Dark & Medullated Fibre Contamination

Australian wool is renowned worldwide for its whiteness and cleanliness and consequently, commands premiums compared to wools from other countries. Such a reputation is hard earned but can be easily lost.

During the 1950's, Australia has seen an increase in the use of so-called “Exotic Breeds”. These breeds have a legitimate place in meat production but bring with them a new and potentially damaging threat to the reputation of Australian wool. These exotic breeds include the Damara, Dorper and Awassi, all of which share the characteristic of shedding their fibre. It is the shedding of their fibre that is causing the major concern as the fibre can be transferred to Merino sheep if they come into contact with Exotic animals.

Research has shown that such contact can lead to excessive levels of dark & medullated fibre in Merino wool, a totally unacceptable position for our customers.  Clearly, a solution to this problem is needed.

The Dark Fibre Testing service introduced by AWTA Ltd iin March 2003 is targeted at growers who are attempting to manage contamination from exotic sheep breeds. It is not a catch-all test. Growers wishing to utilise the DMF service must request it through their wool selling broker or wool selling agent before their lots are sampled for certification for Yield, Vegetable Matter Base and Mean Fibre Diameter.

When the test was first introduced it was labour intensive and slow, and therefore expensive. Research to develop improved technology continued, but the service was implemented based on the best available technology of the time. This technology was developed by CSIRO during the 1980's, initially for the inspection of tops.

The basis for the initial presale test was the IWTO Test Method (DTM-13-01) for the detection of coloured fibres in wool top. This method used the principle of balanced illumination developed by CSIRO. In essence, with this method small (approximately 20g – 100g) samples of top are obtained, from which individual test specimens of 0.2g are selected and spread evenly onto a 20cm square glass plate. A second glass plate is used to compress the wool into a ‘sandwich’ between the two glass plates. The plates are then placed on the Dark Fibre Detector (DFD) and subjected to balanced illumination, which highlights coloured fibres. A graded scale, a CSIRO Dark Fibre Reference Web  is used to provide fibres of standard levels of “darkness” as a basis to grade the darkness level of detected fibres. Fibres of Level 5 or higher are nominated as contaminant fibres.

Research conducted by the South Australian Research & Development Institute (SARDI) demonstrated that the DFD could be used to detect contaminating fibres in core samples of fleece wool from Merinos that had reared Damara crossbred lambs. A subsequent work undertaken by AWTA Ltd and the SARDI, found similar detection rates for contaminant fibres of CSIRO level 6 and darker in when testing commercial presale core samples from selected sale lots. Fibres of level 6 and darker have been shown to cause contamination in fabrics of white or pastel shades (IWTO-55-99). Besides confirming that crossbreeding of Merino ewes with Damara rams exposes the fleece wool from the Merino ewes to the risk of contamination from dark and medullated fibres, this trial also confirmed that testing commercial core samples could provide useful information about the level of DMF contamination arising from crossbreeding of Merinos with so-called exotic breeds.

AWTA Ltd introduced the commercial test method for DMF detection based on the procedures developed in the AWTA/SARDI trial of presale core samples.

Approximately 0.50g of washed and carded core sample was spread thinly between the 20cm square glass plates. The glass plate and wool ‘sandwich’ is placed on a diffuser that is lit from below. The DFD also incorporates a ringed fluorescent light source above the glass plates. When examining for dark fibres the intensity of the illumination from above and below is balanced such that white fibres tend to merge into the background, while the coloured fibres tend to stand out. The operator views successive small (approximately 6.7mm square) sections of the wool through a 2x magnifier, comparing detected fibres to the CSIRO Reference Web Level 6 fibre. If the detected fibre is rated equivalent to, or darker than, the Level 6 reference fibre, and it is greater than 3mm in length, it is counted as a dark fibre.

This procedure was repeated for the detection of medullated fibres, but in this case the lighting is not balanced but comes only from above. With the top-only lighting medullated fibres appear brighter than non-medullated fibres. In the case of medullated fibres there is no IWTO-endorsed reference scale equivalent to the CSIRO Dark Fibre Reference Web, so that a medullated fibre reference scale was developed based on ASTM2968-75, which specifies 60% medullation as the level above which a medullated fibre is regarded as a contaminant.

As part of a research programme funded by AWI, CSIRO achieved a breakthrough in early 2004 in the presentation of samples for detection of contaminant fibres. The researchers at CSIRO were able to demonstrate that test specimens as large as 2g to 3g could be prepared for inspection using a novel technique. This new technique overcame a major limitation of test specimen mass in the balanced illumination method by incorporating a fluid with a Refractive Index (RI) similar to wool.

The use of fluids with matching RI’s to wool is not new, being first described in 1930. A method using benzol alcohol for the detection of medullation in samples of wool was proposed . In 1937 this technique was adapted to include a photocell which could measure degrees of medullation, and later in 1951 this apparatus was further improved. The WRONZ Medulla meter is the modern result of this earlier work.

Unlike the case of the balanced lighting method, this technique relies on the virtual transparency of noncontaminant wool fibres to provide an unobstructed view of coloured or medullated fibres in a mass of wool. In the absence of any pigmentation, wool fibres absorb little incident white light, but appear white in air because of the large difference in RI between air (RI = 1.00) and wool (RI = 1.553). At an interface between two (2) media with different RI’s, light is refracted and reflected. Since there is negligible absorption of the light by wool fibres, it is white light that emanates from a mass of well-lit wool.

CSIRO developed a system of placing the scoured and carded wool specimens inside a plastic bag in a thin layer, filling the bag with benzol alchohol, and compressing the bag and fluid to remove excess solvent and air and to then seal the bag. This sample is then placed on the DFD and the contaminant fibres counted. The technique was further refined by AWTA Ltd to permit a larger sample size (10 g) and a more efficient and ergonometric system for presenting the sample to an operator for inspection.

Using this technology two technicians visually examine two specimens, each of 10 grams, of washed core sample using the Dark Fibre Detector to identify the dark and medullated fibres present. These results are issued as Test Reports and reported according to one of the following options:

• No Fibres Detected
• 1-5 dark fibres (df) per test specimen (ts)
• 6-10 df/ts
• >10 df/ts

A similar convention is used for reporting medullated fibres.

AWI, AWTA Ltd and CSIRO jointly continued to develop an automated imaging system for the detection of dark and medullated fibres to improve the manual system . This is no easy task, especially for greasy wool, due to residual vegetable matter that also contaminates the laboratory sample after scouring, drying and carding.  However this problem is greatly reduced in the case of tops, where most of the vegetable matter has been removed. By 2007 the instrument reached the stage where it could identify and count dark fibres in samples of top.  It is not yet set up to count medullated fibres.  A number of instruments were assembled and  evaluation trial commenced with overseas mills.  Feedback from this evaluation process will be used to refine and improve the technology.