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Quality

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.

In industrial uses of cotton, grades defined by the US Department of Agriculture are generally accepted as the world standards for cotton fibre quality.

- United States cotton standards
- Examples of other national standards
- Quality of cotton fibre

United States cotton standards

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.

Colour

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.
There are seven official leaf grades for American upland cotton designated as "Leaf Grade 1" through "Leaf Grade 7". They are all represented by official physical standards in the custody of the USDA. In addition, there is a descriptive "Below Leaf Grade Cotton" designation for American upland cotton that is lower in leaf grade than Leaf Grade 7.

Official US standards for the leaf grade of American Upland cotton

Leaf Grade	Symbol	Code No.
Leaf Grade 1	LG1	1
Leaf Grade 2	LG2	2
Leaf Grade 3	LG3	3
Leaf Grade 4	LG4	4
Leaf Grade 5	LG5	5
Leaf Grade 6	LG6	6
Leaf Grade 7	LG7	7
Below Leaf Grade	BLG	8

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).

Fibre length

• Length
Fibre length is defined as the average length of the longer one-half of the fibres (upper half mean length). Fibre length is basically an inherited/genetically character of the seed variety. However, weather, nutrient deficiencies, as well as excessive cleaning and/or drying at the gin may also affect the fibre length. By affecting yarn strength and evenness, and the efficiency of the spinning process, the length of the fibre has a great influence on quality and price.
According to USDA's classing methodology, length measurement of American upland cotton is performed by HVI in accordance with standard test methods. The length of staple, measured in inches and fractions of an inch, is classed according to the following codes:

Length (inches)	Code	Length (inches)	Code
< 13/16	24	1-3/16	38
13/16	26	1-7/32	39
7/8	28	1-1/4	40
29/32	29	1-9/32	41
15/16	30	1-5/16	42
31/32	31	1-11/32	43
1	32	1-3/8	44
1-1/32	33	1-13/32	45
1-1/16	34	1-7/16	46
1-3/32	35	1-15/32	47
1-1/8	36	1-1/2	48
1-5/32	37

More information on US standards for length of staple of cotton (PDF, 17.5 KB)

• Uniformity
Length uniformity is the ratio between the mean length and the upper half mean length of the cotton fibres within a sample. It is measured on the same beards of cotton that are used for measuring fibre length and is reported as a percentage. The higher the percentage, the greater the uniformity. If all the fibres in the sample were of the same length, the mean length and the upper half mean length would be the same, and the uniformity index would be 100. The following tabulation can be used as a guide in interpreting length uniformity results. Measurements are performed by HVI. Cotton with a low uniformity index is likely to have a high percentage of short fibres and may be difficult to process

Length uniformity index

Descriptive Designation	Length Uniformity
Very Low	Below 77
Low	77 - 79
Average	80 - 82
High	83 - 85
Very High	Above 85

Source: Cotton Classification - Understanding the Data, USDA, July 2004

• Strength
The fibre strength measurement is made by clamping and breaking a bundle of fibres from the same beards of cotton that are used for measuring fibre length. Results are reported in terms of grams per tex (a tex unit is equal to the weight in grams of 1,000 meters of fibre). It expresses the force required to break a bundle of fibres one tex unit in size. Fibre strength is largely determined by variety. Strength measurements are performed by HVI in accordance with standard test methods. The descriptive terms listed below may be helpful in explaining the measurement results.

Fibre strength table

Descriptive Designation	Strength (grams per tex)
Weak	23 & below
Intermediate	24 - 25
Average	26 - 28
Strong	29 - 30
Very Strong	31 & above

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).

Examples of other national standards

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 efficiency.
The grade of cotton is determined in terms of colour, leaf, and preparation. In reference to these factors, cotton is classed into the groups "TANG" (the superior quality), "GANY" (the fair average quality), and "YIKA" (the inferior quality). These grades (referred to as "Physical grade standards") are all represented by physical samples in the custody of TCB. In addition, there are four "descriptive grade standards" for GANY and one descriptive "under grade" ("UG"). Finally, three descriptive colour standards (named "Slight dull mixed stain" (SDM), "Dull mixed stain" (DM), and "Stain") are introduced for lint contaminated by insects or in the field for a long time after balls opening. Cotton grade is assessed on the basis of the above standards by classers.
For more information on Tanzania cotton classing, refer to: Tanzania Cotton Grade Standards.

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).
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Quality of cotton fibre

Three fibre classing systems exist at the international level: the English system "Na"; the metrical system "Nm"; and the "TEX" system.