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2.4
Applications of wool metrology
2.4.1
Commercial trading
The transformation of wool from a fibre growing on a sheep into a textile
product occurs via a long pipeline where ownership of the fibre may pass
through several pairs of hands. Metrology has a major role to play in this
exchange of ownership since objective measurements now provide most of
the information that determines the value of the fibre. Over the past 50 years
the objectively measured parameters for wool have been incorporated into
the contracts and specifications utilised by the buyers and sellers along this
pipeline. The quality characteristics of raw wool are ranked in order of their
economic importance in
Table 2.2.
Wool metrology has also facilitated the
streamlining of selling systems, resulting in very significant increases in
productivity and reductions in cost.
© 2009 Woodhead Publishing Limited
Advances in wool technology
24
Table 2.1 Definitions of key terms
Accuracy
A measure of the closeness of a test result to the true value.
The difference between accuracy and precision should be
noted. (Also see bias, precision and confidence limits.)
Ash content
The residue of a scoured wool subsample after it has been
subjected to charring followed by heating to 800
°C.
Bias
A constant or systematic difference between a true value and
corresponding test results.
Card sliver
A continuous strand of opened and loosely assembled
scoured wool fibres, together with variable amounts of
vegetable matter. Its linear density is approximately constant
and it is without twist
Coefficient of
A statistical measure of the variability exhibited within a set
variation (CV)
of values. It expresses the standard deviation as a percentage
of the mean; the higher the CV, the greater the variability.
Commercial yield
Any of the yields, calculated from the wool base, vegetable
matter base and hard heads and twigs base, as specified in
Section 3.0 of the IWTO Core Test Regulations. Yields may be
expressed either as net clean mass of a lot or delivery, or as
the net clean mass expressed as a percentage of the net
greasy mass.
Confidence limits
An expression of the precision of the mean of a set of
values, usually associated with a stated probability, most
often 95%. It is the interval around the mean within which,
with the stated probability, the true value is expected to lie.
Core sample
A representative sample of raw wool obtained from each
bale in the lot by coring techniques.
Core test
The series of measurements, typically of wool base,
vegetable matter base and mean fibre diameter, carried out
on a core sample.
Distribution
Distribution can be expressed in the form of a frequency
table or as a frequency histogram with data grouped into
classes of 1
µm size, and integer micrometre values as
midpoints of the class intervals.
Fibre diameter
The thickness of individual fibres; it is customary to quote an
average value (mean fibre diameter or MFD) in micrometres.
Fibre diameter
The distribution of fibre diameter in a wool sample.
Grab sample
A sample taken from within a bale of wool by a mechanically
driven jaw sampling device.
Greasy wool
Wool from the sheep’s back or sheepskins which has not
been scoured, solvent degreased or carbonised or otherwise
processed. It contains grease and suint extruded from the
follicles in the skin and dirt and vegetable matter picked up
from grazing.
Mean fibre diameter The arithmetic mean of all fibre diameter readings in a
sample.
Precision
An indicator of the repeatability of a measurement; it is often
expressed in terms of confidence limits.
© 2009 Woodhead Publishing Limited
The objective measurement of wool fibre quality
25
Raw wool
Wool fibre together with variable amounts of vegetable
matter and extraneous alkali-insoluble substances, mineral
matter, wool waxes, suint and moisture. It includes
(a)
greasy wool;
(b)
wool which has been scoured, carbonised, washed or
solvent degreased;
(c)
scoured skin wools; and
(d)
slipe wools.
Scoured wool
Wool that has been aqueously washed or solvent washed to
remove most of the grease, wax and suit and dirt.
Standard deviation
A statistical measure of dispersion of individual results.
Staple
A well-defined bundle of fibres which has been removed
from a mass of greasy wool as a unit.
Variance
The variance is the square of the standard deviation and is a
measure of the distribution of values around the mean.
Vegetable matter
Burrs (including hard heads), twigs, seeds, leaves and
grasses present in wool.
Vegetable matter
The oven-dry mass of ash-free, ethanol-extractives-free burrs
base (VMB%)
(including hard heads), twigs, seeds, leaves and grasses
present, expressed as a percentage of the mass of the
sample.
Wool base (WB%)
The oven-dry mass of wool fibre free from all impurities, i.e.
ash free from, ethanol extractives and all vegetable matter
and other alkali-insoluble impurities, expressed as a
percentage of the mass of the sample.
Yield
The amount of clean fibre, at a standard regain, that is
expected to be produced when a delivery of raw wool is
processed. The yield may be expressed both as a clean mass
in kilograms and as a percentage of the mass of raw wool
prior to processing.
Table 2.1 Continued
Table 2.2 Significance of single fibre and staple
characteristics in processing
Characteristic
Importance
Yield
****
Vegetable matter
****
Mean fibre diameter
****
Staple length
****
Staple strength/position of break
****
Colour (yellowness)
****
Coloured fibres
****
Fibre diameter variability
**
Staple length variability
**
Cots
**
Crimp/resistance to compression
**
Staple tip
*
**** Major ** Secondary * Minor
© 2009 Woodhead Publishing Limited
Advances in wool technology
26
2.4.2
Quality control
Objective measurements provide the means to monitor and adjust wool
processing machinery to minimise variation within the product. They also
provide tools for monitoring breeding programmes conducted by wool
producers, to ensure that the objectives of these programmes are being met.
Measurements are now integrated into the quality control systems at all the
levels of the industry, extending from the farm to the completion of early
stage processing and beyond.
2.4.3
Research
Modern science and engineering depend upon the ability of scientists and
engineers to measure critical characteristics of the system they are examining.
Wool measurements have been and continue to be used in research programmes
directed towards improving wool production, wool processing and fabric
characteristics.
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