Four factors are considered in the classification of wool fibres; count, staple length, crimp and lustre:
Count: The traditional classification of wool fibre was done using the ‘Bradford Count’ which represented the greatest length of yarn that could be spun from 1lb (454g) of the wool fibre. The fibre was spun into yarn which was then wound into hanks of 560 yards (512 metres), and the number of hanks produced gave the count for that fibre. A spinner can therefore spin 1lb of 56 count wool into 56 hanks each containing 540 yards of yarn, or nearly 17.2 miles (28.5Km). The higher the Bradford Count, the finer the wool fibre.
Fibres are now more commonly classified by the Micron reading for the diameter of the fibres rather than the Bradford Count. 1 micron equates to 0.001mm, therefore the lower the Micron figure the finer the fibre.
Merino, the finest of wool commonly available can have a Micron range of 15 microns (ultrafine) through to the average (but still fine!) 22-23 microns. Note that the count is not consistent across a fleece; the finest wool is at the neck and the coarsest at the britch and belly. Commercially prepared fibres will have been sorted to give a consistent count throughout.
Here are approximate comparisons for Micron Count and the Bradford Count:
The finer the fibre, the more easily it will turn to felt with exposure to damp, heat and alkalis such as soap. This makes fine fibre such as Merino perfect for felting and is why this wool fibre is widely available in a range of dyed colours for felt makers. Although coarser wool fibres can be felted, more effort is required!
Staple: The length of the wool fibres or ‘staple’ varies a little across a fleece, from shorter at the neck to longer in the middle section. The staple length (or Hauteur) of the wool fibre determines how easily it can be spun into a yarn. Fibres with a staple ranging between 80 and 12mm are the easiest to hand spin, typically these are found in the downland breeds.
Longer staples such as Devon Longwool, shown in the image right, has a staple length of 200-250mm and is quite coarse and so is used for making strong rug warp as well as Dolls Hair. By way of contrast in Wensleydale the staple can be up to 300mm, and with a 30-40 Micron count, is spun for fine strong yarns typically used in suits. Long staple fibres require skill in hand spinning, using a long drafting zone and relatively little twist if an even finish is required. Shorter staples such as Black Welsh Mountain, which can be only 50mm, require a high twist to create a viable yarn.
Crimp: The crimp is the number of waves along the length of the wool fibre. Fibres with high crimp, 40-60 per mm are generally finer and therefore can be spun to a finer yarn. They perform well for woollen spinning as they trap air in their structure to give an excellent insulation.
A fibre with a high crimp is less suitable for worsted spinning as it is difficult to compact the fibres enough to give the drape associated with worsted spun yarns. The commercially prepared Bluefaced Leicester fibre, shown in this image, has quite a high crimp and is ideal for woollen spinning. It has a Micron Count of 26 and a staple length o f 85-95mm. The Devon Longwool mentioned above has no crimp and is used for worsted yarns used in belts.
Lustre: At a microscopic level the surface of wool fibres is seen as covered with fine ‘scales’. In the fibres of some sheep breeds, such as Wensleydale and Massam the scales have a very smooth and shiny surface to produce an effect known as lustre, these respond well to dyes giving a strong clear colour. Other breeds such as Cheviot have a rougher and therefore duller surface and dyes show as a matt finish. In general a duller fibre will felt more easily.