Heckling combs are used to remove the fibrous core and impurities from flax. A heckling comb, or heckle is a bed of iron, steel or brass "nails"- sharp, long, tapered, tempered, polished pins driven into wooden blocks at regular spacing. In the heckling process the flax is drawn through the nails, 'combing' it, which parts the locked fibers making the fibers straight and clean. After heckling, the flax is ready to spin. After spinning, the flax is then suitable for weaving into linen.
Dressing is the broad term referring to removing the fibers from the straw and cleaning it enough to be spun, and heckling is the last step in dressing the flax. In the dressing process the flax is broken, scutched and heckled. Heckling involves pulling the fiber through various sized heckling combs. Different sized heckling combs are used, progressing from coarser combs with only a few prongs or nails per inch, to finer combs. Generally around three heckling combs are used, however many more can be used. The finer the final heckling comb, the finer the yarn spun from that flax can be. An example of a progression of five combs is first using a heckling comb with 4 nails per square inch, then one with 12 per inch, then 25, next 48, and finally 80 nails per inch. The first three remove the straw, and the last two split and polish the fibers.
The shorter fibers that remain in the heckling comb after the flax has been combed is called tow. If the heckle is fine enough, the tow can be carded like wool and spun, otherwise it can be spun like the other flax fibers. Tow produces a coarser yarn than the fibers pulled through the heckles because it will still have some straw in it. While this yarn is not suitable for fine linens, it can be used for bagging, rough sheets, cords or ropes.
The term "heckle" is thought to be derived from Old English, with Middle English forms hechele, hetchell (c1300), hekele (c1440), hakell (1485), and later hatchel. The terms "heckle" and "hackel" are used interchangeably at present.