1. Glass a “chip” off the old block

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

3. Additives’ responsibilities Alumina (Al 2 O 3 ) –Aluminum oxide –Units formed lead to improved chemical durability and viscosity Boron Oxide (B 2 O 3 ) –Addition used in borosilicate & aluminoborosilicate glasses. –Very heat resistant

4. Additives’ responsibilities Soda ash (Na 2 CO 3 ) –Lower silica’s melting pt. –Allows metal containers for processing (fluxing agent) Glauber’s salt (Na 2 SO 4 ) –Sometimes used instead of soda ash Lime (CaCO 3 ) –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 Glass TypeDensity (g/cm 3 ) Blue ornamental2.4880 Double glazing2.5125 Lead crystal2.9614 Pyrex2.2300 Quartz2.2106 Red ornamental2.5539 Soda2.4862 Window glass2.5092 Zinc titania2.5310

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 inside the 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 LiquidRefractive index Ethyl acetate1.373 n-butyl alcohol1.402 Olive oil1.467 Corn oil1.473 Castor oil1.482 Methyl salicylate1.522 Clove oil1.543 Canola oil1.465-1.467

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