Why wool pills, and the sweaters that do not
The pill is a knot, and friction ties it
Pilling happens when loose fibers on the fabric surface twist together into small spheres, and the spheres are the pills. The fibers break free from the yarn twist, work their way to the surface, and tangle through friction: the bag strap on the shoulder, the seatbelt across the chest, the desk edge at the elbow, the wash cycle's agitation. Friction is the primary catalyst, and every movement that rubs the fabric contributes.
Wool is particularly prone to it, because wool fibers have microscopic scales along their surface, and the scales catch and pull when the fabric rubs against another surface. The fibers migrate to the surface, and the pills form in the places that get the most wear: the underarms, the side where the bag sits, the cuffs, and the waistband.
The pills show up most in the first weeks of wear, and that is the detail that surprises people. The new sweater sheds its shortest fibers first, and once those are gone, the pilling subsides, because the longer, stronger fibers are all that remain. A new sweater that pills heavily in the first month and then stops is a sweater that has finished shedding, not a sweater that is falling apart.
Short fibers pill, long fibers do not
The length of the fiber is the first determinant. Shorter fibers, such as those in carded yarns, have more ends, and more ends means more fibers that can work their way to the surface. Longer fibers, often called long-staple fibers, can be spun into a stronger, smoother yarn, with fewer fiber ends exposed, and the pilling drops sharply.
High-quality Australian merino is prized for its long staple length, which can exceed 90 millimeters, and the length is the reason the yarn holds together. Shorter fibers, sometimes called carded or woolen-spun yarns, are intentionally loftier and have a fuzzier surface, and the fuzz is the pilling waiting to happen.
The fineness of the fiber is the second determinant, and it cuts the other way from what people expect. An ultrafine merino might be 17.5 microns or less, while a coarser wool could be 25 microns or more. The finer fiber is softer, and the softer fiber is more prone to pilling if the fibers are short, because the fine, short fibers slip free more easily. The finest wools pill the most, and the coarsest wools pill the least, which is the opposite of what the price suggests.
The twist and the knit decide the rest
The construction of the yarn is the third determinant. A tightly twisted yarn, often described as worsted-spun, holds the fibers more securely, preventing them from migrating to the surface. A loosely spun yarn, while it can create a wonderfully soft and airy fabric, allows more fiber movement and thus more pilling. The tighter the twist, the fewer the pills.
The density of the knit is the fourth. A dense, tight knit structure restricts the movement of the yarns and fibers, offering another layer of defence. A loose, open knit, while it offers beautiful drape, provides more opportunity for the friction that leads to pills. A bigger gauge in relation to the weight of the yarn means more space between stitches, and more space means more pills.
The care is the fifth, and it is the one the wearer controls. Turn the garment inside out before washing, so the smoother inside surface takes the friction. Use a gentle cycle or hand wash in cool water with a wool detergent, because agitation is the enemy. Wear a smooth layer underneath, rotate the sweater instead of wearing it every day, and use a fabric shaver or a sweater stone to remove the pills that form. Cashmere pills for the same reason, and the grade that pills fastest is Grade B, at 16 to 19 microns, while Grade A at 14 to 15.5 microns pills less, because the fiber is longer and the yarn is tighter.
Frequently asked questions
Does pilling mean my sweater is poor quality?
No. Pilling happens most often with higher quality, fine fibers like cashmere and merino, because the fibers are short, soft, and silky, and they escape from the twist more easily. Long-staple wools like Icelandic or Shetland, with longer fibers, pill less.
Why does my new sweater pill so much in the first weeks?
Because it is shedding its shortest fibers. The new sweater sheds the shortest, finest fibers first, and once those are gone the pilling subsides, because the longer, stronger fibers are all that remain.
What makes a sweater pill less?
Long-staple fibers, a tightly twisted worsted-spun yarn, and a dense knit. A loose, open knit and a loosely spun yarn allow more fiber movement, and more movement means more pills.
How do I stop wool from pilling?
Turn it inside out before washing, use a gentle cycle or hand wash in cool water, wear a smooth layer underneath, rotate the sweater, and avoid friction from bag straps and seatbelts. Remove the pills that form with a fabric shaver or a sweater stone.