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April 1999 Volume 1
Number 1
Chapter 1 of Forensic Fiber Examination Guidelines
1.0. Scope
This document is an outline
of fiber analysis methods intended for use by forensic fiber
examiners. The particular methods employed by each examiner,
laboratory, or both will depend upon sample size, sample suitability,
laboratory equipment, and examiner training.
2.0.
Reference Documents
SWGMAT Quality Assurance
Guidelines
SWGMAT Trace Evidence Handling Guidelines
3.0. Terminology
Known (Sample): A subset of a larger population
or sample originating from a verifiable source, collected as
representative of that larger grouping; for example, a 2"
x 2" section of carpet from a suspect's living room.
Questioned (Sample): Materials collected as or from items
of evidence that have a known location but an unknown origin;
for example, loose fibers collected from a victim's clothing.
Class Characteristics: Traits that define a group of items
collectively.
Class: A group of items that share properties
or characteristics.
Individual Characteristics: Traits that define and identify
an item as unique and exclusive of all other items.
Individual: A unique item that is identified
as itself to the exclusion of all other items.
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4.0.
Summary of Fiber Analysis Guidelines
The various analytical methods
available for fiber analysis yield different kinds of information.
It is highly desirable to select a combination of methods and
apply them in an order that provides the most exclusionary information
first. By doing this, the examiner optimizes accuracy, precision,
and production while most effectively using the laboratory's
resources.
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5.0.
Significance and Use
5.1. Microscopy
At a minimum, a fiber examiner must employ a stereomicroscope,
a comparison microscope, and a compound light microscope equipped
with polarized light capability. The examiner must view questioned
and known fibers side by side at the same magnifications in visible
light, and alternative lighting, such as polarized light or fluorescent
lighting, although not necessary, is recommended if the equipment
allows. For some analyses, for example, in testing for solubility,
it may be necessary to place questioned and known fibers next
to each other on the same slide or in adjacent wells of a spot
plate. Extreme caution must be exercised with loose fibers in
these circumstances so as not to confuse the source of each fiber.
5.2. Comparison
Typically, fiber examinations involve a comparison of samples
from known and questioned sources to determine whether they are
consistent with having originated from the same source (e.g.,
carpet from a suspect's car compared with foreign fibers removed
from the victim's clothing). This comparison involves the recognition
and evaluation of class characteristics, which associate materials
to a group but never to a single source. Conversely, individual
characteristics allow the association between two or more items
with each other to the exclusion of all other items. For fiber
examiners, this most often occurs when pieces of fabric or cordage
are physically matched.
5.3. Source Determination
Textile fibers can be exchanged between individuals, between
individuals and objects, and between objects. When fibers are
associated with a specific source, such as fabric from the victim,
suspect, or scene, a value is placed on that association. The
probative weight of this value is dependent upon many factors.
The following subsections describe those factors.
5.3.1. Fiber type or types
found;
5.3.2. Fiber color or colors;
5.3.3. Number of fibers found;
5.3.4. Fiber location or locations;
5.3.5. Fabric type or types;
5.3.6. Multiple fiber associations;
5.3.7. Nature of contact; and
5.3.8.Fiber transfer and persistence.
Whether a fiber is transferred
and detected is also dependent on the nature and duration of
the contact between the suspect, the victim, or both and the
persistence of the fibers after they have been transferred.
5.3.1. Fiber Type or
Types. The rarity
or commonness of the fiber types found at a crime scene or on
a victim or suspect affects their probative value. Cotton fibers
are by far the most commonly used plant fibers in textile production.
The type of cotton, the fibers' length, and the degree of twist
contribute to the diversity found in cotton fibers. Processing
techniques, such as mercerization, and color applications also
influence the value of cotton fiber identifications. The presence
of other less common plant fibers at a crime scene or on the
clothing of a victim or suspect increases its significance.
The most common animal fiber
used in textile production is wool originating from sheep. The
fineness or coarseness of woolen fibers often dictates the end
use of wool. The finer woolen fibers are used in the production
of clothing, whereas the coarser fibers are found in carpet.
The diameter and the degree of scale protrusion of the fibers
are other important characteristics. Woolen fibers from other
animals may also be found, including camel, alpaca, cashmere,
and mohair. The identification of less common animal hairs, fibers,
or both at a crime scene or on the clothing of a suspect or victim
would have increased significance.
Over half of all fibers used
in the production of textile materials are manufactured. Some
manufactured fibers originate from natural materials such as
cotton or wood, whereas others originate from synthetic materials.
All nonnaturally occurring fibers are manufactured, but not all
manufactured fibers are synthetic (e.g., rayon). Certain types
of manufactured fibers are more common than others. Polyester
and nylon fibers are the most commonly encountered manufactured
fibers, followed by rayons, acetates, and acrylics. There are
also many other less commonly manufactured fibers. The amount
of production, the end use, the cross-sectional shape, microscopic
characteristics, and other traits of the fiber help to influence
the degree of rarity of a particular fiber type.
5.3.2. Fiber Color
or Colors. One
of the greatest variations seen in textiles is color. Thus, color
greatly influences the significance of a fiber comparison. Synthetic
dyes and pigments belong to 29 different chemical categories
with more than a dozen different application methods (7). Even
simple dyes might require between eight and ten processes to
convert the raw materials into a finished dye. Given that the
total annual production of any particular dye might not amount
to more than 10 tons and that small process batches are becoming
the rule in the dyeing industry, color becomes a powerful discriminant.
Color is particularly significant when the gamut of colors is
spread out over the range of garments and carpeting produced
in any one year and even more so when multiplied by the number
of garments and carpets produced in previous years.
Individual fibers can be
colored before being spun into yarn, yarns can be dyed after
being spun, or the fabric can be dyed before or after its construction.
Color can also be applied to the surface of a fabric by printing.
The absorbance of the dye along the fiber length suggests the
dyes and dyeing method used. Fading and discoloration may also
add increased significance to a fiber association.
5.3.3. Number of Fibers. The number of fibers identified
on the clothing of a victim associated to the clothing of a suspect
is important in determining actual contact. The greater the number
of fibers, the more likely that direct contact occurred between
these individuals. The converse is not necessarily true, however,
and even one fiber association can have probative and scientific
value. Additionally, finding no fibers does not de facto mean
that no contact occurred. Each case is different, and the examiner
must weigh all of the relevant factors before determining the
significance of the evidence.
5.3.4. Fiber Location. Where the fibers are found also
affects the probative value of a particular fiber association.
The location of fibers on different areas of the body or on specific
items at the scene can influence the significance of the fiber
association.
5.3.5. Fabric Type. Fabric construction affects the
number and types of fibers that may be transferred. Tightly woven
or knitted fabrics shed fewer fibers than loosely knit or woven
fabrics. Fabrics composed of filament yarns shed less than fabric
composed of spun yarns. Certain types of fibers also transfer
more readily. The condition and wear of the fabric also affects
the degree of fiber transfers: Newer fabrics may have an abundance
of loosely adhering fibers on the surface of the fabric, whereas
worn fabrics may have damaged areas that easily shed fibers.
Damage to a fabric caused during physical contact greatly increases
the likelihood of fiber transfer.
5.3.6. Multiple Fiber
Associations.
If many different fiber types are associated among the suspect,
victim, and scene, then the likelihood that contact occurred
between these items is greatly increased. Each associated fiber
transfer is considered to be an independent event, and multiple
associations undermine an argument that the fibers were all deposited
by coincidence.
5.3.7. Nature of Contact. The type of physical contact between
a suspect and a victim helps to determine the number of fibers
transferred and the value placed on their discovery. Violent
physical contact of an extended duration may result in many fiber
transfers.
5.3.8. Fiber Transfer
and Persistence.
Textile fibers are transferred to the surface of a fabric either
by direct (primary) transfer or indirect (secondary) transfer.
The likelihood of transfer depends on the types of fabric involved
in the contact and the nature and duration of the contact. Studies
have shown that transferred fibers are lost at a geometric rate,
depending on the types of fabrics involved and on the movement
of the clothing after contact (see endnote 1). For example, the
clothing of a homicide victim may retain transferred fibers for
a longer time because the victim is not moving. Therefore, under
these circumstances it is difficult to predict precisely how
many fibers might remain on the clothing of a living individual
after a given period, but it is important for investigators to
retrieve clothing immediately.
Whenever a fiber is found in relation to a crime scene, victim,
or suspect, it has potential significance. Matching dyed fibers,
whether manufactured or natural, can be very meaningful, whereas
the matching of common fibers such as white cotton or blue denim
cotton can would be less significant. In some situations, however,
the presence of white cotton or blue denim cotton possibly still
has some meaning in resolving the truth of an issue. The discovery
of cross transfers (suspect[s] to victim[s] and vice versa) dramatically
increases the likelihood that two items came into contact and
greatly reduces the likelihood of chance occurrence.
When a fiber examiner associates
a questioned fiber to a known textile item, there are ultimately
two possible explanations: (a) The questioned fiber originated
from the known textile, or (b) the questioned fiber did not originate
from the known textile.
To say that the questioned
fiber originated from the known textile, it either had to be
the only fabric of its type ever produced or now existing, or
the transfer of fibers was directly observed. As neither of these
situations is likely to occur, fiber examiners must conclude
that because the questioned fibers exhibit the same results in
all tested properties as the fibers from the known sample, the
questioned fibers are consistent with originating from the source
textile. Other textile sources that incorporate the same fibers
can be ruled out only by context and availability. In order to
say that a fiber did not originate from a particular textile
is to know the history of the textile or have observed the fiber
transfer from another textile.
5.4. Volume of Fiber Production
It could be argued
that the large volume of fibers produced reduces the significance
of a fiber association discovered in a criminal case. It can
never be stated with certainty that a fiber originated from a
particular textile because other textiles are produced using
the same fiber types and color. The inability to positively associate
a fiber to a particular textile to the exclusion of all others,
however, does not mean that a fiber association is without value.
Considering the volume of textiles produced worldwide each year,
the number of textiles produced with any one fiber type and color
is extremely small. The likelihood of two or more manufacturers
exactly duplicating all of the aspects of the textile is extremely
remote (see endnote 2). Beyond the comments made previously about
color, shade tolerance differs between dyeing companies. Therefore,
color may vary demonstrably from batch to batch. Also, the life
span of a particular fabric must be considered. Only so much
of a given fabric of a particular color and fiber type is produced,
and it will eventually end up being destroyed or dumped in a
landfill.
The world produced approximately
80 billion pounds of fabric in 1995, about half of which was
cotton (5). The other approximately 44 billion pounds of fiber
were manufactured or synthetic. Table 1 provides U.S. fiber production
levels.
Table 1.
U.S. Annual Production for Manufactured Fibers: 1995
(millions of pounds)
(Table 1 [6]). All these fibers
were used in a variety of applications including but not limited
to clothing, household textiles, carpeting, and industrial textiles.
|
Fiber |
Product |
|
Polyester |
3,887 |
|
Nylon |
270 |
|
Olefin |
521 |
|
Rayon/Acetate/Triacetate |
498 |
|
Acrylic/Modacrylic |
432 |
5.4.1. Significance. As an example, given a yarn-dyed
nylon fiber from a knit polo shirt of a specific color, the significance
could be described in the following way:
5.4.1.1. Total fiber production;
5.4.1.2. Total nylon (of that type) production;
5.4.1.3. Total nylon production in staple form;
5.4.1.4. Total production of Item 3 in a particular denier, cross-section,
optical characteristics, and luster;
5.4.1.5. Total amount of Item 4 used in production of garments;
5.4.1.6. Total garments constructed in the same fashion, including
knit specifications, collar, and sleeve incorporating Item 5;
5.4.1.7. Total of Item 6 in a specific color;
5.4.1.8. Total of Item 7 from indistinguishable dye lots;
5.4.1.9. Total of Item 8 available for merchandising;
5.4.1.10. Total of Item 9 sold;
5.4.1.11. Total of Item 10 still in existence;
5.4.1.12. Total of Item 11 available to be connected with a particular
criminal offense; and
5.4.1.13. Total of Item 12 actually connected with a particular
criminal offense (i.e., found and submitted as evidence).
The fiber examiner is still
limited to stating that the questioned fibers are consistent
with originating from the evidence garment, with the understanding
that all other garments listed under Item 11 (subsection 5.4.1.11)
may or may not be distinguishable from the evidence garment by
fiber analysis alone. This argument in no way intimates a positive
match to the evidence garment to the exclusion of all other garments.
Production numbers for textiles may be available for use in interpreting
the significance of evidence in a crime, but the examiner must
be careful to be conservative in all estimates in order to avoid
false inclusions (8). Calculating exact probability statistics
for this type of evidence is problematic at best, and professional
statisticians must be consulted before any calculations are reported
or testified.
5.5. Fiber Source
If questioned fibers
are associated with known fibers, the questioned fibers either
originated from the known textile or from another fabric source,
which not only is composed of fibers of the exact type and color
but also from a fabric that had to be available to contribute
those fibers through direct or indirect contact. The chance is,
therefore, remote to encounter fibers from the environment of
a victim that are identical to fibers from the suspects' environment
or environments in the absence of contact (9). Put another way,
the chance of finding known fibers from a randomly selected suspect
source that match the questioned fibers is remote (see endnote
3).
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6.0.
Sample Handling
Although examiners may be
consulted concerning proper sample size, collection, or packaging,
this may not happen, and the examiner must optimize the evidence
that is submitted. Garfield (1) and others (2) list the following
methods of sampling:
6.1. Probability Sampling
(So-Called "Random Sampling")
Every unit in the population has a known, nonzero probability
of being included in the sample (e.g., collecting about 33 percent
of the fibers from a pillbox or by taping and mounting them);
6.2. Nonprobability or
Judgment Sampling
Every unit in the population either is or is not included in
the sample on the basis of certain characteristics it has in
common with other units of interest (e.g., mounting only red
trilobal carpet-type fibers from the victims' evidence given
the suspect has red carpet as a possible source); and
6.3. Bulk or Lot Sampling
A sampling unit is taken from a larger amount of material that
does not consist of discrete and identifiable units. Special
considerations are involved with bulk sampling, such as where
the sample is taken, how much sample is taken, and if the sample
is considered representative of the lot (e.g., cutting a swatch
from a garment for fabric and fiber examination).
Samples are adequate for
analysis when they are taken in a manner consistent with generally
recognized and accepted sampling techniques and practices within
the context of the proposed analyses. All of the previously mentioned
sampling methods have their place, and one may be more feasible
than another, given crime scene or laboratory constraints. The
examiner must be able to explain how the samples were taken and
why that procedure was used.
Examinations typically should
be conducted in the order of increasing magnification, from gross
inspection to microscopical analysis. If sample size is limited,
nondestructive methods must be exhausted before subjecting the
sample to any destructive tests (e.g., pyrolysis).
It is highly desirable that
the methods be selected in an order that provides the greatest
discrimination between samples. An exclusion precludes further
analysis, thereby maximizing the examiner's time and resources.
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7.0. Analysis
There are three basic activities
involved in an analysis (1): (a) collection of a representative
sample; (b) preparation of the sample for analysis; and (c) analysis
using appropriate methods.
Although these activities
are ostensibly independent of each other, any one can have a
significant effect on another. Because error is possible at each
step, the examiner must be able to identify these errors and
avoid them. Any method of analysis has certain attributes such
as accuracy, precision, specificity, sensitivity, dependability,
and practicality that must be considered when choosing the most
appropriate method to adequately answer the question at hand.
Ultimately, it is the examiner's responsibility to evaluate all
of the available information and decide the level of uncertainty
that is acceptable with a given method on a given set of samples.
7.1. Physical Matches
A physical match occurs when two or more pieces of fabric or
cordage are reconstructed to prove they were previously one continuous
piece of fabric or cordage. This examination is conducted by
describing and documenting any cut, torn, or damaged edges on
questioned items and their correlation to like areas on known
items. Photography is the recommended method of documentation.
Depending upon sample size,
suitability, and exhibited characteristics, it may not be possible
to effect a positive physical match. For descriptions of physical
construction refer to the fabric and cordage guidelines in Chapter 7 of this document.
7.2. Fiber Examinations
Fiber identifications consist of determining the generic class
of fiber type, which generally follows the Federal Trade Commission
Guidelines (3). This analysis requires a sufficient number of
examinations to unequivocally place the fiber in question into
one and only one generic class (see Table
2).
Fiber comparisons consist
of determining if a questioned fiber or fibers exhibits the same
chemical, microscopic, and optical properties as fiber or fibers
comprising part or all of a known sample. A comparison requires
an examiner to complete at least two of the analytical techniques
listed for each of the following categories: generic class, physical
characteristics, and color (see Figure
1). The techniques selected should independently confirm
the results obtained. It should be noted that some techniques
allow greater discrimination than others between apparently similar
samples.
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8.0.
Report Documentation
Laboratory results should
be reported in a uniform and consistent manner. Format, units
of measurement, and accepted calculations should all be documented
in the laboratory's manuals. The contributor of the evidence
must be able to "interpret the results and understand their
significance" (1). The International
Organization for Standardization (ISO) recommends that reports
be clear, accurate, and unambiguous in the presentation of results
(4). Refer to the appropriate sections of the SWGMAT Quality
Assurance Guidelines for further information.
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9.0. References
(1) Garfield, F. M. Quality
Assurance Principles. Association of Official Analytical
Chemists, Arlington, Virginia, 1991.
(2) Levy, P. S. and Lemeshow,
S. Sampling of Populations. John Wiley and Sons, New York,
1991.
(3) Federal Trade Commission
Rules and Regulations Under the Textile Products Identification
Act, Title 15, U.S. Code Section 70, et seq. 16 CFR 303.7.
(4) International Standards
Organization, Guide 25. American National Standards Institute,
New York, 1982.
(5) Layman, P. Growth in
man-made fibers slowed in 1995, Chemical and Engineering News
(May 27, 1996), p. 13.
(6) Fiber Organon,
January 1996.
(7) Aspland, J. R. What are
dyes? What is dyeing? In: AATCC Dyeing Primer. American
Association of Textile Chemists and Colorists, Research Triangle
Park, North Carolina, 1981.
(8) Deadman, H. A. Fiber
evidence and the Wayne Williams Trial, FBI Law Enforcement
Bulletin (March and May 1984).
(9) Grieve, M. C. Fibres
and their examination in forensic science. In: Forensic Science
Progress (Vol. 4). Eds. A. Maehly and R. L. Williams. Springer,
New York, 1990.
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10.0.
Bibliography
ASTM Annual Book of Standards:
(Vol. 7.01-7.02). Textiles.
American Society for Testing and Materials, West Conshohocken,
Pennsylvania, 1996.
Brunello, F. The Art of
Dyeing. Trans. B. Hickey. Neri Pozza Editore, Vicenza, Italy,
1973.
Dictionary of Fiber and
Textile Technology.
Hoechst-Celanese Corporation, Charlotte, North Carolina, 1990.
Technical Manual of the
American Association of Textile Chemists and Colorists. AATCC, Research Triangle Park, North
Carolina, 1997.
Textile Handbook. The American Home Economics Association,
Washington, DC, 1985.
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11.0. Endnotes
1. See, for examples, C.
N. Lowrie and G. Jackson, "Secondary Transfer of Fibers,"
in Forensic Science International (1994) 64:73-82, and
J. Roberston, C. B. M. Kidd, and H. M. P. Parkinson, "The
Persistence of Textile Fibers Transferred During Simulated Contacts,"
in Journal of the Forensic Science Society (1982) 22:353-360.
2. See, for example, W. Bruschweiler
and M. C. Grieve, "A Study on the Random Distribution of
a Red Acrylic Target Fibre," in Science and Justice
(1997) 37:85-90.
3. See D. A. Stoney article
in Journal of Forensic Sciences (1984) 24:473-482.
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