Fabric Fibers and How Each Behaves When You Sew

Fiber is not the same as fabric

Two shirts can look identical on the hanger and behave completely differently under a needle and an iron. One is 100% cotton and forgives almost anything. The other is rayon: it may shrink in the wash, it weakens when wet, and it starts fraying as soon as you cut it. The difference is the fiber: the raw strand that is spun into yarn before it is woven or knitted into cloth.

How the yarns are put together, woven or knitted, is a separate question, covered in woven and knit fabrics. A knit can be cotton or polyester, and so can a woven; you need both answers to plan a repair.

The four fiber groups

Fibers sort into groups by where they come from, and each group shares habits that help you predict how a fabric will act. The University of Georgia Extension sorts them this way:²

• Natural plant (cellulose) fibers: cotton, linen (made from flax) and ramie. They absorb water and tolerate high heat, but wrinkle easily unless treated.
• Natural animal (protein) fibers: wool and silk. Wool can shrink and felt (mat together permanently) in hot water and agitation. Silk is weakened by chlorine bleach and yellows when pressed too hot.
• Manufactured cellulose fibers: rayon (also sold as viscose), acetate and triacetate. These are made from wood pulp. Rayon absorbs water well but loses strength when wet and may shrink or stretch unless treated.
• Synthetic fibers: polyester, nylon, acrylic and spandex, made from petroleum-based chemicals. They are strong and absorb little water. Too much heat softens or melts them.

Heat is the point that drives most sewing decisions. Cotton, linen, rayon and wool do not melt; they can scorch under high, dry heat. Polyester, acrylic and spandex are heat sensitive, and nylon melts and fuses.²

Reading a fiber content label

In the United States, the Textile Fiber Products Identification Act and the FTC rules under it require most clothing to list each fiber by its generic name and its percentage by weight, in order from most to least.¹ A label reading "60% Cotton, 40% Polyester" is exactly that. A single-fiber garment may say "All Cotton" instead of "100% Cotton".¹

A few rules explain labels that look odd. Any fiber under 5% of the weight is usually grouped as "other fiber", except wool, which must be named at any amount, and fibers that are there for a stated purpose, such as spandex for stretch.¹ The stated percentages may be off by up to three percentage points, except on a label that claims a single fiber.¹ The FTC accepts the international ISO names, so "viscose" means rayon and "elastane" means spandex.¹ The label must also name the country of origin and the company, either by name or by an RN number (a registration number issued by the FTC).¹

Brand names such as Lycra or Tencel are trademarks, not fiber names. The label will also carry the generic name, spandex in the first case and lyocell, a type of manufactured cellulose fiber, in the second.

How each fiber behaves when you sew and mend

The table below collects what matters when you sit down to sew. Iron settings come from GINETEX and the Canadian care symbol guide, and general fiber behavior is from UGA Extension.²⁵⁷ Needle types follow the SCHMETZ guide, with Microtex for fine or slippery cloth; the sizes shown are starting points for medium-weight fabric.⁴ Shrinkage and fraying notes describe typical untreated fabric, and a finish applied at the mill can change them.

SCHMETZ describes the Microtex needle as having a slim, sharp point for silk, microfiber, polyester and other fine fabrics, the Jersey needle as having a ballpoint that slips between knit loops, and the Stretch needle as suited to highly elastic materials.⁴ In the needle numbers, such as 80/12, the first figure is the blade diameter in hundredths of a millimeter.⁴ The needle lessons for machines and for hand sewing show how to fit them.

Thread choice works the same way. All-purpose polyester thread suits synthetics, blends and anything that stretches, because it has a little give and does not shrink. Cotton thread suits 100% cotton and linen garments that are pressed hot, and it will break before the fabric does under strain, which some menders prefer on old, fragile cloth.

Blends: the weakest fiber sets the rules

Most everyday clothes are blends, two or more fibers spun together. A blend usually takes some strengths from each fiber, but for heat it takes the limit of the most sensitive one. Set the iron for whichever fiber in the blend needs the lowest temperature.⁶ A cotton and polyester shirt is pressed at the polyester setting, two dots, even though cotton alone could take three.⁵

Blends also shrink less than pure cotton, since the synthetic part holds its size, and beginners often find them easier: they fray less and do not water-spot, and moderate heat presses them flat enough. A 2% or 3% spandex content changes the plan more than the main fiber does, because the fabric now stretches and needs a stretch stitch or a zigzag on the machine.

Worked example: a cotton-poly shirt and a rayon blouse

Take the shirt first. The label says 60% cotton, 40% polyester, and there is a 1 1/2 in (3.8 cm) tear beside the side seam. Because of the polyester, the iron stays at two dots, a maximum of 160 C (320 F) on the GINETEX scale.⁵ The fabric is a plain woven shirting, so a universal 80/12 machine needle or a size 9 hand needle works, with all-purpose polyester thread. For backing the tear, a lightweight fusible interfacing rated for moderate heat will bond at two dots; test one corner first. If the backing fabric is cotton, wash and dry it first; a polyester or blend backing can go on as it is. The repair follows mending a woven tear, and the shirt can go back into a warm wash afterward.

Now the blouse. The label says 100% rayon (or 100% viscose), and the problem is a 2 in (5 cm) seam that has pulled apart where the fabric frayed away from the stitching. Rayon loses strength when wet and can shrink,² so do the repair dry and do not dampen the area to flatten it. Trim no more than the loose threads, then stitch the new seam 1/4 in (6 mm) inside the old line so the needle catches fresh, unfrayed fabric, and finish the raw edges with a zigzag or an overcast stitch so the fraying cannot start again. Use a fine Microtex 60/8 or 70/10 machine needle, or a size 10 or 11 hand needle, and polyester thread. Press at two dots through a press cloth without letting steam hit the fabric directly.⁶

If the rayon blouse needs a patch, buy rayon, and wash and dry it in exactly the way the blouse is cleaned before cutting. Unwashed rayon fabric can shrink noticeably the first time it gets wet, and a patch that shrinks pulls the blouse into puckers.

The burn test, done safely

When there is no label, a burn test can narrow down the fiber. Use a few threads pulled from inside a seam allowance, never the garment itself. The Museum of Fine Arts, Boston's materials database describes rolling 2 to 4 cm (about 3/4 to 1 1/2 in) of fiber into a small ball, holding it in tweezers over a non-flammable surface, lighting it, and removing the flame to watch what happens.³

• Work over a sink, a metal baking tray or a ceramic plate, with a glass of water within reach. Tie back hair and loose sleeves.
• Burn a tiny amount only. Melting synthetics drip hot liquid that sticks to skin.
• Do not breathe the smoke, and work with a window open. Children should do this only with an adult.

The table summarises the museum's descriptions.³ The test works best on fabric made from one fiber; a blend gives mixed signs, such as a paper smell with a hard bead, which itself tells you a synthetic is present. Once you know the fiber, set the iron one step lower than the table above suggests for your first press, then raise it if the test strip stays clean.

Common mistakes

• Pressing a blend at the setting for its main fiber instead of its most heat-sensitive fiber.
• Mending rayon while it is damp, when it is at its weakest and can stretch out of shape.
• Using a heavy universal needle on silk or rayon, which leaves visible holes and pulls threads.
• Assuming "other fiber" on a label means nothing important; small amounts of spandex do change how you sew.
• Patching a wool garment with a cotton patch that will shrink at a different rate when the garment is cleaned.
• Running a burn test on the garment edge instead of on threads from inside a seam allowance.

When in doubt, sew a short test seam on a scrap cut from the hem allowance. Four inches (10 cm) of stitching tells you more about needle, thread and tension on that fiber than any chart can.