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 ) –Addition used in borosilicate & aluminoborosilicate glasses. –Very heat resistant

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

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

Common glass densities Glass TypeDensity (g/cm 3 ) Blue ornamental Double glazing Lead crystal Pyrex Quartz Red ornamental Soda Window glass Zinc titania2.5310

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 RI units The change in RI for solids is in the order of 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 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 oil

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