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FSC239 Jan 6 2011

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Forensic Science

FSC239 Jan 6 th Forensic Glass Examination What is Glass? - glass is a brittle, non-crystalline, usually transparent or translucent material that is generally formed by the melting of sand with soda ash and limestone - an inorganic product of fusion which has cooled to arigid condition without crystallizing What’s in Glass? - sand (silica) – 70-75% - SiO2 - soda ash - limestone - dolomite - alumina, feldspar - chemical composition is the key factor controlling the physical properties of glass Applications of Glass - flat glass – most common – windows and cars - container – bottles and jars - misc – optical, or lenses Types of Glasses - soda-lime-silica glass o contains silica(75%), soda (15%), lime (10%) o the most common glass - borosilicate glass o contains 10-25% boron oxide o used in automotive headlights, lab and industrial glassware (Pyrex), household cookware - leaded glass o contains 35-65% lead oxide o used in crystal tableware, TV tubes Forensic Significance of Glass - types of fracture/direction of impact/sequence of impacts - as trace evidence o Locard’s principle – every contact leaves a trace Types of Fracture - impact - thermal damage – usually fire Direction of Impact - high velocity projectiles penetrating a glass pane will have coning or cratering effect, with a larger opening on the exit side - as projectile velocity decreases the cratering becomes more irregular, while surrounding cracking increases – spiderweb pattern - compressive stress in glass surface (compression) - tensile stress in glass surface (expansion) o when you break a window, it breaks from the back – the opposite side breaks first - slight deformation/bending takes place - radial cracks open on the side opposite to the force - additional force opens concentric cracks which start on the same side as the applied force - apply the 4R rule – ridges on radial cracks are at right angle to the rear Sequence of Impacts - cracks terminate at already-formed cracks caused by a previous impact – cracks don’t jump o usually, only laminated glass will hold together long enough for such a pattern to develop Glass as Trace Evidence - recovered/questioned glass pieces, often recovered from suspect’s/victim’s clothing o jacket, shirt, pant, socks, etc. o Q - comparison/known glass sample – of known origin o K - compare Q to K for the purpose of determining if these could have a shared origin Comparison Sample Collection - sampling of the comparison glass is critical - must ensure that samples of ALL broken glass sources are collected o why? Multiple sources = stronger evidence o ex. each broken window of a vehicle/in a residence o both panes of a windshield or dual pane window - sample the glass from the frame, not from the ground! - full thickness pieces are preferable o thickness is a property we examine - samples should be packaged separately in a rigid container o ex. a film canister o specimen jar o due to the sharp edges, paper folds and plastic bags should be avoided to prevent loss or cross-contamination Glass Reconstruction - photograph - frame submitted if possible (mark in/out) o glass can be taped in place o of pieces are removed from the frame  note the location (with sketch or photograph)  mark the surfaces as in/out - collect all the glass - knowing the inside/outside assists in reconstructing the window and it is an absolute must for the direction of impact examination for any value What is collected? - clothing o jacket, shirt, hat o pants o shoes - hair combings - drop sheet
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