What’s the Dirt on Glass?

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Presentation transcript:

What’s the Dirt on Glass?

What is Glass?

What is Glass? Hard Brittle Amorphous substance Made primarily of sand

Glass Manufacture Melt Form Slow Cool Ingredients Mixed

Glass Ingredients Window +Soda Bottle Glass: Sand Soda (Na2CO3)-lowers melting point Lime (CaO)-makes it water insoluble Combination of metal oxides: Ex. Sodium, calcium, magnesium, aluminum

Glass Ingredients Pyrex+Car Headlight Glass Add boron oxide Vids

Tempered or Safety Glass Car glass Dicing and little splintering Stress glass by rapid heating and cooling =annealing Glass also strong from lamination Thin piece of plastic between glass layers http://www.iihs.org/webvideo_index.html http://auto.howstuffworks.com/question508.htm

Glass and Crime Hit and Run Breaking and Entering Bullet Hole

How Can We Compare Glass? Physical Properties: Weight Volume Density (d=m/v) Color Boiling Point Melting Point

How Can We Compare Glass? Density: Mass/volume Density Gradient Tube Or weigh and calculate volume displacement in graduated cylinder

How Can We Compare Glass? Refractive Index: =light is bent as it travels from one medium to another Refractive index=velocity of light in vacuum/ velocity of light in medium

Becke Line=Bright Halo that disappears when medium and fragment have the same refractive indexes Medium index can be varied by adjusting temperature of stage

Other Methods of Comparison Physical Fit of Edges Look at Glass Ream Patterns

Comparative Studies If R.I.=1.5290 then found in 1/2000 specimens FBI has a data base of from glass manufacturers of densities and refractive indexes =Used for frequency of occurrence issues If R.I.=1.5290 then found in 1/2000 specimens If R.I.=1.5180 then found in 22/2000 specimens

Problem! Most glass is uniform in its making and show no individualistic traits!

Trace Chemical Analysis of Glass

Glass Fractures What Can We Learn?

Glass Fractures Crime Scene Reconstruction: -Force and Direction of Impact -Type of projectile (bullet or rock) Projectiles: Concentric Fractures Radial Fractures

Projectiles+Direction of Force Exit side of bullet is a deep crater 3R Rule =Radial Cracks form a Right angle on the Reverse side of force Concentric fractures originate on the force side of glass

Multiple Bullet Holes A B Which Bullet Came First?

http://www.youtube.com/watch?v=h5HM3y8d0NA