A scutching mill in the flax country of Flanders is a loud place in the months after harvest. Bales of dry, grey-brown flax straw go in at one end; at the other, through a haze of dust and broken stem, come two things of very different value: hanks of long, pale fibre that will become linen thread, and heaps of woody fragments that will not. Linen scutching is the stage that separates them, and it decides more about the finished cloth than its obscurity suggests.

The reason lies in the plant. A flax stem is a slender tube: a thin outer skin, a ring of fibre bundles beneath it running the full height of the stalk, and a woody core inside. The fibre is what the linen trade wants; the core, called shive, is in the way. Flax is pulled from the ground rather than cut, to keep the whole length of the fibre, and left in rows on the field to ret, as described in our guide to linen retting. Retting loosens the natural gums that bind fibre to core. Scutching then removes the core altogether.

In a modern mill the process has two movements. First the straw, with its seed heads removed, passes through pairs of fluted rollers that crush and crack the brittle core along its length; this is called breaking. Then the broken stems are held against fast-turning wheels fitted with blades, which beat and scrape away the shive while the tougher fibre holds together. What comes out is scutched flax: long, silky strands laid parallel, often running most of the length of the original stem. The shorter, tangled fibres knocked loose along the way are gathered separately as tow.

Before machines, all of this was done by hand. The stems were cracked in a hinged wooden brake, then laid over the edge of an upright board and struck with a flat wooden blade, a swingle, until the shive fell away. It was skilled, dusty work, done in barns through the autumn and winter, and in the flax villages of Flanders and northern France it was often a household's winter occupation. The vocabulary survives in the trade: the long fibre is still called line, and the short fibre tow. In Kortrijk, on the river Lys, a museum now keeps that history; the river was once called the Golden River for the colour that retting flax gave its water.

The difference between line and tow is the difference that reaches the wearer. Line fibre, long and evenly aligned, is combed, or hackled, through ever finer steel pins and spun, usually wet, into fine, smooth and strong yarns; this is the route to the cloth of a good linen shirt, explained further in long-staple linen fibre and wet-spun linen. Tow is spun into coarser, less regular yarns for heavier cloth, twine and household textiles. Neither is a defect, but they make different fabrics, and the share of line a mill recovers depends on how well the flax was grown, retted and scutched.

Little is wasted. The shive goes to animal bedding, board for the building trade and fuel; the seed, stripped from its capsules at the mill, is pressed for linseed oil or kept for the next sowing. It is part of why flax is so often described as a plant used whole.

The coastal belt that runs from Normandy through northern France and Flanders into the Netherlands grows the larger part of the world's fibre flax, and much of Europe's scutching takes place within it. The Belgian Linen® certification covers each stage described here, from the field where the flax is pulled to the cloth that leaves the loom; the guide to 100% Belgian linen explains the mark itself. For the Pieter Petros garments woven from certified Belgian linen, that chain ends in the atelier, where the cloth is cut and sewn by hand.

Read more: How Linen Is Made, from the Flax Field  ·  The Properties of Belgian Linen