Last update: April 10, 2012 10:09:44 AM E-mail Print

 

Quality is Important in Merino Wool

C. M. van Wyk 

 

MERINO sheep breeders who have to breed for so many characters, e.g. fertility, conformation and quantity of wool, sometimes wonder whether quality is not perhaps of minor importance. They know that quality is one of the characteristics taken into account by buyers, but the main determinant of the value of quality is the role it plays in manufacture and subsequent use.

That quality is indeed important and is not lost in the scouring or combing processes, will be clear from the following: Quality assessment is based on two main characters: (1) definition and regularity of the crimp, and (2) softness of handle.

 

Crimp

(1) The definition or distinctiveness of the crimp in the staple gives a good indication of the uniformity of the fibres. In any staple there are fibres varying in both length and thickness; these variations can be accurately determined only by measuring the length of a large number of stretched individual fibres and also measuring their diameters with the aid of a microscope. The variations are usually so great that several hundreds of fibres have to be measured. On the other hand, the individual fibres of a staple do not differ much as regards their total number of crimps. It follows, therefore, that, if there is excessive variation in the length of the fibres, the crimps will not coincide. Further, there is a close correlation between fibre length and fibre thickness within a staple, so that a large variation in the fibre length is accompanied by a corresponding variation in fibre thickness. Thus it may be said that a well-defined crimp in the staple indicates a reasonable uniformity among the individual fibres and, conversely, that a poorly defined crimp indicates too great a variation among the fibres.

To the manufacturer it is important that the variation should be as small as possible. The actual count to which wool will spin: is determined mainly by the average fibre thickness, provided the length is sufficient. The spinning count is, however, adversely affected by the variation, and consequently wool with irregular fibres will not spin to the length to be expected from the average fibre thickness. Greatly differing fibres, like hairs, aggravate the position to such an extent that manufacturers of high-grade fabrics will avoid merino wool containing hair.

Regularity in the size of the crimp along the length of the staple is mainly determined by the condition of the sheep, and is indicative of uniformity of the fibres from end to end.

 

Softness of Handle

(2) Softness of handle is influenced by various factors, one of which is the pliability of the fibres. Most important among the factors determining pliability is the fibre thickness. The resistance of a fibre, to bending is in direct proportion to the fourth power of its thickness. Thus, if the thickness is doubled, the resistance is increased sixteen fold. If thickness were the only factor, the fibres of fine wools would be expected to be far more pliable than those of strong wools. As stated before, fibres within a staple vary in thickness and the presence of excessively thick fibres like hair will reduce the pliability of the staple as a whole to an extent out of all proportion to their number. So, for example, a bundle consisting of 100 fibres of a certain thickness offers the same resistance to bending as one consisting of only six fibres twice as thick. It is the presence of excessively thick fibres, lacking the pliability of the other fibres in the sample, which accounts for the fact that woollen underwear, manufactured from poorly selected wool, is sometimes irritating to the skin. It is clear, therefore, that uniformity in fibre thickness and the absence of hair are essential to softness of handle, quite apart from the effect of the variation on the spinning count, already referred to.

Although fibre thickness plays an important role, wool has another valuable property which influences softness of handle, viz., the crimp. The crimp imparts a bulkiness or volume far in excess of the volume of the fibres, which accounts for its characteristic springiness. A mass of wool fibres, when compressed with the hand, can therefore yield considerably.

The role played by crimp in determining the softness of handle may be summarized as follows: The resistance to compression increases with increasing fibre thickness and decreases as the size of the crimp increases. Owing to the correlation between fineness and crimp, the greater resistance to compression of a strong wool is reduced by the broader crimp, whereas the smaller resistance of a fine wool is increased by the finer crimp. Wool samples of different thicknesses can therefore offer the same resistance to compression, provided their crimp is in the correct proportion to the fibre thickness. A fine wool with fine crimps is comparable with a strong wool with broad crimps.

In practice, however, there is a considerable deviation from the average relationship between fineness and crimp. The position, then, is that the resistance of a strong wool with fine crimps is above normal and that of a fine wool with broad crimps below normal.

Too high a resistance produces harshness of handle and too low a resistance, flabbiness. It is clear, therefore, that a certain relationship between fineness and crimp is essential to softness of handle coupled with bulk elasticity.

The size of the crimp is, however, not the only important factor. The shape is also of importance. An examination of an individual fibre reveals a far greater variety of crimp forms on the same fibre than is discernable in the staple. By stretching the fibre, the behaviour of the different shapes can be observed. Due to their smaller resistance, the larger crimps are the first to disappear. In crimps of similar size, those with smooth curves are found to offer the least resistance, the greatest resistance being offered by those with sharp curves, some of which are even angular. It is obvious, therefore, that a sample may have the ideal size of crimp for its fibre thickness but, if the crimps contain exceptionally sharp curves, the resistance to compression will be increased in much the same way as if the crimps were too fine for the fibre thickness.

Another factor influencing-handle is the fibre surface. The serrations or scale-like projections on the surface of a wool fibre cause a certain frictional resistance. Increased friction, artificially induced, is clearly discernable to the touch and results in harshness of handle. It is possible, therefore, that differences in surface friction may also cause differences in the handle of different samples, but this aspect has not yet been fully investigated.

From the above it appears, therefore, that a good definition of the crimp and softness of handle indicate certain valuable properties which should be aimed at. Although the quality of the wool must not be regarded as the prime requirement in the merino, it is nevertheless important and, should be accorded its rightful place in our breeding policy, if we are to cater for a fastidious market.

 

Published

Farming in South Africa 27