17.1: Soil

• Soil –  earth material, either natural or man-made (concrete, gravel, other building materials), that is transferred from a crime scene to a person or object, or vice versa.

• Soil contains both organic and inorganic materials. The organic materials are essentially decayed and decaying vegetative and animal matter.

• Humus – a collective organic fraction of soil.

• The inorganic part of the soil is generally crushed rock and clay materials — made up of minerals.

17.2: Analysis of Soils

The majority of forensic soil cases consist of footwear where someone has left a shoe print in soil or automobile cases where a tire tread has been imprinted in soil – though it doesn’t involve analysis.

• One of the most important tests done on soils is to determine the particle size distribution.

• Physical properties are fairly easy to measure, and the tests are inexpensive and not too consumptive of material.

• The color of soil is affected by moisture content, mineral distribution, and location.

  • Dusty dry soils – tend to be light tan or white owing to lack of moisture.

  • Agricultural or tropical soils – dark brown owing to the high humid content.

• Microscope – the most important tool in the forensic soil scientist’s toolbox.

  • Solid should be examined through a stereomicroscope.

• Particle analysis – the key to understanding the nature of the samples under examination

• Chemical analysis of soil is performed less frequently than physical analysis.

  • Infrared spectrophotometry is commonly used for chemical analysis.

17.3: Glass

• Glass – an amorphous solid; a hard, brittle material that is usually transparent, but which lacks the ordered arrangement of atoms.

• Common glass – is made up largely of oxides of silicons that have been doped with other materials to give it its familiar properties.

• Glass is manufactured by melting sand and other desired ingredients and then allowing them to cool without crystallizing.

• Float glass – made by pouring the molten glass from a furnace into a chamber that contains a bed of molten tin.

• Soda Lime Glass – is made when calcium oxide and sodium carbonate are added.

• Borosilicate Glass – is used in cookware, thermometers, and automobile headlights because it can take fast, extreme changes in temperature without cracking or shattering.

• Tempering – a process whereby the glass is heated and cooled rapidly, producing deliberate stress on the surface.

Three major types of glass were encountered as evidence in cases:

1. Flat glass – is used to make windows and windshields and can also be shaped to form light bulbs, headlights, and other materials.

2. Container glass – used to make bottles and drinking glasses.

3. Glass fibers –  used to make fiberglass and fiber optic cables, as well as glass–plastic composite materials.

• Optical glass –  used to make eyeglass lenses and similar materials may be encountered in forensic cases.

17.4: Forensic Examination of Glass

• Mechanical Fit – a.k.a. Fracture match; where glass can be individualized to a particular object.

  • This occurs when a piece of glass breaks into relatively large pieces that have at least one, good, intact edge that can be fitted to the edge of another piece from the same source.
    

• The most important of these is the identification of the material as glass, density, and refractive index. Other tests include color, thickness, flatness, surface features, and fluorescence.
  

• Preliminary tests include color, surface characteristics, flatness, thickness, and fluorescence. These tests are also valuable in comparing known and unknown samples.
  

• Becke Line Method – a technique in optical mineralogy that helps determine the relative refractive index of two materials.

  • It is very accurate and precise, but it is always advisable to get as much data as possible before rendering an opinion about the association of evidence.

• Glass Refractive Index Measuring Instrument (GRIM) – used in most forensic science laboratories to determine the refractive index of small glass particles.

17.5: The Effects of Projectiles on Glass

• If the glass is very thin or the projectile is large or powerful, the glass will shatter.

• When a high-speed projectile, such as a bullet, passes through a piece of glass, a crater will form in the glass that is larger on the exit side of the glass.

• Radial Cracks – will form on the side of the glass opposite to the side of the impact.

• Concentric Cracks –  fractures forming in an approximately circular pattern around the point of impact.