Pilling is caused by friction – the scales on short fibres work their way to the surface and lock together where the friction is occurring, which then become matted into balls. They often occur in areas of high friction, like the armpits and cuffs of jumpers. Things like the breed of the sheep and how the fleece is prepared and spun will influence whether a garment is more susceptible to pilling.
Short-fibred fleece that goes through the process of carding and spinning causes the fibres to become randomly arranged, making it easier for the short fibres to make their way to the surface. Long-fibred fleece is typically combed to remove the shorter fibres, which is then spun in a style that keeps the long fibres parallel and smoothed down, which deters pilling. A densely woven fabric will hold the fibres firmly and have less tendency to pill, while a loose, lighter weave will allow them to escape.
When shopping for your next woollen garment, it’s a good idea to examine the fabric to see how dense it is. If you do have a problem with an existing garment, a pilling comb or a battery-operated de-pilling device can be used to gently remove them, which are low cost and easy to use. To prevent pilling, it’s best to wash garments gently, inside out, using a specially formulated wool wash.