Forensic Glass and Paint Analysis

Why is glass an amorphous solid?

Atoms are arranged in a random fashion/ variety of fracture patterns

What are the main components of glass?

Sand, Lime or Calcium Oxide, Sodium Oxide

Soda-lime

• basic, common, inexpensive, easy to make

• windows and glass bottles

leaded

• lead oxide (makes denser)

• sparkles as light passes

• fine glassware & art glass

tempered

• strong

• strengthened by stress

• breaks by fragments

• side & rear windows of automobile

laminated

• bonding 2 sheets w/ a plastic film

• windshield

How can forensic scientists obtain individual characteristics of glass evidence collected from a crime scene?

If glass edges fit like a puzzle piece

Class characteristics of glass evidence samples from crime scenes involve what physical properties of glass?

density and refractive index

Describe what is performed in the crime lab when making a flotation comparison

A glass sample sinks into a liquid containing an exact volume of a dense liquid, then a denser liquid is added in drops until glass attains neutral buoyancy

Explain how the refractive index of glass and other media are used to compare glass samples from a crime scene.

Every liquid and glass have their own refractive index and when a piece of glass is placed in a liquid with the same refractive index, the glass seems to disappear.

Radial Fractures

• Produced first

• forms on opposite side of entry point

• sun rays

Concentric fracture

• produced second

• forms on the same side as entry point

• circle

How can you determine an entrance hole vs. the exit hole in glass at a crime scene?

Exit hole is bigger. Glass always blows back in the direction of impact

How should glass evidence be collected from a crime scene?

collect largest fragments w/ forceps, avoid cross contamination and put in box

What characteristics does a forensic scientist look for when comparing paint chips under a microscope

Sequence of color layers

What is the most important characteristic in evaluating the significance of paint evidence

Color

Electrocoat Primer

apply to steel body for corrosion resistance

Primer Surfacer

apply to primer to smooth it out

basecoat

color, acrylic-based polymer

clearcoat

unpigmented, improves gloss, duability and appearance

What purpose does pyrolysis gas chromatography serve? Describe how this method is used in a lab.

Many solids cannot be injected into a gas chromatograph so items must be pyralized to high temps so they will decompose into gaseous products

How does infrared spectrophotometry help to analyze a paint specimen?

Binders absorb infrared radiation to yield a spectrum that is characteristic to that specimen

Why is emission spectroscopy used to analyze paint pigments?

b/c they can detect 15-20 elements in auto paints simultaneously & some are common to all paints but others have significant forensic uniqueness.

Why must pain collected from a vehicle involved in a hit-and-run accident be taken from the area of the car suspected of being in contact with the victim

to tell if the pain