Chapter 17: Soil and Glass

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

Flat glass – is used to make windows and windshields and can also be shaped to form light bulbs, headlights, and other materials.
Container glass – used to make bottles and drinking glasses.
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. \n
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. \n
Preliminary tests include color, surface characteristics, flatness, thickness, and fluorescence. These tests are also valuable in comparing known and unknown samples. \n
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.
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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.