Glass

a “chip” off the old block

What is glass?

• Mixture of:– Sand– Soda– Lime– Other trace elements

Additives’ responsibilities

• Alumina (Al2O3)

– Aluminum oxide– Units formed lead to improved chemical

durability and viscosity

• Boron Oxide (B2O3)

– Addition used in borosilicate & aluminoborosilicate glasses.

– Very heat resistant

Additives’ responsibilities

• Soda ash (Na2CO3)– Lower silica’s melting pt.– Allows metal containers for processing (fluxing agent)

• Glauber’s salt (Na2SO4)– Sometimes used instead of soda ash

• Lime (CaCO3)– Added to improve hardness & chemical durability

• Lead oxide (PbO)– High lead content lowers melting pt.= decreased

hardness, but increases refractive index

Types of glass• Aluminosilicate & borosilicate

– Can withstand high temps.

• Laminated glass– Glass w/ plastic layer - used in car windshields

• Lead glass– Fine crystal

• Soda lime glass– Plate & window glass, glass containers, electric light bulbs, art

objects

• Tempered (stressed) glass– Side & back windows of cars; breaks into tiny pieces

Forensic analysis of glass

• Physical, fracture matching– Best evidence, but very rare

• Density determination– Float test– Results subjective

• Refractive index (R.I.) determination• Chemical testing, destructive

– Test for silicates, metal oxides, trace evidence

Density

• Can be measured

• Can be “observed” by placing in a “density tower”– May not be the most accurate, subjective

Dm

v

Common glass densitiesGlass Type Density (g/cm3)

Blue ornamental 2.4880

Double glazing 2.5125

Lead crystal 2.9614

Pyrex 2.2300

Quartz 2.2106

Red ornamental 2.5539

Soda 2.4862

Window glass 2.5092

Zinc titania 2.5310

Refractive index (R.I.) measurement

• Becke - immersion method

• Emmons & Winchell - temp. variation method

• Saylor - double diaphragm contrast method

• Oettle - phase contrast method

Refractive index measurement

• Refraction is the change of direction and/or speed of light as it passes from one medium to another

• RI is a ratio of the speed of light between two mediums (e.g. glass & oil)

• RI is dependant on:– The wavelength of light– The temperature of the medium

RI measurement technique

• RI of material varies with temp.• The change in RI for liquids is in the order of 10-4

RI units• The change in RI for solids is in the order of 10-6

RI units• What does this mean?

– When the temp. of a liquid is changed, the RI changes rapidly, but the RI of an immersed solid will not

RI measurement technique

• Silicon oil usually used • Oil is calibrated so RI can be determined

from its temp.• Sample glass is immersed in oil• Oil is heated/cooled to determine match

temp.– Glass “disappears”

• Oil RI = Glass RI

RI measurement using Becke line

• Bright halo observed around glass when oil has higher RI

• Bright halo observed inside the edge of the glass when oil has lower RI

• When oil & glass = RI, glass “disappears”

Becke line

• Glass has higher refractive index

Becke line

• Glass has lower refractive index

Common liquid refractive indices

Liquid Refractive index

Ethyl acetate 1.373

n-butyl alcohol 1.402

Olive oil 1.467

Corn oil 1.473

Castor oil 1.482

Methyl salicylate 1.522

Clove oil 1.543

Canola oil 1.465-1.467

Glass Fracture Patterns

• Two parts:– Radial– Concentric

• Can tell direction of force from these

Determining direction of force

• Radial edge of glass– 3 R’s

• Radial cracks have Right angles

on the Reverse side of applied force

• Concentric edge of glass

Direction of force

Direction of force