Cotton is a natural fibre, like silk, wool, or linen.
Alongside natural fibres, there are artificial and synthetic man-made
fibres. Artificial fibres (such as viscose rayon and acetates) are made
from organic polymers derived from natural raw materials, mainly cellulose.
Synthetic fibres (including acrylics, polyamides, and polyesters) are
generally derived from petrochemicals.
Developments in cotton classification standards
Prior to the development of official standards, cotton was marketed primarily on the basis of its variety and where it was grown, although some physical standards for cotton classification (sets of physical samples) were used privately. The United States Cotton Futures Act of 1914 authorised the Department of Agriculture to establish physical standards as a means of determining colour grade, staple length and strength, and other qualities and properties. These standards were thereafter agreed upon and accepted by the leading European cotton associations and exchanges. They were accordingly termed and referred to as the "Universal Standards for American Cotton." Indeed, when in 1923 the US Department of Agriculture (USDA) signed the Universal Cotton Standards Agreement with nine leading cotton associations in seven major European countries, the US classing system entered into increasingly global use. Under the auspices of the Agreement, the currently twenty-four signatory cotton associations representing twenty-one countries agreed to use only Universal Standards to arbitrate US grown American upland cotton. In addition to use by signatory countries, Universal Standards are routinely used in over twenty-five non-signatory countries as the standard for US and non-US grown cottons. Whereas other countries started developing their own classification system, the USDA kept committed to continual development and improvement efforts in the area of cotton classification standards. Since 1991, USDA cotton classification has relied on instrumental measurements (in addition to or as a substitute for human vision) for fibre length, strength and length, micronaire (a measure of the cotton's fineness), colour grade, colour Rd (reflectance), colour +b (yellowness), and trash percent area. All instrument measurements currently utilised in USDA are performed by High Volume Instrument (HVI) patented by Uster Technologies, a leading company in textile quality controlling. Given the international acceptance of HVI testing, in 1996 the Universal Cotton Standards Agreement was amended to recognize USDA-produced HVI calibration cotton standards for strength, length and uniformity index. The new standards were named Universal HVI Calibration Cotton Standards and continue to serve today as the most recognized standards for HVI calibration. USDA is continuing its effort toward global HVI standardisation.
The quality of the cotton fibre is determined by three factors, namely, the colour of ginned cotton, purity (the absence of foreign matter) and quality of the ginning process, and the length of fibres. Practically all cotton grown in the United States is classed by USDA at the request of producers. While classification is not mandatory, growers generally find it essential to marketing their crop and for participation in the USDA price support program. For additional information on USDA cotton classification standards, the reader is referred to the official USDA website sections on classification, the HVI system, and standardisation.
The colour of cotton fibres is primarily determined by conditions of temperature and/or humidity, cotton lint exposure to sunlight, and cotton varieties. Action by parasites or micro-organism, as well as technical defects in harvesting and subsequent storage and transport, may all affect the colour of cotton.
The colour of cotton ranges from white to yellowish and is classed into the groups "White", "Light Spotted", "Spotted Tinged" and "Yellow Stained", in descending order of quality. There are 25 official colour grades of American upland cotton, ranging from "Good Middling" colour through "Middling Yellow Stained" colour. In addition, there is a descriptive "Below Colour Grade" standard for 5 categories of American upland cotton. Fifteen of these grades are each within the range represented by a set of physical samples in the custody of the United States Department of Agriculture (physical standards), whereas the remaining 10 grades (the six "Light Spotted" grades, "Good Middling Spotted Colour", "Strict Middling Tinged Colour", and the two "Yellow Stained" grades), as well as the 5 "below grade" categories, are descriptions based on the physical colour grade standards (descriptive standards).
Source: UNCTAD secretariat, based on USDA, United States Standards for the Color Grade ofAmerican Upland Cotton, August 1993
HVI classing has been available on an optional basis to all growers since 1981. The colour of cotton is measured by the degree of reflectance (Rd) and yellowness (+b). Reflectance indicates how bright or dull a sample is, and yellowness indicates the degree of colour pigment. A three-digit colour code is used to indicate the colour grade. This colour grade is determined by locating the quadrant of the colour chart in which the Rd and +b values intersect. For example, a sample with an Rd value of 72 and a +b value of 9.0 would have a colour code of 41-3. In cotton classification, the colour grade of American upland cotton is determined using the HVI Colour Chart (instrument measurement), and referenced to colour grade standards that are in the custody of the USDA (the abovementioned Universal Cotton Standards used by human classers to determine official colour grade).
For further information, please refer to United
States Standards for the Color Grade of American Upland Cotton, USDA,
August 1993 (PDF, 22.4 KB).
Leaf grade and extraneous matter
Leaf grade describes the leaf or trash content in the
cotton. Purity as regards the presence of foreign
matter (waste such as leaves or earth) is of the utmost importance.
Official US standards for the leaf grade of American Upland cotton
Source: United States Standards for the Leaf Grade of American Upland Cotton, USDA, August 1993
Other foreign matter (such as seed coat fragments), as well as the degree of smoothness or roughness with which cotton is ginned, may all affect the purity of the cotton lint. Additional explanatory terms considered necessary to describe adequately the condition of the cotton may thus be entered on classification memorandums or certificates.
An HVI trash measurement is also available, although the traditional method of classer determination for leaf grade and extraneous matter continues to be included as part of USDA's official cotton classification. Trash in raw cotton is measured by a video scanner, commonly referred to as a trash meter. It is a measure of both leaf and other elements such as grass and bark. The surface of the cotton sample is scanned by the camera and the percentage of the surface area occupied by trash particles is calculated.
For more information: United States Standards for the Leaf Grade of American Upland Cotton, USDA, August 1993 (PDF, 15.8 KB).
More information on US standards for length of staple of cotton (PDF, 17.5 KB)
Length uniformity index
Source: Cotton Classification - Understanding the Data, USDA, July 2004
Fibre strength table
Source: Cotton Classification - Understanding the Data, USDA, July 2004
More on HVI measurement of fibre uniformity and fibre strength: USDA, Cotton Classification - Understanding the Data, July 2004 (.doc, 2.56 MB).
Other properties that are of great importance in the industrial uses of cotton, including fibre fineness and maturity, are measured in accordance with standard test methods. Classing methodology is constantly updated to include state-of-the-art methods and equipment. Fibre properties are also measured for American pima cotton.
While the basic testing procedures for
American Pima cotton are the same as for American upland cotton, different
grade standards are used. For more information, refer to the Classification
of Cotton (Cotton incorporated).
Official cotton standards have been enacted by other countries.
In Tanzania, for example, the Tanzania Cotton Board (TCB) has established
quality standards for measuring those physical attributes of raw cotton
that affect the quality of the finished product and/or manufacturing
National standards and testing procedures allegedly reflect
domestic conditions and are suitable to local actors. The coexistence
of national specifications and universal (US) standard as a point of
reference in international trade does not necessarily engender confusion,
to the extent that some comparability is assured. Benin for example
has developed its own cotton labels for different quality grades. Cotton
is classed into three groups, "Kaba" (the superior quality),
"Zana, "Kene" and "Bati" (inferior qualities).
Three fibre classing systems exist at the international level: the English system "Na"; the metrical system "Nm"; and the "TEX" system.