What Is Silk?

How is silk made?

Silk is made by rearing caterpillars until they spin cocoons, unwinding the cocoon filament into thread, twisting that thread into yarn and weaving or knitting it into cloth. Most commercial silk comes from the domesticated silkworm, Bombyx mori, which feeds only on mulberry leaves and is reared indoors. According to the International Sericultural Commission (ISC), this mulberry silk makes up around 90 per cent of world silk production.

1. Rearing. The ISC gives a rearing period of about 25 to 28 days for mulberry silkworms. Spinning the cocoon is completed within three days, and the cocoons can be harvested after five days.
2. Reeling. Filaments drawn from several separate cocoons are combined into one thread. The ISC describes the result, raw silk, as a compact, untwisted and undegummed thread.
3. Throwing. The raw silk receives a twist and is wound onto bobbins.
4. Degumming. Raw silk is two strands held together by a protein gum, sericin, which is removed by boiling in soapy water or a dilute alkali.

Sources disagree on how long the filament is. The Museum of Fine Arts, Boston's CAMEO database gives a filament length of 250–750 m; Encyclopaedia Britannica puts the thread in one cocoon at roughly 1 km. Raw silk that cannot be unwound, and so is unsuited to throwing, becomes silk waste: the longer filaments are spun into yarn called spun silk (or schappe), and the shorter ones become silk noil.

Mulberry and wild silks

What are the properties of silk?

Silk is a fine, lustrous protein filament that stretches by a fifth to a quarter of its length before it breaks, regains 11 per cent moisture from the air, and is weakened by sunlight and by alkalis. Unlike cotton and wool, which are spun from short fibres, reeled silk is a continuous filament. Figures below are for Bombyx mori silk, from CAMEO.

• Shape and lustre: a fine, lustrous fibre with a triangular cross-section.
• Stretch: elongation of 20–25 per cent in normal conditions and about 30 per cent when wet.
• Moisture regain: 11 per cent, the moisture a dry fibre takes back from the air under standard conditions.
• Density: 1.25–1.34 g/ml.
• Heat: CAMEO lists 175 °C as the point at which silk decomposes; it burns readily with an unsteady flame.
• Light and chemicals: degraded by sunlight, damaged even by weak alkali solutions such as soap, decomposed by strong acids, and unaffected by most organic solvents.

Britannica notes that during and after the Second World War, man-made fibres such as nylon replaced silk in hosiery and other garments, which greatly reduced the silk industry.

How is silk graded or measured?

Silk is measured in two different ways: raw silk yarn is graded by the trade, and finished cloth is described by its weight in momme. Neither figure has to appear on a clothing label.

• Raw silk grades. The ISC says raw silk is classified by a grade derived from its denier (a measure of yarn fineness) and other tests, expressed in the order 4A, 3A, 2A, A and B.
• Momme. Some silk cloths are sold by weight in momme. CAMEO's entry for habutai, a soft plain-weave silk, gives 3–4 momme as sheer and 12 momme as heavyweight. Within the same weave, a higher momme number means a heavier cloth.

How does silk appear on a clothing label?

On a clothing label silk appears simply as “silk” with its percentage by weight; the label names the fibre, not the cloth.

Under US rules

• Labels must give the generic names and percentages by weight of the fibres (16 CFR 303.16). “Silk” is one of the generic names the rules use as an example (16 CFR 303.6(a)).
• Under the Textile Act rules, a fibre under 5 per cent of the total fibre weight is listed as “other fiber” unless it has a clearly established and definite functional significance (16 CFR 303.3(a)).
• The label must also give the name or registered identification number (RN) of the manufacturer or another company marketing or handling the product, and the country where it was processed or manufactured (16 CFR 303.16).
• In a blend, the stated percentages may be off by up to 3 per cent of the total fibre weight, so “60% silk” may mean 57–63 per cent (16 CFR 303.43).

Under EU rules

• Only the fibre names in Annex I of Regulation (EU) No 1007/2011 may be used. There, “silk” means “fibre obtained exclusively from silk-secreting insects”, so the one name covers mulberry and wild silks alike.
• “100 %”, “pure” or “all” may only be used for products made exclusively of one fibre, although up to 2 per cent by weight of extraneous fibres (5 per cent for products that have been carded) is allowed under Article 7, if it is technically unavoidable in good manufacturing practice and not added as a matter of routine.
• A multi-fibre product must list every fibre with its percentage by weight, in descending order (Article 9).

What the label tells you

Chiffon, georgette, crêpe, charmeuse, organza and satin are names of cloths, not fibres, and none of them is an Annex I fibre name. The fibre line is what separates a silk satin from one made of acetate, polyester or viscose, each of which has its own legal name.

How do you tell good silk from poor silk?

Judge silk by its fibre line, its weight within the same weave, a burn test on a spare thread, and, on older pieces, signs of weighting or light damage.

1. Read the fibre line first. Look for “silk” and its percentage. On an EU label the fibres run in descending order, so silk listed second is not the main fibre.
2. Ask for the momme weight and compare it only against the same weave.
3. Burn a single loose thread, for example one trimmed from a seam allowance, over a non-flammable dish. Silk burns readily with an unsteady flame, smells like burnt hair and leaves an ash that crumbles easily. Treat it as a check against look-alikes, not as proof.
4. Look at undyed wild silk colour. Muga is naturally golden-yellow, eri white or brick-red, and the Japanese tasar silkworm gives green thread. Dyeing hides these differences.

Red flag: weighting. Silk readily absorbs soluble salts and is often treated with metal salts such as tin chloride to increase its density. CAMEO says a moderately weighted silk contains about 25–50 per cent salt, and that weighted silks degrade more rapidly.

Red flag: light damage. Sunlight degrades silk, so check the parts of an older garment that were most exposed to light before you buy it.

How do you care for silk?

Care for silk by following its care label, keeping it out of strong sunlight and avoiding alkaline products such as soap, both of which damage the fibre.

• Washing. If the label allows it, avoid ordinary soap and other alkaline products, and keep strong acids away: they decompose silk.
• Storage. Keep silk covered or in a closed wardrobe, out of the light.
• Press with care. Silk decomposes at about 175 °C, so use the iron's silk setting rather than a high heat.

Is silk sustainable?

The sources here do not support a blanket claim that silk is low-impact; its footprint depends on how it is farmed, processed and finished.

• Feed crop. According to the ISC, mulberry is a perennial crop that can yield for 25–30 years, with the plants generally pruned twice a year and the leaves harvested five times a year.
• Water in processing. The ISC estimates that wet processing uses, on average, almost 100 litres of water for 1 kg of silk goods.
• The moth. In eri production, the ISC says, the moths are allowed to emerge and the pierced cocoons are spun into yarn.

Question any unexplained “sustainable” claim for silk.