Forensic Glass Analysis

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Information about Forensic Glass Analysis

Published on January 9, 2008

Author: Dionigi


Forensic Glass Analysis:  Forensic Glass Analysis Continuing Education Program in Forensic Science April 5, 2004 Glass:  Glass “An inorganic product of fusion which has cooled to a rigid condition without crystallizing” Does not have a specific m.p. Softens over a temperature range Glass Components:  Glass Components Formers – Network Formation SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5 Fluxes – Softeners Na2O, K2O, LiO, Al2O3, B2O3, Cs2O Stabilizers – Provide Chemical Resistance CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO Major Types of Glass:  Major Types of Glass Soda lime silicate glass SiO2 + Na2O / K2O + CaO / Al2O3 / MgO Flat glass, container glass, electric light bulbs Borosilicate glass > 5% B2O3 (in place of Na2O) Lab glassware, thermometers, cookware, sealed-beam headlights Slide5:  Aluminosilicate glass Higher percentage of aluminum Useful at higher temperatures than borosilicate Aluminoborosilicate – 50:50 Al/B Labware, cookware, and glass fibers Lead alkali silicate (leaded glass) Up to 80% PbO Low softening temp, high refractive index “Crystal” tableware, Major Forensic Glass Sources:  Major Forensic Glass Sources Flat Glass – Soda lime silicate Drawing, Rolling, or Floating Tempered: Rapid Cooling Adds Strength Dices when broken Automotive windows & security windows Float Glass floresces when excited at 254 nm. Slide7:  Coated: Surface modification Mirrors Laminated: Sandwiched around plastic Automotive windshields Headlights: Sometimes borosilicate Light bulbs: Soda lime glass Heat absorbing/ UV filtering Tinted Photochromic (Light Sensitive) Eyeglasses Slide8:  Container Glass Lower magnesium, higher sodium Clear vs. greenish (window) Glass Fibers Fiberglass insulation Alumino-borosilicate Binder (red or yellow) to hold fibers in bundles Is It Glass?:  Is It Glass? Isotropic vs. Anisotropic Glass: Isotropic Remains dark when rotated under crossed polarizers Crystalline solids and plastics: Anisotropic Change brightness when rotated under crossed polarizers Hardness: Pin test Solubility (distinguish salt) Comparison of Physical Characteristics:  Comparison of Physical Characteristics Color (Microscopy) Flatness (Microscopy, Interferometry) Thickness Full thickness required 0.79 – 6.2 mm Fluorescence – Float Glass 254 nm, note color Physical Properties:  Physical Properties Density Sink-Float Method CHBr3, C2H2Br4, Sodium Polytungstate Density Range 2.465-2.540 g/cm3 Slide12:  Index of Refraction: symbol n Measured by Na-D line 589nm at 20°C Other wavelengths Hydrogen-C 656.3 nm Hydrogen-F 486.1 nm θi θr Slide13:  Dispersion (V) – Change in Refractive Index with wavelength Chemical Composition – Inductively Coupled Plasma (ICP) SEM with EDX Neutron Activation Analysis X-Ray Fluorescence Determining Refractive Index:  Determining Refractive Index Liquid Immersion When glass and liquid have same refractive index, particle will be (nearly) invisible Becke Line = Halo around particle Moves to medium of higher index of refraction when microscope is defocused by moving the stage down. Becke Line Slide15:  Method of Continuous Variation Glass RI: 1.470 – 1.560 Immersion Liquids: Silicone Oils Organic Compounds Cyclohexanone 1.448 m-xylene 1.494 Methylsalicylate 1.522 Benzaldehyde 1.544 Benzyl benzoate 1.568

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