Chapter 4: Analysis of Glass

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basic composition of glass

sand
metal oxides
soda
lime

sand

silicon oxides

metal oxides

CAMS"
calcium
aluminum
magnesium
sodium

soda

sodium carbonate
-lowers the melting temperature

lime

calcium oxides
-prevents dissolving in water

formation of glass

sand and other metals are mixed
mixture is heated to melting
mixture is cooled to a rigid condition w/o crystallization

types of glass

soda-lime glass
borosilicate glass
tempered glass
laminated glass

soda-lime glass

basic window and bottle glass

borosilicate glass

boron oxide is added to make glass heat resistant
-car headlights and pyrex glassware

tempered glass

increase strength of glass by introducing stress by rapid heating and cooling
-shatters into small square fragments (dicing) instead of breaking
-side and rear car windows

laminated glass

increase strength of glass by sandwiching a layer of plastic between two layers of window glass
-car windshields

alanlysis of glass

compare properties of glass that will associate one glass fragment with another while minimizing or eliminating the possible existence of other sources
-matching of fragments
-density
-refractive index
-chemical analysis
-fracture analysis

matching of fragments

fragments that fit together are unlikely to come from different sources

density

mass per unit volume
m/v

refractive index

ratio of the speed of light in a vacuum to its speed in a given medium

chemical analysis

identification of chemical composition may not be too useful as glass is basically uniform across manufactures

fracture analysis

observing characteristics of broken glass

flotation (density of glass)

a glass fragment is placed in a liquid mixture, typically bromoform and bromobenzene

process of flotation

1. A standard/reference sample of glass is placed in the mixture
2. The amounts of bromoform and bromobenzene are adjusted until the fragment is suspended in the liquid, meaning the liquid and fragment have the same density
3. An unknown fragment is then

0.0003 g/ml

a single sheet of window glass can vary by ___ in flotation

0.001 g/ml

flotation can distinguish densities by ____

refractive index of glass (RI)

immersion: fragments are immersed in a liquid whose refractive index is adjusted until it equals that of the glass particles
-use temp to adjust refractive index of liquid

hot-stage microscope (immersion)

used to observe refractive index
-heats silicone oil to boiling and then increase heat at 0.2 degrees C per minute

match point (immersion)

when the disappearance of Becke lines is observed ("halo" around fragment immersed in a liquid)

0.0002

single sheet of glass can vary by ___
-thus for comparison, the differences in refractive index must exceed this amount immersion

density and refractive values

FBI has a data base of the ____ and ____ of glass in the U.S.
-during investigation they correlate the frequency of occurrence of the glass within the U.S.

1/2000, 22/2000

FBI asses the probability that glass came from a specific source
-glass with RI of 1.5290 occurs ___ times
-glass with RI of 1.5180 occurs ___ times

tempered, nontempered glass

both glasses are annealed to change the RI
-___ glass has a higher rate of change in RI after annealing when compared to ___ glass

radical fractures

cracks that extend outward like the spoke of a wheel from the point of impact
-occurs on opposite side of impact

concentric fractures

cracks that form a rough circle around the point of impact
-occurs on same side as impact

high velocity impact

___ will leave a round crater shaped that is wider on the exit side
-helps determine direction of impact
-as velocity decreases the irregularity of hole increases making it difficult to determine direction of impact

stress marks

arches that are on the sides of broken glass and curve from one side to the other and can be used to determine direction of impact
-radical
-concentric

radical fracture stress marks

the perpendicular end of arch is found on opposite side of impact
-3R rule

3R rule

radial cracks form a right angle on the reverse side of the force

concentric fracture stress marks

the perpendicular end of arch is found on same side as impact

sequencing multiple impacts

fracture lines will end at an existing fracture line

collecting glass evidence

-all glass should be collected
-standard/reference samples should be collected from glass remaining in window or door frames
-fragments should be placed in solid containers to prevent further breakage
-shoes and clothing should be wrapped individually in

trace evidence

hair, fibers, skin, blood, paint, soil