Glass

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Glass

• a chip off the old block

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What is glass?

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

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Additives responsibilities

• Alumina (Al2O3)
• Aluminum oxide
• Units formed lead to improved chemical durability
  and viscosity
• Boron Oxide (B2O3)
• Addition used in borosilicate
  aluminoborosilicate glases.
• Very heat resistant

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Additives responsibilities

• Soda ash (Na2CO3)
• Lower silicas melting pt.
• Allows metal containers for processing (fluxing
  agent)
• Glaubers 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

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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

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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

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Density

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

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Common glass densities
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Refractive index (R.I.) measurement

• Becke - immersion method
• Emmons Winchell - temp. variation method
• Saylor - double diaphragm contrast method
• Oettle - phase contrast method

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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

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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

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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

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RI measurement using Becke line

• Bright halo observed around glass when oil has
  lower RI
• Bright halo observed inside the edge of the glass
  when oil has higher RI
• When oil glass RI, glass disappears

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Becke line

• Glass has higher refractive index

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Becke line

• Glass has lower refractive index

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Common liquid refractive indices
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Glass Fracture Patterns

• Two parts
• Radial
• Concentric
• Can tell direction of force from these

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Determining direction of force

• Radial edge of glass
• 3 Rs
• Radial cracks have Right angles
• on theReverse side of applied force
• Concentric edge of glass

Direction of force
Direction of force