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Presale testing for Yield and Mean Fibre Diameter has been an integral part of the Australian wool selling system since the introduction of Sale by Sample in July, 1972. Today virtually all Australian wool is tested for Yield and Diameter, irrespective of the method of marketing, and is sold to the processor on test results. In fact, failure to provide presale test information results in reduced competition and in severe price penalties.
Steps in Determining Yield
1. Coring
Under the supervision of AWTA Ltd Sampling Officers, a coresample of approximately 1000g is taken from each sale lot. All bales in the lot are sampled, and the bale weights recorded at the time of sampling. At the same time, a grab sample is taken from the side of each bale for Length & Strength testing. The role of the Sampling Officer is to ensure that sampling is conducted in accordance with IWTO standards, and to ensure the samples are secured and returned to the laboratory.
2. Batching
Once samples are received in the laboratory, AWTA Ltd Testing Officers place samples with similar testing requirements into a “batch”. The batches then go through the testing process as a group.
3. Greasy Sub-Sampling
The coresamples are tipped into a bucket where they are weighed, blended by air, and weighed again. The difference between the first and second weights is used to calculate a correction factor to compensate for any weight change as a result of the blending process.
Testing Officers then remove 2 or 3 sub-samples that weigh 150g each. Classed grower lots require only 2 sub-samples. However, bulk class, interlots and lots containing 40 bales or more have 3 sub-samples taken because of their increased greater variability.
The sub-samples are put into containers, together with their accompanying paperwork and sent onto scouring. From then on each sub-sample is treated individually.
After removing the 150g sub-samples from the bulk coresample, the remainder is packaged into an air-tight container and stored along with 30,000 other samples. Known as “keeper samples”, they allow further testing, if required, on the lot without the need to recore the bales.
4. Scouring
Sub-samples are placed in a washing machine that operates along the same principles as a commercial scour. Sub-samples are washed in hot water and detergent before being rinsed twice in cold water.
Scouring removes all the suint and 98% or so of the mineral content (sand & dirt) and wool grease. A fine mesh at the base of the scour retains all the wool and all the vegetable matter (VM). After washing, all the wool fibre and VM are removed from the scour and placed in a drying can. The cans are loaded into a centrifuge, which operates like the spin cycle on a washing machine, and spun to remove excess water.
5. Drying
The drying cans are then loaded onto ovens that force hot air through the sample at 105°C. Electronic dryer monitors indicate when all the moisture has been removed from the wool. Each sub-sample is then removed from the oven, weighed and the oven-dried scoured weight recorded. By this stage the weight of the sub-sample is approximately 90g with about 60g having been removed from the wool as dirt, suint, grease and moisture.
6. Residual Sub-Sampling
After scouring and drying, 3 contaminants remain in the wool. These include all the VM that was in the sub-sample, and a small amount of residual grease and residual dirt not removed during the scouring process. In order to determine Wool Base (WB), these 3 contaminants need to be quantified. At residual sub-sampling, specimens are taken from the dry scoured sub-sample and sent to the relevant area for further testing.
7. Dissolving and Vegetable Matter (VM) Dissection
40g specimens of wool are placed into each bowl of a dissolver. 600ml of a 10% solution of boiling caustic soda is injected into each bowl and agitated for 3 minutes. After this time, the wool has completely dissolved, but the VM is relatively unaffected. The solution is poured through a sieve to retain the VM. After rinsing, the VM is dried in an oven at 110°C.
The caustic soda affects the VM. The finer and softer particles of VM are more affected by the caustic soda than the harder ones. In order to correct for this, the VM, after drying, is separated into 3 categories, namely:
• Burrs (B);
• Seed and shive (which loses most weight during dissolving) (S); and
• Hard Heads and Twigs (which are less affected by caustic soda) (H).
Correction factors for each category have been pre-determined and are applied to the weights of the 3 categories identified so as to gain an estimate of the percentage of each VM category present. The total weight is used to calculate the Vegetable Matter Base (VMB).
In addition to the above VM categories, Testing Officers also identify and separate dag, sand, other alkali insolubles e.g. lamb marking rings, and pack material contamination. If Testing Officers find traces of contamination in the VM specimen (e.g. baling twine, pieces of cloth etc), action is taken with the broker that may see the offending bales unpacked in an attempt to find the source of the contamination.
8. Grease Extraction
About 99% of the grease present in wool is removed during the scouring process. To determine the quantity of residual grease remaining in the scoured wool, a 10g specimen is taken from the scoured sub-sample. Grease is washed from the specimen by hot alcohol, then the grease and alcohol are siphoned down into a clean pre-weighed flask. The grease accumulates in the flask as a result of this distillation process.
20 siphonings are required to remove all the residual grease from the wool. After distillation, an amount of moist grease remains in the flask. Flasks are placed into a hot oven for 6 minutes to rapidly dry the flask and grease before re-weighing. The actual amount of grease can then be determined.
AWTA Ltd has now introduced modern technology to replace this traditional grease extraction method. The laboratories use Near Infrared Reflection (NIR) Spectrophotometry to quickly measure the grease content remaining in the scoured sub-sample. The instruments are calibrated to a range of samples with different grease values. Thus, when an unknown sample is presented to the NIR, it rapidly predicts the grease value of that sample. The process is safer and quicker than the traditional method.
9. Wool and VM Ash
As with grease, about 99% of dirt present in a wool sub-sample is removed in the scouring process. To determine the quantity of residual dirt (mineral content) remaining in the scoured wool, a 10g specimen is removed from the dry scoured sub-sample. The specimen is packed into a crucible and then combusted in a furnace at a temperature of 800°C. The crucibles take about 2-3 hours to pass through the furnace during which time all organic matter is burned off, leaving only the mineral matter in the ash. The crucible contents are weighed and expressed as a percentage of the wool specimen weight.
10. Wool Base Calculation
Having accounted for all the residual contamination in the wool, the amount of clean, dry wool fibre or Wool Base (WB) in each sub-sample can now be calculated. The WB is calculated by expressing the weight of wool with all impurities removed as a percentage of the original greasy wool weight. The WB results of the 2 or 3 sub-samples are then averaged to give a final result for that particular sale lot. If the range in test results between the sub-samples is excessive, then a check test will be initiated from the “keeper” sample. By using this procedure, AWTA Ltd is automatically putting a quality control check in place to minimise errors. |