Average Yellowness

Before measuring the Average Clean Colour a sub-sample from the scoured core sample is carded by a Shirley Analyser to remove any Vegetable Matter contamination that may be present.

Average Colour is described by three variables (X, Y, Z). The value of X describes the Brightness ( the higher the number the brighter the sample). The difference Y-Z describes the Average Yellowness ( the higher the number the more yellow the wool). These parameters are measured by placing the carded sample in the measuring cell (see centre above) and then using a spectrophotometer (left above) to measure the spectrum of the reflected light. This spectrum is analysed by a computer (right above) to determine the values of X, Y and Z.

The importance of Average Colour to the Dyer is illustrated above. The left of the illustration shows three samples of scoured and carded wool, where the yellowness increases progressively. The top sample is clearly quite bright and white. The second sample is duller and more yellow and the third sample is even more yellow. Each sample has been dyed using exactly the same dying process and exactly the same dye. The colour of the products are clearly quite different. Topmakers are under increasing pressure from spinners and dyers to ensure that they produce unform brightness and yellowness in the tops they deliver, so that the colour of batch lots of dyed yarns and fabrics can be reliably maintained.

The third phase of the objective specification of Raw Wool came when AWTA Ltd introduced a service for the measurement of Average Yellowness (and conversely, the Whiteness) of wool after scouring in the late 1980's.

Average Yellowness is an important characteristic of wool because it is related to dyeing performance and it is not necessarily related to the colour of Greasy Wool.

AWTA Ltd assisted the industry to become familiar with the measurements by the conduct of extensive commercial trials with wool brokers across Australia which gave wool exporters access to Average Yellowness measurements in sale catalogues. The Company also facilitated the adoption of Average Yellowness measurement by the development of innovative Test Fee packages. Test volumes assumed commercial quantities in 1993.

However, since then the uptake of the measurement has been slow, largely because there is a widespread belief that the bulk of Australian wool is white and bright and therefore the Yellowness measurement adds little value. This belief is only partly correct. Considerable quantities of wool are typed as H1 (Light Unscourable Colour), H2 (Medium Unscourable Colour) or H3 (Heavy Unscourable Colour) based purely on visual assessment of the greasy wool. A number of independent research studies have shown that the colour of greasy wool is an extremely poor indicator of the clean colour of the same wool when it is scoured. This is perfectly understandable given that the greasy colour is strongly affected by wool grease and dust, both of which can generally be removed by scouring.

The benefit of testing the average clean colour of greasy wool presale is that it provides predicability of the processing potential of the wool. Many scourers are already aware that the difference between AWTA Ltd's Clean Colour Measurement and a Colour measurement made on their scoured product is a reliable indicator of their scouring performance.

However, the greatest beneficiary of the colour information is the dyer. The colour of the dyed product will be determined by the colour of the clean wool and the that of the added dye. Topmakers are now increasingly measuring the colour of their tops and providing this information to their customers (including dyers). Contracts already exist between topmakers and spinners that include limits on the colour measurement of the top. In New Zealand all auction wool, including merino, is presale tested for clean colour measurement.

The development of wool colour measurement originally was driven by New Zealand, where colour is far more important than in Australia due to the different wool types and climatic conditions. In the initial stages calibration of the instruments used to measure colour relied upon wool standards distributed by the then Wool Research Organisation of New Zealand (WRONZ).

However, this lead to the wool industry adopting a completely different colour space to the rest of the world (colour measurement is critically important to a huge number of quite different industries, and International Standards have existed for many years for defining the measurement of colour). The stability of this standardisation system over a long period of time was also shown to be deficient.

Consequently, AWTA Ltd's Research & Development Division, in collaboration with other scientists developed a new calibration system which brings the wool industry in line with the rest of the world. This received IWTO approval in 1999 and was implemented by AWTA Ltd in January, 2000.

This changes the colour scale from the now obsolete C/2 to the scale universally used by dye houses (D65/10). This will be similar to changing from inches to centimetres for length measurements. Now this is complete there there is a common language from greasy wool specification to the dyer for the first time.