Chapter 14 Glass Evidence

Transcription

1 Chapter 14 Glass Evidence By the end f this chapter yu will be able t: Explain hw glass is frmed List sme f the characteristics f glass Prvide examples f different types f glass Calculate the density f glass Use the refractive index t identify different types f glass Describe hw glass fractures Analyze glass fracture patterns 1 All Rights Reserved Suth-Western / Cengage Learning 2012, 2009 Frensic Science: Fundamentals & Investigatins, Chapter 14

2 Intrductin and Histry f Glass Egypt circa 2500 B.C. The earliest knwn human-made glass bjects (beads) 1st Century B.C. glass blwing begins 13th Century specialized glass prductin was an art, a science, and a state secret in the republic f Venice 2 Frensic Science: Fundamentals & Investigatins, Chapter 14

3 Intrductin and Histry f Glass 14th Century glass-making spreads thrugh Eurpe The industrial revlutin applies mass prductin t many types f glass Analysis f glass fund at a crime scene can yield trace evidence 3 Frensic Science: Fundamentals & Investigatins, Chapter 14

4 What Is Glass? Materials used t create (sda-lime) glass: Silicn dixide (SiO 2 ) Sdium xide (Na 2 0) Calcium xide (CaO) Once cled, glass can be plished, grund, r cut 4 Frensic Science: Fundamentals & Investigatins, Chapter 14

5 What Is Glass? Crystalline slids have a regular atmic structure Glass is an amrphus slid and s has an irregular atmic structure Therefre, glass breaks in a variety f fracture patterns 5 Frensic Science: Fundamentals & Investigatins, Chapter 14

6 Types f Glass Many types f glass Adding metal xides yields different clrs Different densities Refractin indexes These characteristics allw cmparisns 6 Frensic Science: Fundamentals & Investigatins, Chapter 14

7 Density The frmula fr calculating density is: D = m / V m = mass, measured by a balance beam device V = vlume, place the glass fragment int a beaker filled with water and measure the verflw D = density, divide the mass (in grams) by the vlume (in milliliters) 7 Frensic Science: Fundamentals & Investigatins, Chapter 14

8 Density Cmmn Examples Type f Glass Density (g/ml) Bttle glass 2.50 Windw glass 2.53 Lead crystal Pyrex 2.27 Tempered (aut) 2.98 Flint 3.70 Crwn Frensic Science: Fundamentals & Investigatins, Chapter 14

9 Refractive Index 9 When a beam f light mves frm ne medium int anther: The speed changes The directin bends Refractive Index a tl used t study hw light bends as it passes frm ne substance t anther Nrmal line is perpendicular t the glass surface Frensic Science: Fundamentals & Investigatins, Chapter 14

10 Refractive Index When a beam f light mves frm less dense medium (air) int a mre dense medium (water): Its speed slws, and Bends light tward the nrmal line 10 Frensic Science: Fundamentals & Investigatins, Chapter 14

11 Refractive Index When a beam f light mves frm a mre dense medium (glass) int a less denser medium (air): Its speed increases And bends light away frm the nrmal line 11 Frensic Science: Fundamentals & Investigatins, Chapter 14

12 Applicatin f Refractive Index t Frensics Submersin methd used when glass fragments fund at the crime scene are small 12 Frensic Science: Fundamentals & Investigatins, Chapter 14

13 Applicatin f Refractive Index t Frensics Place the glass fragment int different liquids f knwn refractive indexes The glass fragment will seem t disappear when placed in a liquid f the same refractive index 13 Frensic Science: Fundamentals & Investigatins, Chapter 14

14 Applicatin f Refractive Index t Frensics Becke Line a hal-like effect appearing at the edges f a glass fragment when the reflective index f the glass and liquid are different If the line is inside the glass perimeter, the glass index is higher than the index f the liquid If the line is utside the glass perimeter, the glass index is lwer 14 Frensic Science: Fundamentals & Investigatins, Chapter 14

15 Fracture Patterns in Brken Glass Being an amrphus slid, glass will nt break int regular pieces with straight line fractures Fracture patterns prvide clues abut the directin, rate, and sequence f the impacts 15 Frensic Science: Fundamentals & Investigatins, Chapter 14

16 Why Radial and Cncentric Fractures Frm 16 Impacted glass is cmpressed n the side it is hit. It will stretch n the ppsite side f the glass, and the tensin there will radiate breaks in the glass utward frm the pint f impact. Then fractures frm in the shape f cncentric circles n the same side f the impact. Frensic Science: Fundamentals & Investigatins, Chapter 14

17 Why Radial and Cncentric Fractures Frm Glass after an impact shws radial fractures (red) and cncentric circle fractures (blue) 17 Frensic Science: Fundamentals & Investigatins, Chapter 14

18 Why Radial and Cncentric Fractures Frm 18 Frensic Science: Fundamentals & Investigatins, Chapter 14

19 Bullet Fractures 19 As a bullet passes thrugh glass, it pushes a cne shaped piece f glass ut f the glass ahead f it The exit side f the hle is larger than the entrance side f the hle Radiating fracture lines frm a subsequent sht will stp at the edge f the fracture lines already present in the glass Frensic Science: Fundamentals & Investigatins, Chapter 14

20 Path f a Bullet Passing thrugh Windw Glass perpendicular t the glass sht frm the left sht frm the right 20 The angles at which bullets enter windw glass help lcate the psitin f the shter Bits f the glass can fly backward (backscatter), creating trace evidence Frensic Science: Fundamentals & Investigatins, Chapter 14

21 Handling f Crime Scene Glass Samples 1. Identify and phtgraph any glass samples befre mving them. 2. Cllect the largest fragments that can be reasnably cllected. 3. Identify the utside and inside surface f any glass. 4. If multiple panes are invlved, make a diagram. 5. Nte trace evidence such as skin, hair, bld, r fibers. 6. Package all materials cllected t maintain the chain f custdy. 21 Frensic Science: Fundamentals & Investigatins, Chapter 14