Test 9: Soil Glass Paint Analysis, Impressions and Ballistics

Soil Material

• Partly organic

  • Decaying matter

  • Ex: peat - 100% organic

• Partly inorganic

  • Minerals

  • Ex: sand - 100% inorganic

Components of Soil

• Rocks

• Minerals

• Vegetation

• Animal waste

• Glass 

• construction debris

• Asphalt

How do Soils Differ?

• Color

  • 1000 different colors

  • Wet soils appear dark

• Texture

• Particle size

• pH

• Density

• Fossil types (Diatoms)

• Mineral and rock content

• pesticides/herbicides

Forensic Geology: Geotracking

different parts of the world have different soil

Minerals

naturally occurring crystals (2200 known)

• Look at color, shape, density, refractive index

• Fluorescence

Rocks

contain a combination of different minerals

Ingredients of Glass

1. Sand (mostly silica sand)

2. Soda (Na₂CO₃) - lowers melting point

3. Lime (CaO) - makes it water insoluble

• Combination of metal oxides:

  • Ex: Sodium, Calcium, Magnesium, Aluminum

• Melting point between 1,500 and 2,500 degrees celsius

Tempered (Safety) Glass

• glass dices (shatters into small pieces, not splintering) on impact

• strong from lamination (plastic between glass layers)

Pyrex Glass

contains boron oxides to be able to withstand high temperatures

Glass Fractures

able to determine the direction of a shot through glass

• Concentric — circular fractures that originate on the force side of glass

• Radial — right angle fractures that originate on the reverse side of force

Multiple Shot Rule

cracks don’t cross cracks

Problem with Glass Identification

• most glass is uniform in its making (with respect to its refractive index)

Density Determination

D = m/V

• sink or float

• density gradient tubes

• volume displacement method

Refractive Index Determination

Becke Line — bright halo that disappears when medium and fragment have the same refractive indexes

Birefringence (aka Double Refraction)

• Refraction of light in 2 directions

• Sandwich between polarizing filters

Paint and Locard’s Principle

• Small pieces of paint are often unwittingly transferred between objects during

  • Vehicle accidents

  • Burglaries

  • Robberies

  • Assaults

  • Homicides

  • Even from simple contact with freshly painted surfaces during a crime

Paint Composition

Pigment: very tiny particles of organic and inorganic colored compounds that give the paint its characteristic hue

Binder: suspends the pigment particles and helps to firmly fix them to the surface

Solvent: such as water or an organic liquid, provides a consistency suitable for spreading the paint on the surface

Recording Impressions

• take a photo

• make a cast

Info Learned from Shoe Impressions

1. Class Info

   1. manufacturer

   2. make

   3. model of footwear (based on sole pattern)

      1. aided by SICAR and TreadMark databases

      2. footwear impressions may be individualized by identifying imperfections

2. Determination of how many people and objects were involved

3. description of the movements of the participants and objects

4. establish a timeline and sequence of actions that occurred during an incident

5. support or refute eyewitness, suspect, and victim accounts of what occurred

Information Learned from Tire Marks

1. Class Traits

   1. manufacturer

   2. mode

   3. year of manufacture

2. Individual Traits

   1. rocks

   2. nails

   3. etc

Tool Marks

Individualized Marks

1. Class Traits (Typical Features)

2. Individual Traits (Accidental of Identifying Features)

   1. cuts

   2. tears

   3. gouges

   4. wear marks

Rifling

grooves in the barrel of a gun that spin the bullet to aim it straight

Lands

raised portion of rifling

Grooves

recessed portion of rifling

Striations

linear marks on bullets after they have been fired

Caliber

diameter of a rifle or handgun barrel between opposite lands

Bullet to Bullet to Gun Comparison

use a comparison microscope to compare striation patterns on bullets

Rifles and Handguns

rifled firearms

• leaves striations

Shotguns

non-rifled firearms

• no striations

• pellets as ammunition

Gauge

diameter of barrel of shotgun

Ballistic Mechanics

1. Trigger pulled

2. Activates firing pin

3. Hits primer

4. Gunpowder ignited

   1. gunpowder stored in cartridge

5. Gases propel bullet through barrel

6. Spent casing bashed against breechblock

7. Spent cartridge removed from barrel by an extractor

8. Spent cartridge thrown out of gun by the ejector

   1. extractor and ejector are a source of markings

Close Range, <6 in

• muzzle fire burns or melts clothes

• concentrated burn marks and shot residue immediately around hole

Shot 12 in Away

halo of smoke-soot around hole

About 18 in Away

scattered specks of shot residue with less soot around hole

Greiss Test

used to determine the presence of gunpowder residue on clothes

• press a treated photo paper against questioned surface with an iron

• nitrate residue stuck and chemically enhanced

Bullet Trajectory Analysis

used to determine from where a shot was fired

Integrated Ballistic Identification System (IBIS)

combined FBI and ATF database of fired bullets and cartridges

• input and recover digital images of the markings found on fired bullets and cartridge cases either recovered from crime scenes or from test-fired weapons

• digital images are then compared with those previously stored

• computer matching system