Glass Analysis Flashcards
what is tempered glass?
- type of flat glass
- surface of glass heated and cooled rapidly (cool on top)
- much more resistant to breakage than regular glass
- must be cut and shaped prior to tempering
- in back and side windows of cars (tiny, non-sharp pieces)
what is laminated glass?
- type of flat glass
- layer of plastic heat-sealed between two layers of non-tempered, heat-strengthened glass
- retains its integrity (makes hole rather than shattering)
- in windshields of cars
what is continuous filament used for?
- type of glass fiber
- fiber optics and fiber-reinforced plastic composites
what is non-continuous filament used for?
- type of glass fiber
- insulation
- three types: fiberglass wool (glass wool), mineral wool (slag and rock wools), and glass ceramic fibers
where does glass fracture from first?
tension side at surface defect called the “stress concentrator”
what is the 4R rule?
Radial Ridges form Right angles from the Rear (conchoidal ridges form right angles on the side opposite of where the force was applied)
what are the ways you can examine glass?
- physical comparison of fractured glass
- non-optical physical properties (density and thickness)
- optical physical properties (absorption/color, fluorescence, RI/dispersion)
- chemical properties (elemental analysis through ICP)
what type of glass examination is the most probative?
physical comparison of large fragments (fit together)
what are the major factors affecting glass transfer and persistence?
- position of person relative to window
- nature of clothing worn by person
what did glass transfer studies reveal?
- transfer process is highly variable
- 3-4 m max
- thick/wired glass produce more particles
- force influences number of particles backspattered
where are glass particles most likely to be found in relation to clothing?
footwear/socks rather than pants
what did glass persistence studies reveal?
- as activity vigor increases, the size of retained particles decreases
- very small proportion of fragments on one item are likely to be transferred to a second item by contact between them
- two stage process: initial loss within 30-60 min is very rapid and second process is slower
what can you say with an expert opinion about glass?
- inference of source
- individualization or conclusion
- often consistent but not I or C…match/consistent with/etc.
how is RI measured in the lab?
- use of Becke line (immersion method)
- halo effect around particle that moves into medium of higher RI when stage is lowered
what is an isotropic material?
- a single RI value (n1 = n2 = n3)
- light is NOT able to pass through with crossed polars (no rotation of polarity)