Glass Evidence

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

How is Glass Made?

• Melt at high temperature:– sand (SiO2) – lime (CaO) – makes glass insoluble– soda(Na2CO3) – lowers melting pt

• hard, brittle, amorphous solid

Major Types of Glass

1. Soda-lime glass2. Borosilicate glass3. Leaded glass4. Tempered glass5. Laminated glass6. Bulletproof glass

1. Soda-lime Glass

• Window glass (aka float glass)– Cooled on molten tin

• Bottle glass

2. Borosilicate Glass• Soda lime + B2O3

– Low coefficient of expansion– resists thermal shock

• Uses– Automobile headlamps– Pyrex, – cookware and – lab glassware

3. Leaded Glass• PbO for CaO– denser– higher refractive index– lower melting temperature

• Uses– decorative

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

windows

5. Laminated Glass

• Glass-plastic-glass sandwich

• Car Windshields

6. Bulletproof Glass

• Layers -close refractive indices• ¾” to 3” thick

Key Glass Properties

• Fragments• Color• Thickness• Fluorescence• Markings – striations, dimples, etc.• Density• Refractive Index

Glass as Forensic Evidence - Strategy

Use glass properties to associate a glass fragment with another while minimizing or eliminating the existence of other sources

Flowchart for Glass Analysis

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

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

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

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

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

Estimating the Refractive Index

Becke Lines

Match Point and Becke Line

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

An example

Sequencing Patterns

Glass as Evidence

• Class Evidence• physical properties• chemical properties• chemical composition• Individual Evidence• Fragments fit together and are unique in their fit

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.

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)!