ch4: glass

Crystalline Solid

A structure where the atoms are arranged in a regular pattern; a crystalline solid

• EX) Diamonds and graphite

Amorphous Solid

GLASS IS AN AMORPHOUS SOLID 

Solids with no set pattern

• Brittle

• Shows conchoidal (circular) fracture pattern

  • When glass breaks, impact spot is moving in a circular pattern from said spot 

• Vary in hardness and elasticity depending on shape since their pattern is not the same all throughout 

• Primarily composed of silica dioxide (SiO2) & other elemental oxides

Soda-Lime glass

One of the 5 types of glass

• Most common commercial glass (90% of total production)

• Plate and window glass, electric light bulbs

• Least expensive

• Oldest type of glass: Traces back to the Mesopotamia (2’500 B.C.), common in Egypt & Rome

Composed of…

• Sand (silica, SiO2)

• Soda (Na2O)

  • Reduces melting point; easier to work with 

• Lime (CaO) 

  • Insoluble in aqueous solution

Uses…

• Windows

• Glasses

• Containers

Borosilicate Glass

One of the 5 types of glass

• Strong

• Heat resistant 

• Contains boron oxides (B2O3)

  • Better at handling temperature changes

USES

• Beaker glass

• flasks 

Tempered Glass

One of the 5 types of glass

• Made stronger than soda-lime glass by rapid heating and cooling 

• When broken; they break into a lot of smaller pieces that are not AS sharp (still sharp); the entire piece of glass breaks TOGETHER

USES

• Side & rear windows in cars

• Shower/backdoors

  • More safe in case it shatters

Laminated Glass

One of the 5 types of glass

Two pieces of glass glued to a piece of plastic between & some resin

• If the glass were to break, it’s shards wouldn’t fly all over the place; more SAFE and SECURE 

USES

• Windshields 

Soda-Lead Glass

One of the 5 types of glass, also known as crystal or lead glass

• Expensive

• Heavy (duh, lead)

• Contains a small piece of lead that can bounce light across different services; can look very shiny

  • Lead increases density + refractive index is greater (sparkle)

USES

• Finest tableware

• Art objects

• Radiation protection

Jigsaw Fit

If a bunch of pieces of glass fit together, it is individual evidence 

Physical Characteristics Making up CLASS Evidence (6)

1) Fractures: How well do glass fragments match with one another

2) Color

3) Thickness

4) Fluorescence: Different kinds of glass will glow different colors under black light

5) Markings: Striations, dimples, etc.

6) Density: Mass/Volume

How to Determine Density

FLOTATION: Use the known density of an object to compare with an unknown object 

• Float glass particles in liquids of known densities 

Refraction

The BENDING of light as it passes through an object

• Can mess with distance perception

Refractive Index (RI)

The value that measures the REFRACTION; all substances (no matter the state), as an index

• RI = (velocity of light in vacuum)/(velocity of light in medium) 

  • Light in a vacuum is CONSTANT 3.0×10^8

  • Light going through a medium is ALWAYS SLOWER; always a fraction

Immersion Method

A method used to find the RI of a very small shard of glass 

• Submerge a piece of glass in different substances; if it disappears, then that substance shares a RI with the shard

• Becke Line: A band of white line visible outside the grain (via microscope)

  • Wherever the HIGHER RI IS

• Match Point: Where the Becke line disappears, and the RI of he glass shard is equal to the substance it is submerged in

Data Bank

• Provides densities, RI, & relative frequencies in the glass population of U.S. 

• Makes it easier to match types of glass

Radial Fragments

One of the two key types of fracture patterns, also known as “first fractures”

• Radiate OUT from the origin of the impact

• Begin on the OPPOSITE side of the force

• Occurs first, then other kinds of fractures follow

• Breaks on the OPPOSITE SIDE of the force app

Concentric Fractures

One of the two key fracture patterns, also known as “next fractures” 

• Circular lines AROUND the point of impact

• Begin on the SAME SIDE as the force 

Breaks on the SAME side of the force

How does glass break?

The applied force comes from the BACKSIDE of the direction of the glass fracture; the force pushes OUT against the glass, which pushes the glass outwards

High Velocity Projectiles (Bullets)

Leaves a wider hole on the EXIT side of the class

• Helps determine the direction of impact

• The wider hole (exit side) also going to feel rough to the touch

• The SMALLER the velocity of a bullet, the MORE damage is done— greater CONE

Stress Markings

If the entire piece of broken class is shattered and there is no coherent shape, the SIDE of a glass can be used to help determine force direction

• Wallner Lines: Direction of rib marks (stress marks on broken edges perpendicular to one side of glass

  •  RADIAL cracks form a RIGHT angle on the REVERSE side of the force

  • CONCENTRIC fracture is on the SAME SIDE as the impact

Fracture Termination

Determining which fracture was made first 

• Wherever they overlap, it can be used to determine which was first then second 

Blockback

The backscattering of particles toward the DIRECTION of the force

• If you shoot a gun at a piece of glass, particles are going to fly toward the shooter

Fluorescence

A characteristic of glass; when glass absorbs energy, they get excited and jump up orbitals. When they jump back down, they release energy in the form of LIGHT.

• LEAD: White/blue under UV

• Uranium: Green

• Could also be clear