Forensic Examination of Glass Flashcards
What are the levels of the analyze of a glass sample?
- preliminary examination
- measurements
- physical match
- physical properties
- elemental analysis
What goes into Preliminary Examinations?
- note shape (flat, cylindrical, spherical)
- note surface texture and markings
note nature of fracture (fracture ridges, wavy thermal fractures, dicing) - note color (under both room light and UV)
What goes into Measurements?
- thickness
- curvature
What qualifies as a Physical match?
- usually worthwhile if the smaller of the two samples contains large glass flakes (1/4 in in diameter or larger)
- if this is successful no further examinations are needed
- pain, putty, rust stains and scratches may aid in physical matching
- ream marks may be used
What are some Physical Properties of Glass?
- density
- R.I.
- Dispersion
- Color
- Hardness
How is Density determined when examining glass?
- bouyancy
- flotation
density gradient tube - sink-float method
How is Density determined when examining glass?
- buoyancy
- flotation
density gradient tube - sink-float method
How is R.I. determined when examining glass?
- becke line, hot-stage
Why measure Density of Glass?
- can be used as a screening technique with large numbers of fragments
- useful in identifying multiple sources present in the known and/or questioned samples
- it is nondestructive and an intensive property (not dependant on sample mass)
- need to measure very precisely in parts per hundred or thousand or better
What is Displacement?
- a way to measure density
- directly determining mass and volume
- comparison by flotation
- comparison using a density gradient column
What is the Gradient Column Method?
- Fragments of different densities settle at different levels in the column of liquid of varying density
- technique is not accurate for fragments that are cracked or contain an inclusion
Explain the Density by the Flotation Method?
- glass particle is immersed in a liquid
- density of liquid is adjusted by the addition of small amounts of another liquid untill glass chip remains suspended
- when suspended, glass will have the same density as the liquid medium and can be compared to other relevant pieces of glass which will remain suspended, sink, or float
What is R.I.?
- the ratio of the velocity of light in a vacuum to the velocity of light in any other medium
- an intensive property but it can be measured very precisely (0.0002) & does not destroy the sample
- R.I. of glass varies with small changes in composition or by ow it is manufactured
What goes in Elemental Analysis?
- atomic absorption
- atomic emission (ICP-AES)
- Mass Spectrometry (ICP-MS)
What are the Match Criteria?
- range test
- confidence interval
- student’s t test or Welch’s test
What is a Range Test?
- questioned sample values fall within range defined by the minimum and maximum values obtained from known sample
What is a Confidence Interval?
- mean 2 or 3 standard deviations
- mean and standard deviation determined from measurements on known sample
What is the American Interpretive Approach of glass evidence?
What is the probability that a randomly selected sample of glass will match the questioned sample?
What is the Britch Interpretive Approach of glass evidence?
What is the probability of finding a sample of glass the same size as the questioned sample and sharing the same properties on a person’s clothing?
