Lecture 9: Physical Evidence III (Paint) Flashcards
What is paint?
A liquid spread over rigid surfaces to form a thin, hard coating
What does paint dry into?
A hard film consisting of pigments and additives suspended in the binder
In what manner is paint usually applied?
In a series of layers
What is the significance of the PDQ database?
It is an automobile paint comparison database
What common situation results in paint evidence submitted to a lab?
Hit-and-run cases involving automobiles
Collect victim vehicle, paint transfer, suspected source
What are the three types of paint analysis mentioned?
- Microscopic comparison (layers, primer, coatings, scratches, damage)
- Infrared spectroscopy
- Pyrolysis gas chromatography
What does a stereoscopic microscope compare in paint analysis?
Color, surface texture, and color layer sequence
What is the value of soil as evidence based on?
Its prevalence at crime scenes and transferability between the scene and the criminal
What is the first step in soil sample collection?
Sampling vertically and horizontally
What is a common practice in soil lab examination?
Visual comparison of color and texture under a microscope
What is the purpose of density-gradient tubes in soil analysis?
To compare soils based on their density
What is glass?
An amorphous fusion of mineral compounds that produces a transparent solid when cooled
What are the physical properties of glass?
Hard, elastic, brittle, insulative (thermal and electrical)
What are the chemical properties of glass?
Resistant to all but fluorine and very strong bases
What is soda-lime glass commonly used for?
Windowpanes and containers
Has sodium oxide (Na2O) to reduce M.P., and quicklime (CaO) to increase durability
What is borosilicate glass commonly used for?
Cockware, laboratory glassware, light bulbs
Lime replaced with boron oxide (B2O3) to increase resistance to thermal shock
What is lead glass?
Known as crystal for decorative glassware, radiation protection
Lime replaced with lead oxide (PbO) to increase refractive index and reduce M.P. & viscosity
What does laminated glass consist of?
Two sheets of glass with plastic between them
Used in windshield and skylights
What makes tempered glass stronger?
It is treated by heating and quickly cooling or chemically treating the surface, replacing Na+ with K+
What are the six types of glass classified based on composition?
- Soda-lime
- Borosilicate
- Lead (crystal)
- Flat
- Laminated
- Tempered
How do glass windows break?
- Radial cracks form first
- Propagated in short segments on opposite side of force
- Concentric cracks
What does the 3R rule in glass fractures state?
Radial cracks form at
Right angles on the
Reverse side of the force
What are Wallner lines?
- “Stress” marks on edges of broken pieces of glass
- Perpendicular to unloaded side and parallel to loaded side
What are the three exceptions to the three R rule?
- Tempered glass (dices without forming ridges)
- Small windows held tightly in frame (cannot bend or bulge)
- Windows broken by heat or explosion (no “point of impact”)
What indicates the direction of a bullet’s impact on glass?
The exit side is wider than the entry side
What is the goal of forensic examination of glass?
To determine types of glass, how it was fractured, and individualize it to a source
What method is used to determine the refractive index of glass?
Submersion method in liquids of known refractive indexes
What is the formula for calculating density?
π = m/V
What is the significance of measuring the refractive index?
It can be measured very precisely and is nondestructive
What happens to light when it moves from air into water?
Its speed slows and bends light toward the normal line
What are Becke lines used for in glass analysis?
To indicate differences in refractive index between glass and surrounding liquid
E.g. white line inside means higher refractive index, white line outside means lower refractive index
Fill in the blank: Most paint evidence submitted to a lab comes from _______ cases.
hit-and-run
True or False: The sequence of impacts on glass can be determined based on crack propagation.
True
