forensics glass

What is glass?

An amorphous solid made from silica, calcium oxide, and sodium oxide.

What is silicas chemical name

silicon dioxide

amorphous solids (glass)

an irregular atomic shape; will break into random shapes

crystalline solids

regular atomic structures; will break into known shapes

prehistoric humans

used obsidian (volcano) glass as cutting tools

syria

accidentally made glass by cooking on rocks on top of sand

1st century BC

glass blowing began

egypt

the earliest known human- made glass objects (beads)

14th century BC

knowledge of glassmaking spreads throughout Europe

13th century 1291 (Italy)

specialized glass production was an art, science, and secret. Glassmaking moved to an island off coast to protect secrets.

soda-lime glass

most common because it is inexpensive and to make

crystal (lead glass)

Pb substituted for calcium oxide. Used to make fine glassware and decorative art. Adding lead makes glass dense and sparkly.

pyrex/limax

Used for ovenware or labware. Chemicals are added during manufacturing so glass can withstand temperature.

Colored glass

Adding metal oxides to change color

What type of crime scene is glass evidence typically found at?

Automobile accidents, break-ins, and shots fired through window

Forensic examination of glass

class evidence, if reconstructed perfectly into a whole object, its a match

goals in examining evidence

1. Determine type of glass

2. Determine how glass was manufactured

3. Use physical characteristics to classify it

4. Individualize glass to a source

compare physical and chemical characters

1. Optical properties: color and refractive index

2. Non-optical properties: surface wear, striations from manufacturing, thickness,

3. Chemical properties: additive or trace elements

Why measure density

It can be used as a screening technique with large number of fragments, useful in identifying multiple sources, and is a Non-destructive and intensive property

Formula for density

D=mass/volume

physical properties of glass

hard, elastic, brittle, non-conductor, density, refractive index

chemical properties of glass

resistant to all but fluoride and very strong bases

refraction defintion

bending of light waves b/c change in velocity

refractive index

ratio of velocity of light in a vacuum to that in medium under examination

As a bullet passes…

the bullet pushes cone shaped piece of glass out ahead

exit side of hole is…

larger than entrance side of hole

Radiating fracture lines from subsequent shots will…

stop at the edge of a fracture line already present

colors of metal oxides

nickel (yellow to purple), cobalt (purple/blue), and selenium (red).

Density can be measured by…

determining mass/volume, comparison by flotation, comparison using a density gradient column

glass wont break into regular pieces due to…

being an amorphous solid

fracture patterns provide cludes on..

direction, velocity, and sequence of impact

becke line

halo-like effect, appearing at edge of glass when refractive index of glas and liquid are different

submersion method

A technique used to determine the refractive index of a glass fragment. The glass is placed in a liquid whose refractive index is known and adjusted. When the glass fragment 'disappears' in the liquid, it means the refractive index of the glass (nglass) is equal to the refractive index of the liquid (nliquid), and the Becke line will vanish.

(n) normal line

perpendicular to the surface

(b) angle of refraction

The angle formed between the normal line (n) and the refracted light ray as it travels through the new medium.

(a) angle of incidence

The angle formed between the line (n) and the light ray as it strikes the surface.

density gradient column method (not accurate for fragments that are cracled or contain inclusions)

A method used to compare glass fragments by placing them in a tube containing layers of liquids with progressively higher densities. The fragment will remain suspended at the level where its density is equal to that of the liquid (Dglass​=Dliquid​).

when a beam of light moves from one medium into another…

speed changes and direction bends light towards normal line