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Measurement of Yield and Fibre Diameter

AUTHOR: Jock Simmonds, Rowan Park, NSW

Yield Measurement

The cashmere goat is two coated, producing a coarse outer coat of guard hair and a fine undercoat called down. Processors remove the guard hairs and then use the down to produce fabric/yarn etc. Down yield is the amount (by weight) of down fibre measured as a percentage of the raw sample. Yield is used as an estimator of down weight because visual appraisal of the amount of down on an animal is extremely difficult and inaccurate.

In the early 1980s, the Australian Wool Testing Authority Ltd and Bob Couchman (Vic. Dept. of Agriculture) separately and jointly began investigating alternative methods of determining yield. Processors are somewhat secretive regarding their dehairing procedures but based on snippets of information, it was decided that dehairing related to three major factors, namely:

the difference in fibre weights, ie. down is light, guard hair is heavy;
the rigidity of fibre - guard hair is more rigid than down; and
the moisture absorption differential between the fibres.

It became obvious that the Shirley Analyser, used in wool and cotton testing, relied on certain of these factors to remove vegetable fault/trash from wool and cotton. Cashmere fibre was trialed and it was found that provided certain modifications were made to speeds and settings, cashmere could be dehaired with the Shirley Analyser.

Trials were run between AWTA Ltd laboratories and the Kinross Cashmere Company, which showed the Analyser to be equal to the commercial dehairer. The modified Shirley Analyser is now an accepted piece of equipment for cashmere dehairing.

Shirley Analyser procedures.

Following sub-sampling, three specimens are randomly drawn. These specimens weigh between 10 and 15 grams each. They are scoured in a solvent, air dried and then conditioned at 20 degrees celsius and 65% humidity for at least four hours.
Two of the specimens are weighed and the weight of each recorded. The third is retained as a reference sample.
The first specimen is passed through the Shirley Analyser until all guard hair is removed. This will normally take between 6-12 passes.
The second specimen is similarly tested. Both the down and the guard hair are recovered.
The down and guard hair recovered are weighed for each specimen. The yield for each specimen is determined separately and the results compared. If the results are compatible they are averaged and a result issued. If, however, the results are not compatible, the reference sample is dehaired and a result obtained. The third result is then compared to the first two results. If one result is obviously in error it is discarded, else the three results are averaged and the result issued.

Yield% = (Weigh_of_dehaired_fibre / weight_of_raw_sample) * 100

Shorn yields generally average around 20-40%. Combed yields will sometimes be as high as 80%. The yield figure is then used to calculate the weight of down in the parcel from which the sub-samples were taken.

Weight_of_Down =(Total_Fleece_Weight * Yield%) / 100

Fibre diameter was then measured using by microprojection or the Fine Fibre Distribution Analyser (described below).

Simultaneous Measurement of Yield and Diameter.

Both the dehairing of samples on the Shirley Analyser and measurement of fibre diameter by microprojection was extremely slow and laborious, and, therefore, expensive. Thus these methods were usually restricted to bulk sale quantities of fibre or the selection of breeding bucks. Breeding does were normally selected by the subjective assessment of essential fleece criteria.

The need for a rapid and inexpensive method for the simultaneous estimation of cashmere yield and mean fibre diameter in raw cashmere fleeces without prior separation of the hair and the down became evident. This would allow the measurement of these important selection criteria on all animals, thus ultimately ensuring the fastest rate of genetic gain. This led to the development of the Optical Fibre Diameter (OFDA) method, employing new image analysis technology.

OFDA can measure fibres up to 150 micron compared with FFDA, which is limited to 64 micron. By means of OFDA, the fibre diameters are measured automatically, providing the fibre diameter distribution of the sample. Using this distribution, the fibre characteristics of the fine as well as coarse hair content can be determined and used for the calculation of down yield. Fibres with diameters of 1-35 micron are classified as down. Above 35 micron is classified as hair.

OFDA Procedures.

Fleece sub-samples are scoured in detergent and hot water, and dried. These are minicored twice and the 2 mm snippets then conditioned at 20 degrees celsius and 65% relative humidity for 24 hours.

All snippets are spread onto 2 or 3 OFDA slides using a wire mesh box suspended over the slide. The fibre distribution of each slide is measured by OFDA, and the yield calculated using the combined diameter distribution from all of the slides from the same sub-sample. Fibre diameter is used to estimate of the weight of the fibres since the fibre length is constant (2 mm), and it is assumed that the specific gravity of cashmere down and guard hair are the same.

Laserscan.

The AWTA Ltd. has adapted the OFDA theory and technology for use with Laserscan. The procedures used are very similar to OFDA, and the results are comparable.

Fibre Diameter Measurement

The importance of fibre diameter is illustrated by the prices received for cashmere, with the highest prices per kilogram being for the finest fibre diameters. Therefore, it is essential that the diameter be ascertained by objective methods.

The techniques of diameter measurement are sufficiently covered in the above discussion on yield. There remains only the need to understand the tolerances that apply to test house results, and to recognise the need to take account of these tolerances when analysing fleece test results.

Accuracy of Yield and Diameter Measurement

(Proper sampling of fibre for testing is very important, see Goat Note F 7 – Sampling the Fleece for Testing).

Although different accuracies may be quoted for the different measurement techniques, all of the methods correlate sufficiently well for the grower to accept single figures for yield and micron.

Yield measurements are expected to be within plus or minus 3.5 of the figure obtained on 50% of occasions. That means that, given a yield test result of 30%, the true result has as much chance of being outside of the band 29.5% to 33.5% as being within it. There is a 95% probability of being within a band of plus or minus 10.0%.

Fibre diameter measurements are expected to be within plus or minus 0.2 micron on 50% of occasions. That means that, given a test result of 16.7 micron there is as much chance of being outside of the band 16.5 to 16.9 micron as being within it. There is a 95% chance of being within a band of 0.6 micron.

These bands of error are the best that can be achieved, and are unlikely to improve much, with current technology. We must use the tests, but do not ignore these bands when considering or comparing fleece test results.

Conclusions

As the accuracy of the different test methods are relatively comparable, choice of test houses will probably depend on the cost of tests and confidence in the test house. Note that systemic errors have occurred in test houses due to failure to maintain proper laboratory standards.

The importance of sampling should never be forgotten, and bias in sampling should be avoided as far as possible. Similarly the expected bands of error should not be overlooked when making decisions based on test results.

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