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Fiber Guidelines, Chapter 7 (FSC, April 1999)

Fiber Guidelines, Chapter 7 (FSC, April 1999)

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Masthead of Forensic Science Communications
April 1999   Volume 1   Number 1


Chapter 7 of Forensic Fiber Examination Guidelines

Fabrics and Cordage

Read about …

1.0. Scope
2.0. Reference Documents
3.0. Terminology
4.0. Summary of Guidelines
5.0. Significance and Use
6.0. Sample Handling
7.0. Analysis
Figure 2
8.0. Report Documentation
9.0. References

10.0. Bibliography

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

 

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