Chapter 7 glass & glass fragments


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Forensic Class in the Philipines

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Chapter 7 glass & glass fragments

  1. 1. What is a Glass? A super cooled liquid which possesses highviscosity and rigidity. It is a non-crystalline inorganicsubstance.Importance in the study of glass: as physical evidence, breakage of pieces are scattered at the crime scene and on the suspect a common type of carried away evidence in burglary (window glass), hit and run (fragments of headlights), and a case which a bullet was fired
  2. 2. It is composed of oxides like SiO (silica), BO(boric oxide), PO (phosphorous pentoxide).1. Silica – most important oxide for commercial use - the base of commercial glasses2. Oxide – for fluxing, durability and reduction ofviscosity Glass, like window and plate which are made inmass production is fairly uniform in composition.This may contain impurities and the presence ofthese substances is invaluable for the identificationand comparison of glass by spectrographic analysis.
  3. 3. Gas have presence of trace elements whichmaybe sufficient to establish or negate the fact of acommon source for two samples of glass. OXIDES FUNCTION1.) Silica (SiO2) -------------------- base of commercial glass2.) Soda Na2O) --------------------- acts as flux for silica3.) Lime (CaO) --------------------- gives the glass chemical durability which it otherwise lack because of the water-soluble Na2O
  4. 4. 4.) Magnesia (MgO) ----------------- present as impurity or substitute for CaO5.) Alumina (Al2O3) ----------------- gives the glass greater chemical durability lower coefficient expansion, and greater freedom from devitrification. The most important problem commonlyreferred to a forensic chemist is the comparison oftwo or more samples of glass.
  5. 5. 1.) Spectrographic Test – an instrumental methodof analysis which determines the presence oftrace elements.2.) X-ray Diffraction Test – determines the type ofpattern of the glass but not as effective asspectrographic test. Depends upon thecomposition of glass.3.) Physical Properties Examination – the mostsensitive method of determining differences ofcomposition in glass samples and depends uponthe physical properties of glass.
  6. 6. 4.) Ultraviolet Properties Examination –determines the differences in theappearance of the fluorescent thusindication of physical and chemicaldifferences.5.) Polish Marks Test – optical glass and otherfine glass wares are usually polished. Finemarks are left for the basis of comparison.
  7. 7. Floatation Method – a rapid and convenient method ofdetermining the density of small fragments. Procedure andprinciple is the same as in soil. Immersion method – use to measure the refractive index of a glass - difficult to distinguish between two samples of glass by density and refractive index
  8. 8. The surface is cleaned with alcohol and thenetched by spraying with 20 to 25% hydrochloricacid. The acid is permitted to remain on thesurface for several minutes. The glass is againwashed with alcohol and dried. If the surface isilluminated by oblique light, a photograph can bemade to show the polish marks.
  9. 9. In the field of forensic chemistry the emphasis is placed on:1.) Automobile glass in case of hit and run.2.) Broken windows cause by pressure, blow or bullet incase of robbery3.) Broken bottles, drinking glasses, spectacles found at thescene of an assault or other crimes of violence, which wouldsuggest examination of the soles and heels of a suspect forembedded fragments.
  10. 10. (HOW GLASS FORMS CRACKS WHEN A BLOW OR PRESSURE IS APPLIED ON ONE OF ITS SURFACE) When the blow strikes the glass on one of its surface,the front for example, the glass first bends a little owing toits elasticity. When the limit of elasticity is reached, the glassbreaks along radial lines starting form the point where theportion or surface which is more subjected to stretching bybending. The front surface is only pushed. While the radialfractures are taking place, the newly created glass trianglebetween the radial rays also bend away from the direction ofthe destroying force. By this bending the bend is stretchedalong the front surface and when the limit of elasticity isreached, the glass breaks in concentric cracks. Theseoriginate on the front glass because of stretching.
  11. 11. Hit and run accidents represent a good percentage ofcrimes. If an automobile or ay vehicle from that vehicle forthat matter discovered in which fragments of the lens canbe found, a comparison maybe with the fragments found atthe scene of accident employing the methods of analysis ofglass. Examination of window fragments in robbery cases is important when there is a question of “as to whether the glass was broken from the outside or the inside.” Since our penal law specifically provides the mode of entrance before a crime maybe classified as robbery, this particular kind of examination becomes
  12. 12. very important. The general procedure to determine whetherthe glass was broken from the outside or inside or todetermine the side from which a pane of glass was broken isto collect and piece together as much of the glass as possiblein order to study the patterns of the cracks and to be able toorient the pieces in their original position.1.) On one side of the hole, numerous small flakes of glass willbe found to have been blown away giving the hole theappearance of a volcano crater. Such appearance indicatesthat the bullet was fired from the opposite direction of thehole from which flakes are missing.
  13. 13. 2.) If the shot was fired perpendicular to the windowpane the flake marks a re evenly distributed aroundthe hole.3.) If the shot was fired at an angle from the right,the left side will suffer more flaking from the right.4.) Excessive flaking on the right side of awindowpane would indicate a shot fired at anglefrom the left.(The direction is taken from the person shooting)
  14. 14. low-speed impact high-speed projectile
  15. 15. - states “stress lines on a radial crack will be at right angle to the rear side of the glass”.- states “stress lines on a concentric crack will be at a rightangle to the front side”, that is the side from which the blowcame rather than the rear side. * the rule for concentric crack is the reverse of the 3R’s rule provided the concentric cracks can be examined is near preferably adjacent to the point of impact.
  16. 16. Where there are two bullet holes in awindow, one from each side, the problem ofwhich one was first becomes important todetermine who is the aggressor. It will befound that the fractures caused by the firstwill complete especially the radial cracks,whereas the fractures from the second will beinterrupted and end-stopped at point wherethey intersect those from the first.
  17. 17. Laminated glass that is now being usedin automobiles does not shatter whenstruck sharply. Frequently the cracking ofsafety glass is not complete. The radialcracks do not extend to the side of impactand the spiral cracks do not extend to theother side.
  18. 18. The cracking into radial lines divides the pane into anumber of triangles. These triangles are pushed out from apoint of impact by initial impulse. The main body of a glass thatis fairly rigid resists the pushing out. The effect of a torqued isproduced, and if the force is sufficient, the glass is now pushedin the opposite direction until again the limit of elasticity isexceeded and the glass begin to break on the side where theblow was struck. The cracking now takes place along the quasi-circle concentric with the point of impact. It was demonstratedthat the number of spiral cracks present in a fracture dependsupon the nature of impringing force. A rapid dynamic forceproduces more spiral pattern than a slow, relative static forceeven though the total energy involved is the same in both cases. If radial or concentric cracks cannot be definitelyestablished the side from which the blow came cannot bedetermined.