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)
what is an anisotropic material?
- more than one RI value (birefringent)
- uniaxial (n1 = n2 x n3)
- biaxial (n1 x n2 x n3)
- light is able to pass through with crossed polars (rotation of polarity)
- no light when aligned at axes (extinction)
why does birefringence produce color?
- two paths from two RIs = one faster = change color
- ordinary ray (continues straight path) vs extraordinary ray (bends)
- slow ray = when light separates into two rays, the ray that experiences higher RI
what are extinction angles?
- when one of the axes of the indicatrix is aligned with either analyzer or polarizer, the light passing through sample only experiences one RI (extinct)
- when sample is rotated midway between extinction angles, light passing through experiences two RIs (maximum birefringence, bright)
what is parallel extinction?
- when length of sample is aligned with one of the polarizing filters
- optic axis is aligned either parallel or perpendicular to sample (dark on crosshairs)
what is symmetric extinction?
sample appears extinct midway between either polarizing filter
what is oblique or inclined extinction?
sample appears extinct when sample is slightly off any axis (not parallel or symmetrical)
what is undulose extinction?
- sample is twisted, bent, or warped
- only a portion of the sample which is aligned with extinction will be dark
- rotating the sample causes the extinguished area to change
what is sign of elongation?
- relationship between physical structure of anisotropic crystal and the optical indicatrix
- only for crystals that exhibit parallel or oblique extinction
what is positive and negative sign of elongation?
positive = length slow, higher RI (aligned with accessory plate/length of fiber)
negative = length slow aligned with width of fiber
in a biaxial crystal, how is optic axis determined?
- two planes have same RI in all directions
- direction perpendicular to each planes defines optic axis
- angle between two axes = 2V
- if 2V is along z-axis = positive
- if 2V is along x-axis = negative
what is the advantage of conoscopic illumination over orthoscopic?
- swing-out lens causes most of the light rays to pass through sample at an angle (except those that pass through center of lens)
- most of the light rays experience two different RIs as they pass through sample
- Bertrand lens refocuses these off-axis rays allowing image to be focused normally (can observe interference figures)
for optic sign, what does gray or white observed in spaces between interference figure mean?
crystal is first-order
for optic sign, what does blue observed in the upper right and lower left after accessory plate is inserted mean?
crystal is positive
for optic sign, what does blue observed in the upper left and lower right after accessory plate is inserted mean?
crystal is negative
how can interference figures help you determine if a crystal is uniaxial or biaxial?
if you rotate the stage 45 degrees, an uniaxial crystal will keep a cross shape while a biaxial crystal will have a baseball stitch shape (isogyres separate)
for optic sign, what does blue observed in middle of the baseball stitch shape (or convex side) mean?
crystal is positive
for optic sign, what does blue observed on outsides of the baseball stitch shape (or concave side) mean?
crystal is negative
