Glass Analysis Flashcards

1
Q

what is tempered glass?

A
  • 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)
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2
Q

what is laminated glass?

A
  • 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
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3
Q

what is continuous filament used for?

A
  • type of glass fiber
  • fiber optics and fiber-reinforced plastic composites
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4
Q

what is non-continuous filament used for?

A
  • type of glass fiber
  • insulation
  • three types: fiberglass wool (glass wool), mineral wool (slag and rock wools), and glass ceramic fibers
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5
Q

where does glass fracture from first?

A

tension side at surface defect called the “stress concentrator”

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6
Q

what is the 4R rule?

A

Radial Ridges form Right angles from the Rear (conchoidal ridges form right angles on the side opposite of where the force was applied)

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7
Q

what are the ways you can examine glass?

A
  • 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)
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8
Q

what type of glass examination is the most probative?

A

physical comparison of large fragments (fit together)

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9
Q

what are the major factors affecting glass transfer and persistence?

A
  • position of person relative to window
  • nature of clothing worn by person
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10
Q

what did glass transfer studies reveal?

A
  • transfer process is highly variable
  • 3-4 m max
  • thick/wired glass produce more particles
  • force influences number of particles backspattered
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11
Q

where are glass particles most likely to be found in relation to clothing?

A

footwear/socks rather than pants

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12
Q

what did glass persistence studies reveal?

A
  • 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
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13
Q

what can you say with an expert opinion about glass?

A
  • inference of source
  • individualization or conclusion
  • often consistent but not I or C…match/consistent with/etc.
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14
Q

how is RI measured in the lab?

A
  • use of Becke line (immersion method)
  • halo effect around particle that moves into medium of higher RI when stage is lowered
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15
Q

what is an isotropic material?

A
  • a single RI value (n1 = n2 = n3)
  • light is NOT able to pass through with crossed polars (no rotation of polarity)
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16
Q

what is an anisotropic material?

A
  • 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)
17
Q

why does birefringence produce color?

A
  • 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
18
Q

what are extinction angles?

A
  • 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)
19
Q

what is parallel extinction?

A
  • 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)
20
Q

what is symmetric extinction?

A

sample appears extinct midway between either polarizing filter

21
Q

what is oblique or inclined extinction?

A

sample appears extinct when sample is slightly off any axis (not parallel or symmetrical)

22
Q

what is undulose extinction?

A
  • 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
23
Q

what is sign of elongation?

A
  • relationship between physical structure of anisotropic crystal and the optical indicatrix
  • only for crystals that exhibit parallel or oblique extinction
24
Q

what is positive and negative sign of elongation?

A

positive = length slow, higher RI (aligned with accessory plate/length of fiber)
negative = length slow aligned with width of fiber

25
Q

in a biaxial crystal, how is optic axis determined?

A
  • 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
26
Q

what is the advantage of conoscopic illumination over orthoscopic?

A
  • 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)
27
Q

for optic sign, what does gray or white observed in spaces between interference figure mean?

A

crystal is first-order

28
Q

for optic sign, what does blue observed in the upper right and lower left after accessory plate is inserted mean?

A

crystal is positive

29
Q

for optic sign, what does blue observed in the upper left and lower right after accessory plate is inserted mean?

A

crystal is negative

30
Q

how can interference figures help you determine if a crystal is uniaxial or biaxial?

A

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)

31
Q

for optic sign, what does blue observed in middle of the baseball stitch shape (or convex side) mean?

A

crystal is positive

32
Q

for optic sign, what does blue observed on outsides of the baseball stitch shape (or concave side) mean?

A

crystal is negative