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Glass

Glass. a “chip” off the old block. What is glass?. Mixture of: Sand Soda Lime Other trace elements. Additives’ responsibilities. Alumina (Al 2 O 3 ) Aluminum oxide Units formed lead to improved chemical durability and viscosity Boron Oxide (B 2 O 3 )

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Glass

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  1. Glass a “chip” off the old block

  2. What is glass? • Mixture of: • Sand • Soda • Lime • Other trace elements

  3. 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

  4. 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

  5. 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

  6. 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

  7. Density • Can be measured • Can be “observed” by placing in a “density tower” • May not be the most accurate, subjective

  8. Common glass densities

  9. Refractive index (R.I.) measurement • Becke - immersion method • Emmons & Winchell - temp. variation method • Saylor - double diaphragm contrast method • Oettle - phase contrast method

  10. 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

  11. 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

  12. 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

  13. RI measurement using Becke line • Bright halo observed around glass when oil has higher RI • Bright halo observed insidethe edge of the glass when oil has lower RI • When oil & glass = RI, glass “disappears”

  14. Becke line • Glass has higher refractive index

  15. Becke line • Glass has lower refractive index

  16. Common liquid refractive indices

  17. Glass Fracture Patterns • Two parts: • Radial • Concentric • Can tell direction of force from these

  18. 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

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