1. Scope
This document outlines the methods intended for use by forensic
glass examiners for the collection, handling, and identification
of glass. The particular methods employed by each examiner and/or
laboratory will depend upon sample size, sample suitability, laboratory
equipment, and the purpose of the examination.
2. Reference Documents
2.1. |
Scientific Working Group for Materials Analysis
Documents
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Quality assurance guidelines
Trace evidence recovery guidelines
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2.2. |
American Society for Testing and Materials
Standards |
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D1395 Standard Definitions of Terms Relating
to Glass
E1492 Practice for Receiving, Documenting, Storing, and Retrieving
Evidence in a Forensic Laboratory |
3. Terminology
Anisotropy is the property of having different refractive
indices depending on the vibration direction of light.
A conchoidal type of fracture is observed when glass breaks
to give irregularly curved and usually striated surfaces.
Extinction is the condition that occurs when the vibration
directions of the specimen are parallel to the direction of either
of the two crossed polars. At extinction, the specimen will appear
dark.
Glass is an inorganic product of fusion that has cooled to
a rigid condition without crystallizing.
Interference colors are colors produced by the interference
of rays of white light that are out of phase.
Isotropy is the property of having the same refractive index
regardless of the direction of vibration of light passing through
the material. An isotropic particle will exhibit extinction at all
orientations between crossed polars.
Known sample is a subset of a larger population or sample
originating from an identifiable source, collected as being representative
of that larger grouping (e.g., fragments removed from a broken window
at the scene of a crime).
Polarized light microscope (PLM) is a microscope equipped
with two polarizing elements, one (polarizer) located between the
light source and the sample and the other (analyzer) between the
sample and the observer.
Questioned sample is material of an unknown source collected
from a known location either as, or from, items of evidence (e.g.,
fragments recovered from a suspect's clothing).
Stereomicroscope is a microscope containing two separate
optical paths, resulting in a three-dimensional view of a specimen.
4. Summary of Guideline
This guideline covers the collection and handling of glass specimens,
the recovery of questioned particles from suspect items such as
clothing and tools, the cleaning of questioned particles, and procedures
for the identification of glass particles.
5. Significance and Use
Glass samples should be collected in a manner consistent with generally
recognized and accepted practices such that they are adequate for
analysis and interpretation of results within the context of an investigation.
All of the sampling techniques listed below are acceptable. However,
one may be more appropriate than another depending on circumstances,
such as the nature of the crime scene or analytical methodologies
to be employed. For additional information see to the Scientific
Working Group for Materials Analysis Trace Evidence Recovery Guidelines.
6. Sample Handling
6.1.
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Considerations for Collection
of a Glass Sample |
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6.1.1.
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The collector must consider that fragments within
a questioned sample may have multiple origins. If possible,
the collector should attempt an initial separation based on
physical properties. |
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6.1.2.
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The collector must consider the possibility there
may be a physical match to a known sample (e.g., a piece of
glass to a fractured vehicle headlamp). When an attempt to make
a physical match is made at the site of collection, the collector
should take precautions to avoid mixing of the known and questioned
samples. |
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6.1.3.
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Any glass samples collected should
be documented, marked (if necessary), packaged, and labeled
as recommended in the Scientific Working Group for Materials
Analysis Trace Evidence Recovery Guidelines. |
6.2.
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Considerations for Receiving Glass
Evidence |
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6.2.1.
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Check the evidence packaging for
the presence of damage (i.e., tears, cuts) and make a notation
of any damage detected. |
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6.2.2.
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Maintain a separation of all known and unknown
samples, avoiding contamination at all steps of collection and
examination. |
6.3.
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Collecting the Sample |
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6.3.1.
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The glass sample should consist of
the largest amount that can be practically collected from each
broken object and packaged separately. The sample should be
removed from the structure (e.g., window frame, light assembly).
The inside and outside surfaces of the known sample should be
labeled if a determination of direction of breakage or reconstruction
of the pane is desired. If multiple panes are sampled, a diagram
to show relative positions from which samples are taken can
be helpful. |
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6.3.2.
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When multiple broken glass sources are identified,
it is necessary to sample all sources. For instance, when multiple
vehicle windows are involved, a sample from each of the windows
should be collected (e.g., both sides of a laminated windshield,
each side window, mirrors, and headlights). |
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6.3.3.
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A sample should be collected from various locations
throughout the broken portion of the object in order to be as
representative as possible. |
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6.3.4.
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The sample should be collected with consideration
being given to the presence of other types of evidence on that
sample (e.g., fibers, blood). |
6.4.
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Collecting Glass Samples from
Garments, Footwear, and Other Items of Evidence |
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6.4.1.
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The evidentiary item to be examined is removed
from its packaging and transferred to a clean surface.
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6.4.2.
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The packaging that contained the
item should be examined for the presence of glass or other trace
evidence. |
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6.4.3.
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Garments and other textile products should be
examined and their condition noted. Particular attention should
be paid to damage such as tears and cuts. Any cuts or holes
in the soles of footwear should be probed for the presence of
glass. Any glass fragments that are found should be documented
with their locations and recovered, if appropriate. Cuffs or
pockets should be examined and turned out to free any glass
that may be present. The item can then be tugged, shaken, or
scraped over the collection surface. Other sample collection
techniques may be employed, as appropriate. Anything detected
on the collection surface should be recovered as recommended
in the Scientific Working Group for Materials Analysis Trace
Evidence Recovery Guidelines. |
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6.4.4.
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Items such as tools or bats should be examined
microscopically, giving special consideration to any damaged
area or powdered substance. Any glass fragments that are found
should be noted and recovered, if appropriate. |
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6.4.5.
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The area from which any sample is collected should
be documented as specifically as possible. |
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Cleaning Glass Samples |
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6.5.1.
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The condition of the glass should be noted,
as well as consideration given to the fact that any material
removed from the glass may have its own evidentiary value.
The fragment must be cleaned to remove dirt, grease, and other
debris that may have an effect on any analytical procedure
to be performed on the fragment.
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6.5.2.
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The fragments can be cleaned manually
using an appropriate solvent. The fragment can also be cleaned
using detergent in an ultrasonicator, then rinsed, and dried. |
6.6.
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Crushing Glass
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6.6.1.
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If crushing the fragment is required, it should
be in a manner that will allow for the maximum recovery and
minimum introduction of foreign materials. Consideration should
be given to obtaining crushed samples of specific interest
(e.g., near surface, bulk fragments from known samples) and
maintaining the evidentially significant features of the original
fragment (e.g., original surfaces on questioned fragments).
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6.6.2.
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It may be necessary to fracture the
original fragment to remove a sufficient piece for further crushing
in order to obtain particles of proper dimension. |
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6.6.3.
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If the sample is limited, nondestructive
methods should be used before subjecting the sample to any destructive
testing. |
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6.6.4.
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Unless the sample is consumed during
analysis, the examiner should retain a portion of every sample
analyzed and return it to the submitter or preserve it, as appropriate. |
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6.6.5.
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If the entire sample is required
for analysis, attempts should be made to maintain the sample
in its tested state. |
7. Analysis
7.1.
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Characterizing Particles as Glass
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7.1.1.
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Glass fragments can be distinguished from
plastic by their relative hardness. If the careful application
of pressure from a needle causes deformation of a particle,
it is not glass.
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7.1.2.
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Some plastics are soluble in some
organic solvents. If the questioned particle exhibits this characteristic,
it is not glass. |
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7.1.3.
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Because glass is an amorphous material,
it breaks in a random manner. Some glass fragments can be recognized
by their conchoidal fracture.
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7.1.4.
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Glass is an isotropic material. An
examiner can determine if an unstrained particle is isotropic
by placing or mounting it on a glass microscope slide and observing
it with a polarized light microscope. |
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7.1.4.1.
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Insert the analyzer (cross the polars). If the
particle is unstrained glass, it should appear extinct. Rotate
the microscope stage 360 degrees. If the particle remains extinct
at all orientations, the particle is isotropic. If interference
colors are observed, either the particle is anistropic and therefore
not glass, or it is glass that has been subjected to mechancical
or thermal stress. |
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7.1.4.2.
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A stereomicroscope with a polarizing attachment
may also be used without mounting a particle in an immersion
medium. This is an extremely useful technique for screening
a large number of samples. |
7.2.
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Upon the completion of the initial screening,
separation, and cleaning, the samples can then be analyzed
or retained for later analysis.
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8. Considerations
The number of glass fragments recovered from a questioned source
may be important to the findings in a case. The number, or approximate
number, of fragments recovered should be noted. The location of
a recovered fragment of glass may also be relevant to the findings
and, as such, should be noted when possible and applicable.
9. Bibliography
Koons, R. D., Buscaglia, J., Bottrell, M., and Miller, E. T. Forensic
glass comparisons. In: Forensic Science Handbook. 2nd ed.
R. Saferstein, ed. Prentice-Hall, Upper Saddle River, New Jersey,
2002, Volume I, pp. 161-213.
McCrone, W. C., McCrone, L. B., and Delly, J. G. Polarized Light
Microscopy. McCrone Research Institute, Chicago, 1984.
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