Shattered: Forensic Glass Analysis. What is Glass?. “An inorganic product of fusion which has cooled to a rigid condition without crystallizing” Uniform amorphous solid No specific m.p. Softens over a temperature range. 'Ordinary' sheet glass
-made by passing the molten glass through rollers; process gives an almost flat finish but with distortions.
-glass is used in glazing greenhouses and garden sheds where the visual distortions do not matter.
Float glass (plate)
Float glass gets its name from the method of production used to manufacture it.
-molten glass is 'floated' onto a bed of molten tin
-produces a glass which is flat and distortion free
-process imparts a fluorescence to the glass [UV light]
Float Glass Process
-glass is produced by applying a special treatment to ordinary float glass after it has been cut to size and finished.
-treatment involves heating the glass so that it begins to soften (about 620 C) and then rapidly cooling it.
-produces a glass which, if broken, breaks into small pieces without sharp edges.
-glass is made up of a sandwich of two or more sheets of glass (or plastic), bonded together by a flexible, normally transparent material.
-if cracked or broken, the flexible material is designed to hold the glass fragments in place.
SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5
Na2O, K2O, LiO, Al2O3, B2O3, Cs2O
CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO
SiO2 + Na2O / K2O + CaO / Al2O3 / MgO
> 5% B2O3 (replaces Na2O)
Lab glassware, thermometers, cookware, sealed-beam headlights
Different glasses-domestic windows, car headlamps, light bulb glass-are produced differently from different chemistries.
-Rolling or Floating
Fundamental Question: Is it Glass?
When 2 substances have different RI, light passing through them produces a Becke Line
A Becke line is a band or rim of light visible along a grain/crystal boundary in plane-polarized light.
performed using our old
friend the PLM
-the goal of glass analysis
by PLM is to individualize an
evidence glass fragment to an
exemplar from a crime scene
[primary or secondary]
Forensic Fracture Analysis
Begin at impact point and radiate
outward from point of impact
-initial crack is on the side
opposite the applied force
-Cracks seeming to make a circle around point of impact
-Motion of the projectile through the glass
puts tension on the front surface of the glass, causing concentric fractures.
-high-speed projectile [a rock] penetrates the glass, it leaves an exit hole larger than it‘s entrance hole.
-helps to determine the direction on impact.
Conchoidal Lines – stress marks shaped like arches that are perpendicular to 1 glass surface and curved nearly parallel to the opposite surface; perpendicular surface faces side where crack originates; primarily found with bullet holes
Determination of the sequence of bullet holes can be made by noting the radial fractures.
Radial fractures caused by the passage of a bullet stop at pre-existing fracture.