Chapter 15 Glass Evidence By the end of this chapter you will be able to:

Transcription

1 1 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Glass Evidence By the end f this chapter yu will be able t: 15.1 Describe the three majr cmpnents f glass Cmpare and cntrast sda glass, lead glass (crystal), and heat-resistant glass List and describe the physical prperties f glass Calculate the density f glass samples. 2 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

2 Glass Evidence By the end f this chapter yu will be able t: 15.5 Estimate the refractive index f glass using the submersin methd and Becke lines Distinguish between radial and cncentric fractures in terms f their appearance, hw they are frmed, and their lcatin n fractured glass Summarize and describe the infrmatin that can be gained by analyzing bullet hle(s) in fractured glass. 3 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Glass Evidence By the end f this chapter yu will be able t: 15.8 Cmpare and cntrast laminated, tempered r safety glass, and bullet-resistant glass in terms f structure, use, and fracture pattern Describe hw t prperly cllect and dcument glass evidence Summarize the ways t determine whether tw glass fragments are cnsistent. 4 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

3 Vcabulary amrphus lead glass (crystal) backscatter nrmal line bullet-resistant ( bulletprf ) glass cncentric fracture density glass laminated glass radiating fracture refractin refractive index silicn dixide tempered glass 5 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Intrductin Glass evidence can be fund at many crime scenes. Autmbile cllisins Stre break-ins Assaults Glass is typically cnsidered class evidence. Mass prduced Only with fracture match can be individual evidence 6 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

4 Glass as Frensic Evidence Glass is frensically imprtant It is fund at mst crime scenes It can be carried away frm scene undetected It is stable, des nt decay As class evidence it cntributes t the pl f evidence Imprtant t cllect prper knwn samples s incidental glass can be eliminated Frensic Science: Fundamentals & Investigatins, 2e 7 Cengage/NGL/Suth-Western 2016 What is Glass? Glass is a hard, amrphus, brittle material made by melting the fllwing materials at very high temperatures: Silicn dixide, r silica Calcium xide Sdium xide 8 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

5 Flat Glass Flat glass r sda lime glass Windws, picture frames, flat glass bjects SiO 2 with sdium carbnate (Na 2 CO 3 and calcium xide CaO) 9 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Laminated Glass Tw r mre panes f glass bnded by a plastic middle layer are called laminated glass. Resists breakage 10 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

6 Tempered Glass Glass that has been subjected t extreme temperatures r chemical treatments t imprve its strength is called tempered glass, r safety glass. Same chemical makeup as sda lime glass When brken this glass fractures int small blunt pieces. 11 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Bullet-resistant Glass Bullet-resistant ( bulletprf ) glass is a laminated and tempered glass cmpsed f tw layers, ne hard and ne sft. 12 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

7 Other Types f Glass Brsilicate glass- lab glassware, kitchen items (Pyrex) SiO 2 with Brn Tinted glass- has clrants that reduce glare / heat penetratin r fr decrative use SiO 2 with additives/clrants Leaded glass (crystal)- wine glasses, fine dining, figurines SiO 2 with lead xide 13 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Thickness 14 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

8 Density 15 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

9 Refractive Index Light changes directin as it passes frm ne medium, such as air, int anther medium, such as water. The change in directin is called refractin. The change in directin is because the light travels at a different speed thrugh the tw mediums. 17 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Refractive Index (cntinued) The perpendicular line that frms at the interface f tw media is called the nrmal line. In this figure, as the beam f light mves frm the air int the glass, the light slws dwn and bends tward the nrmal line. 18 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

10 Refractive Index (cntinued) In this figure, light speeds up as it mves frm the il int the air. The beam f light bends away frm the nrmal line. 19 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Refractin 20 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

11 Frensic Use f Refractive Index The submersin methd is used t determine the refractive index f small glass fragments. 21 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Becke Lines If a submerged glass fragment has a refractive index that differs frm the liquid medium that it is suspended within, a hallike ring appears arund the edge f the glass. This hal-like effect is called a Becke line. 22 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

12 23 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Refractive Index Determinatin f a Small Glass Fragment Crime labs als use a ht stage micrscpe that allws determinatin f refractive index Use liquid with slightly higher RI than the glass As liquid heats, RI decreases When glass disappears, the RI is determined Ht Stage Micrscpe Frensic Science: Fundamentals & Investigatins, 2e 24 Cengage/NGL/Suth-Western

13 Analyzing Brken Glass Brken glass analysis is frensically imprtant t the recnstructin f events in a criminal act Analysis can include: The sequence f the fractures rder f events The directin f the frce which caused the fracture did the break ccur frm the inside r utside Frensic Science: Fundamentals & Investigatins, 2e 25 Cengage/NGL/Suth-Western 2016 Radial and Cncentric Fractures Radial fracture lines start at the pint f impact and mve utward. 26 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

14 27 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Sequencing Fractures Fracture lines: Radial and Cncentric Radial fracture lines ccur first extending utward frm the break pint, prduced when the ppsite side f impact fails first Cncentric fracture lines frm a circle abut the break pint and are prduced by the side f impact failing first A B Cncentric Fractures Radial Fractures Frensic Science: Fundamentals & Investigatins, 2e 28 Cengage/NGL/Suth-Western

15 Sequencing Fractures Radial lines always end in existing radial lines Order in pht: A->B->C A B C 29 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 What Are The Pssible Fracture Sequences? It can be nted: B was first C was after E E was after A D was after B B D Pssible Sequences: B, D, A, E, C B, A, D, E, C B, A, E, D, C B, A, E, C, D A C E Frensic Science: Fundamentals & Investigatins, 2e 30 Cengage/NGL/Suth-Western

16 Bullet Fractures Bullet hles in glass can be distinguished frm ther impacts by the greater number f cncentric fractures that result frm high-velcity impact. 31 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Evidence frm Bullet Fractures The angle at which a bullet enters windw glass can help determine the psitin f the shter. 32 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

17 Heat Fractures Heat fracturing prduces breakage patterns n glass that are different frm breakage patterns caused by an impact. Wavy fracture lines develp n glass that has been expsed t high heat. Glass tends t crack n the surface expsed t the higher temperature. 33 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Scratches Windshield wipers may leave unique scratches n a windshield. 34 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

18 Scratches (cntinued) Any dirt r hard particles embedded in the rubber insulatin f a car windw may leave scratch marks when the windw is pened and clsed. 35 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Cllectin and Dcumenting Glass Evidence Crime-scene investigatrs shuld initially separate cllected glass by physical prperties including: Size Clr Texture 36 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

19 Frensic Glass Analysis Technlgy Scanning electrn micrscpic (SEM) X-ray flurescence (XFR) Inductively cupled plasma-mass spectrmetry (ICP-MS) 37 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Summary Glass is an amrphus slid usually made frm silica, calcium xide, and sdium xide. Because mst glass is mass-prduced, it is difficult t prve that tw different glass pieces came frm the same surce. Glass is almst always class evidence. Laminated glass is used in windshields. Tempered glass is used in side and back windws. These types f glass prtect passengers frm the dangers f flying shards. Bulletprf glass is laminated and tempered. Characteristics that prvide class evidence f glass include the type f glass, its density, its thickness, and its refractive index. 38 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

20 Summary (cntinued) The density f an bject is calculated by dividing its mass (usually in grams) by its vlume (usually in milliliters). The refractive index f a material is a measure f hw much light bends, r refracts, as it travels thrugh that material. Light will bend, r refract, when it mves between media with different refractive indexes. The submersin methd can be used t estimate the refractive index f a glass sample. 39 Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western 2016 Summary (cntinued) 40 The psitin f a Becke line is anther methd used t estimate the refractive index f a piece f glass. When glass is hit, it first stretches, and then breaks, frming radial fracture and cncentric fracture patterns. Radial fracture patterns ccur n the side f the glass ppsite the pint f impact. Cncentric fracture patterns ccur n the same side as the pint f impact. Fire fractures in glass appear wavy and tend t appear n the surface expsed t higher temperatures. Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western

21 Summary (cntinued) 41 When a bullet is sht thrugh glass, the exit hle is larger than the entry hle. The angle f entrance f a bullet int glass can be estimated frm the pattern f glass fragments left near the bullet s exit hle. Frensic glass-analysis methds include fracture matching, scanning electrn micrscpy f headlight filaments after cllisins, and the use f X-ray flurescence (XRF) and inductively cupled plasmamass spectrmetry (ICP-MS), which help determine prperties f glass evidence samples. Frensic Science: Fundamentals & Investigatins, 2e Cengage/NGL/Suth-Western