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
m
D
v
• Can be “observed” by placing in a “density
tower”
– May not be the most accurate, subjective
Common glass densities
Glass Type
Blue ornamental
Double glazing
Lead crystal
Pyrex
Quartz
Red ornamental
Soda
Window glass
Zinc titania
Density (g/cm3)
2.4880
2.5125
2.9614
2.2300
2.2106
2.5539
2.4862
2.5092
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
Ethyl acetate
n-butyl alcohol
Olive oil
Corn oil
Castor oil
Methyl salicylate
Clove oil
Canola oil
Refractive index
1.373
1.402
1.467
1.473
1.482
1.522
1.543
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
Direction of
force
• Concentric edge of glass
Direction of
force