water1.33 Olive Oil1.467 Borosilicate (pyrex)1.47 Glycerin1.473 Castor Oil1.482 Headlamp Glass1.47-1.49 Soda lime glass (bottles)1.51-1.52 Soda lime glass (window)1.51-1.52 Tempered Glass (auto glass)1.52 Clove Oil1.543 Leaded Glass1.56-1.61 Cinnamon Oil1.619

19. Estimating the Refractive Index

20. Becke Lines

21. Match Point and Becke Line

22. 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

23. An example

24. Sequencing Patterns

25. Glass as Evidence Class Evidence physical properties chemical properties chemical composition Individual Evidence Fragments fit together and are unique in their fit

26. 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.

27. 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)!