- Slides: 22
Types of Glass Saferstein - Chapter 4
l Glass – an l Si. O 2 – silicon dioxide amorphous solid (sand) which lacks the crystal l Na 2 CO 3 – sodium lattice arrangement of carbonate (soda) atoms usually found in l Ca. O – calcium oxide solids. (lime) l B 2 O 3 – boron oxide
Soda Lime glass l “soda-lime” – sand + soda (Na 2 CO 3) + lime (Ca. O) ¡ Used for windows and bottle glass ¡ Makes up 90% of all glass products.
Borosilicate glass l Borosilicates – includes boron oxide ¡ Adds heat resistance ¡ Used for Pyrex cooking and lab glassware and for automobile headlights
Tempered Glass l Tempered glass – stronger than other types because of repeated stress from heating and cooling ¡Does not splinter ¡Used for side and rear windows in US-made cars; also used for windshields in some European cars
Laminated Glass l Laminated glass has a layer of plastic between two pieces of ordinary window glass (windshield)
Matching glass fragments l Fracture Match - Individualized if 2 pieces can be matched together by shape, like puzzle pieces l Type of glass indicates class characteristics – can determine probable source of the glass, but not the exact car, window, etc. l Best matches made through refractive index and density
Calculating density l Density = mass/volume ¡Can be calculated using immersion in a liquid. ¡Place known amount of water in a graduated cylinder, add the glass fragment, and record the difference in beginning and ending volume. ¡Mass fragment on a balance ¡Result is in grams/ml or grams/cm 3
Refractive Index l Refractive index – indicates the extent of the bending of light by an object or medium ¡Measured using a hot-stage microscope ¡Use liquid of known RI, such as silicone-based medium ¡Fragment will show Becke lines – halo-like lines around the border of the fragment
l Becke lines form a halo-like effect around the edge of the glass fragment
Becke lines disappear at the refractive index of the glass.
Glass fractures l Glass fractures ¡ Radial or concentric ¡ Formed by penetration of the glass by a bullet or other projectile. ¡ Radial fractures form first. ¡ Concentric fractures form second.
l Radial – extend outward from the center of the penetration (i. e. radius) l Concentric – form circles surrounding the point of penetration
l The order in which the fractures occurred can be determined from the radial fractures. ¡ A new fracture always terminates at an existing line of fracture. ¡ Which bullet hole occurred first in the picture below?
Stress marks l The 3 R rule – l Radial fracture form Right angles on the Reverse side of impact l Stress marks can tell you whether the impact came from the inside or outside of a window.
Perpendicular marks (~90 degree angles) are found on the edge opposite the way that the force was pushing
l Point of impact can also be determined by the entrance and exit of the projectile. l Exit holes are larger than entrance holes, forming a small crater in the glass on the opposite side of impact.
Collection of glass evidence l Collect as many fragments as possible in case they can be reassembled. l For headlights, collect filaments to determine if lights were on or off. l Collect control samples l Package in solid containers or wrap in paper. l Identify interior/exterior if possible