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April 1999 Volume 1
Number 1
Chapter 7 of Forensic
Fiber Examination Guidelines
1.0. Scope
These guidelines are intended
to assist individuals and laboratories that conduct examinations
of fabrics and cordage. They are not intended as a detailed methods
description or protocol for the analysis and comparison of fabric
and cordage evidence but as guidelines to an acceptable procedure
to strengthen the usefulness of the analytical method.
2.0.
Reference Documents
SWGMAT Quality Assurance
Guidelines
SWGMAT Trace Evidence Handling Guidelines
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3.0. Terminology
Core: A fiber or fibers running lengthwise
through the center of a cordage.
Course: The row of loops or stitches running
across a knit fabric, corresponding to the filling in woven fabrics.
Crown: The raised portion of a strand in
a twisted cordage.
Knit Fabric: A structure produced by interlooping
one or more ends of yarn or comparable material.
Pitch: The number of crowns per inch of
the same strand.
Ply: The number of single yarns twisted together
to form a plied cord. An individual yarn in a plied yarn or cord.
Selvage: The narrow edge of woven fabric
that runs parallel to the warp. It is made with stronger yarns
in a tighter construction than the body of the fabric to prevent
raveling.
Strand: A single fiber, filament, or monofilament.
Twist (Lay): The direction of twist in yarns
is indicated by the capital letters S and Z. Yarn has an S-twist
if when it is held vertically the spirals around its central
axis slope in the same direction as the middle portion of the
letter S, and it has a Z-twist if the spirals slope in the same
direction as the middle portion of the letter Z.
Wale: A column of loops lying lengthwise
in a knit fabric.
Warp: The set of yarn in all woven fabrics
that runs lengthwise and parallel to the selvage. It is interwoven
with the filling.
Weft (Filling): In a woven fabric, the yarn running
from selvage to selvage at right angles to the warp.
Woven Fabric: Generally used to refer to fabric
composed of two sets of yarns, warp and weft (filling), that
is formed by weaving, which is the interlacing of these sets
of yarns.
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4.0.
Summary of Guidelines
Because of their general
availability, fabric and cordage are often encountered by forensic
scientists, who must compare these types of evidence in order
to determine if the two pieces could have originated from the
same source. Structural details such as design, construction,
and composition can provide information that may assist the examiner
in reaching a conclusion.
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5.0.
Significance and Use
The construction, composition,
and color of textiles involved in crimes are useful comparison
characteristics for forensic examinations. Textiles appear in
a variety of weaves, knits, and nonwoven constructions, and a
combination of fabric types can occur in any one textile. The
range of colors in which textiles are offered in the marketplace
is enormous. Therefore, the construction, composition, and color
of a textile can aid the examiner in including or excluding a
textile for consideration in a forensic examination.
A complete characterization
of the fabrics including their construction and other materials
used in the completion of a textile (e.g., sewing thread) is
a critical component of a comprehensive forensic fabric or cordage
examination.
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6.0.
Sample Handling
Photograph the item prior
to conducting any analyses in order to provide documentation
of original condition. Document and remove other evidence (e.g.,
hair, blood, and paint) that may require additional analysis.
Document and record descriptions of any physical damage (e.g.,
worn, cut, broken, and frayed). The following general macroscopic
characteristics should be observed and documented:
6.1. Severed ends for possible physical matches;
6.2. Knots, ligatures, or both;
6.3. Dimensions: size, length, diameter, etc.;
6.4. Components: number, type, and twist;
6.5. Color;
6.6. Dyed; and
6.7. Natural.
Do not bring a questioned
specimen (e.g., a piece of fabric, yarn, and tuft of fibers)
in contact with the known fabric from which it is suspected to
have originated until you have performed a preliminary examination
of the questioned specimen.
Do not alter the condition
of a questioned specimen (e.g., shape, position, layers, or relation
of one yarn to another) before a preliminary examination and
before receiving a known sample for comparison.
Do not cut a sample to be
used for composition testing from ends of yarn or edges of fabric
if there is a possibility of physically matching a questioned
specimen to a known specimen. Take the known sample away from
the existing edge or edges and mark the location as known taken.
Fabric and cordage may be
a source of other types of physical evidence (hairs, fibers,
blood, etc.). In addition, cuts, tears, knots, and severed ends
may be of forensic value. Therefore, fabric and cordage evidence
should be examined in a manner that preserves these types of
evidence.
All pertinent data collected
on questioned and standard samples should be placed into or referenced
within the specific case file.
Reference samples should
be maintained. These reference samples should be supplied by
the primary manufacturer. If not purchased from a primary source
(manufacturer), structural components must be verified by a secondary
source.
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7.0. Analysis
Preliminary examination of
fibers composing a fabric or cordage, with any adhering matter,
should include its general appearance under a low-power microscope
before a sample is mounted on a slide. Any adhesives or other
material used in bonding fabrics, carpet backings, and so forth
should also be noted.
Physical matches should always
be considered if two pieces of fabric or cordage need to be compared.
If the ends have been cut or torn, a physical match may be possible.
A physical match must be documented by photography. Additionally,
describing the condition of corresponding threads and their relative
positions in the damaged area on the questioned and known pieces
(so-called longs and shorts) provides a detailed corroborative
description.
If a physical match is not
possible, comparison of the parameters determined in the checklist
will assist the examiner in determining if the two pieces could
have originated from the same source.
7.1. Fabric
Fabric examinations are primarily a process of deconstructing
the fabric by dissecting its constituent elements. Each of these
elements can have a number of sub-elements, all of which must
be characterized to complete the examination. These elements
include the following:
7.1.1. Construction (woven, knit, nonwoven);
7.1.2. Threads per inch in warp and weft direction;
7.1.3. Staple or continuous fibers in yarns;
7.1.4. Yarn twist;
7.1.5. Number of plies;
7.1.6. Direction of twist of plies;
7.1.7. Number of filaments in each ply;
7.1.8. Composition of yarn;
7.1.9. All fiber types composing the fabric;
7.1.10. Colors and design;
7.1.11. Blend of two or more types of fibers within each ply;
and
7.1.12. Sewing threads, buttons, decorations, and so forth as
detailed previously.
The information contained
on tags in textiles should also be recorded, especially the registered
number (RN) and the woolen products label (WPL) number. These
refer to the manufacturer of the textile and can assist the examiner
with tracking a particular textile or garment (4, 5).
7.2. Cordage
The initial step in the identification of rope and cordage is
to determine its construction and assembly. Wiggins (2) recommends
that a laboratory develop a checklist for this purpose. The checklist
should include, but is not limited to, the following characteristics
(see Figure 2):
7.2.1. Diameter;
7.2.2. Staple or filament fibers;
7.2.3. Twisted, braided, or nontwisted;
7.2.4. Twist;
7.2.5. Crowns or turns per inch;
7.2.6. Number of plies or braids;
7.2.7. Twist of each ply or braid;
7.2.8. Crowns or turns per inch; and
7.2.9. Filaments in each ply or braid, which are evaluated for
the following characteristics:
7.2.9.1 Core, if any;
7.2.9.2. Twist;
7.2.9.3. Crowns or turns per inch;
7.2.9.4. Number of filaments;
7.2.9.5. Color or colors;
7.2.9.6. Coatings, if any;
7.2.9.7. Tracers, if any; and
7.2.9.8. Coatings.
After the construction has
been established, then the constituent fibers should be analyzed
with the appropriate microscopic and instrumental techniques
(q.v.). Additional characteristics may be used if necessary to
adequately describe the cordage (3).
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8.0.
Report Documentation
Physical matches should be
reported so they indicate if the two or more pieces of material
were at one time a continuous piece of fabric or cordage. If
no physical match is possible, a complete fiber comparison, including
construction, must be performed. If the items are the same in
all tested characteristics, then the examiner would report that
the two objects exhibit the same color, construction, and composition
and are consistent with originating from the same source.
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9.0. References
(1) Hatch, K. L. Textile
Science. West Publishing Company, Minneapolis, Minnesota,
1993.
(2) Wiggins, K. Recognition,
identification, and comparison of rope and twine, Science
and Justice (1995) 35:53-58.
(3) Himmelfarb D. The
Technology of Cordage Fibres and Rope. Textile Book Service,
Metuchen, New Jersey, 1957.
(4) -—. The Cordage
Directory. The Cordage Institute, Hingham, Massachusetts,
1998.
(5) —. RN/WPL Encyclopedia.
Reed Reference Publishing, New Providence, New Jersey, 1996.
(6) —. Davison's
Textile Blue Book. Ed., B. W. Nealy. Davison Publishing Company,
Concord, North Carolina, 1996.
(7) Budworth, G. Knots
and Crime. Police Review Publishing Co., London, 1985.
(8) Hearle, J. W. S., Lomas,
B., Cooke, W. D., and Duerden, I. J. Fibre Failure and Wear
of Materials: An Atlas of Fracture, Fatigue, and Durability.
John Wiley and Sons, Inc., New York, 1989.
(9) Mahall, K. Quality
Assessment of Textiles: Damage Detection by Microscopy. Springer-Verlag,
New York, 1993.
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10.0.
Bibliography
—. Dictionary of
Fiber and Textile Technology. Hoechst-Celanese Corporation,
Charlotte, North Carolina, 1990.
Hatch, K. L. Textile Science.
West Publishing Company, Minneapolis, Minnesota, 1993.
Oelsner G. H. A Handbook
of Weaves. Dover Publications, Inc., New York, 1952.
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FORENSIC SCIENCE COMMUNICATIONS APRIL 1999 VOLUME 1 NUMBER 1 |
