FOR10002 Lecture Notes - Lecture 9: Viscous Liquid, Lead Glass, Boron Trioxide
FORENSIC SCIENCE - FOR10002
WEEK 10
MATERIAL SCIENCE AND
FIREARMS
Trace evidence will often involve materials such as glass, paints or fibres !
These materials are mostly considered to be manufactured materials !
•Natural or man-made !
•Unique structure and source !
•Once identified then possible to be traced!
Glass!
Glass is hard brittle material manufactured by cooling melts consisting of silica (SiO2) mixed with
other oxides. It is most commonly sodium or potassium (or both) but may also contain calcium,
magnesium, aluminium or lead oxides. Glass is a supercooled liquid which doesn’t have a
crystalline structure.!
•Soda-lime glass is the most frequently encountered — eg. windows and containers such as
bottles and drinking glasses !
•Heat resistant glasses include high levels of boron oxide (B2O3) — eg. Pyrex !
•Lead crystal contains high levels of lead oxide !
•Decorative glass due to high refractive index !
TYPES OF SAFETY GLASS
•Tempered – rapid heating and cooling stresses the glass structure — “annealing” causes the
glass to break into smaller cube-like pieces !
•Laminated – a layer of plastic film holds two layers of glass together !
•Reinforced – a metal screen is embedded in the glass!
FORENSIC COLLECTION OF GLASS
•Breakage of glass often occurs during the process of a crime !
-Theft from or of vehicles and burglary !
•Types of glass involved typically include !
-Windows and lights (both household and automotive) !
-Containers !
-The fragmentation pattern and the actual fragments are important evidence !
•Collection of all fragments enable the original piece of glass to be reassembled – missing
fragments might have been carried away !
-A “physical match” describes a physical fit between fragments !
-This is the only scenario that can conclusively determine that two pieces of glass
originated from the same source!
REFRACTIVE INDEX OF GLASS
Refractive Index measures the relative speed of light
within a substance. Light travels through a substance
by a continuous absorption- reemission process.
Refractive index of materials varies with the wavelength
of light — A slightly different refractive index for each
colour!
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In a forensic examination it is measured by immersing the glass fragment in oil of known refractive
index. The temperature of the oil is increased until the refractive indices of both the oil and the
glass are the same. This point is determined microscopically by noting the temperature where the
“Becke line”, a halo of light surrounding the glass, disappears. Measurements are usually taken at
four sites and averaged!
DENSITY OF GLASS
Density is a non-destructive and informative test.!
Density is an additional piece of data to identify a fragment. Most glasses have a density of ~2.5
grams per cm3. Density is measured using a mixture of two liquids. !
1. One higher than 2.5 g/cm3 and one lower. !
2. The liquids are mixed in varying proportions until the glass fragment becomes suspended !
3. The proportions of the two liquids then determines the density !
4. The test fragment is then replaced with a comparision sample !
5. If it floats or sinks then it is not the same as the test specimen!
!
FRAGMENTATION OF GLASS
The direction of impact can be determined from the distribution of
fragments !
•Large fragments will travel in the direction of impact !
•Small fragments will “blow backwards” and may deposit on
perpetrator !
The order of impact for multiple impacts can be determined !
•Radial cracks will spread until terminated by an existing crack!
A low velocity impact may create concentric rings as well as
radial cracks !
A small, fast moving projectile will create a crater !
•Point of entry smaller than point of exit!
CHEMICAL ANALYSIS OF GLASS
The composition of glass is dependent on the raw materials used to manufacture the glass.
Additives are included that affect colour and properties !
•Red due to cadmium, blue due to cobalt, etc !
Slight variation in trace components within a large pane of glass but usually not great enough to
prevent comparisons. It sometimes results in destructive testing. It is also technique dependent.!
Acid digestion for Atomic Absorption Spectroscopy (AAS) — Expensive equipment requiring
trained technicians !
SEM-EDX – Scanning Electron Microscopy/ Energy-dispersive X-ray spectroscopy!
SEM-EDX
A Scanning Electron Microscope uses a beam of electrons instead of light to probe the surface of
a sample. Higher magnification than an optical microscope (x 1 000 000). X-rays generated from
the sample enable elemental analysis (EDX). Can be used for glass, paint and gun-shot residue
(GSR) analysis.!
SUMMARY
•Amorphous structure results in random shapes when broken !
-Reconstruct original item !
•Large density and refractive index databases available !
-Useful for quickly grouping glass type !
•Pattern of broken glass reveals type and direction of impact !
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-Small particles easily transferred !
•Fracture pattern displays order of multiple impacts !
-Second crack terminated by first cracking !
•Range of additives used to produce glasses with different properties !
-Unique chemical composition enables traceability!
Hair and Fibre Analysis
Fibres can be classified depending on their nature !
•Databases exist that contain information about colour, cross-sectional shape, refractive index
and melting point that enable identification!
Hair is a specialised fibre produced by mammals !
•Growth occurs below the skin level !
•Follicle contains the living portions!
•The follicle is associated with a sebaceous gland !
•This gland produces oil !
•The shaft of the hair is keratin !
•Tremendously durable protein !
•Resistant to degradation and chemical attack!
HAIR ANALYSIS
Visual examination is the primary method of analysis
for hairs !
•Possible to observe characteristics such as colour, length, growth stage and condition !
-Identify body region source due to known variation of characteristics !
-Scalp hair is morphologically different to pubic hair !
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find more resources at oneclass.com
Document Summary
Trace evidence will often involve materials such as glass, paints or bres. These materials are mostly considered to be manufactured materials: natural or man-made, unique structure and source, once identi ed then possible to be traced. Glass is hard brittle material manufactured by cooling melts consisting of silica (sio2) mixed with other oxides. It is most commonly sodium or potassium (or both) but may also contain calcium, magnesium, aluminium or lead oxides. Forensic collection of glass: breakage of glass often occurs during the process of a crime. Theft from or of vehicles and burglary: types of glass involved typically include. Windows and lights (both household and automotive) The fragmentation pattern and the actual fragments are important evidence: collection of all fragments enable the original piece of glass to be reassembled missing fragments might have been carried away. A physical match describes a physical t between fragments.
