How Silk Is Reeled From a Cocoon

One Cocoon, One Filament

A silkworm cocoon is not a tangle of short fibres. It is a single continuous filament, laid down in overlapping arcs. That is the entire commercial value of the animal: unwind it carefully and you get one unbroken thread.

Reeling

The process is called reeling, and it is exactly what it sounds like. The end of the filament is located, and several cocoons are unwound together — sometimes with a slight twist — so their filaments combine into a single strand.

A single silkworm filament is far too fine to use alone. Combining several during reeling is how a usable yarn is produced, and how thickness is controlled.

Denier

Silk thickness is measured in denier: the number of grams of weight per 9,000 metres of length. A low denier is a fine yarn; a high one is heavier. It is a mass-per-length measure rather than a diameter, which suits a fibre that is not perfectly round.

Degumming

Reeled silk still carries its sericin coating, which is stiff and dulls the surface. Removing it is called degumming, and it is what produces the handle and shine people associate with silk.

Globally the numbers are substantial: an estimated 400,000 tonnes of dry cocoons produce around 50,000 tonnes of sericin — which the textile industry has traditionally washed away into wastewater.

Why the Moth Cannot Be Allowed to Emerge

This is the uncomfortable part of reeled silk and it follows directly from the biology. A moth emerging from a cocoon breaks the filament to get out, and a broken filament cannot be reeled as one continuous thread.

Silks where the moth is allowed to emerge — eri silk being the best-known — are spun from shorter lengths instead, producing a woollier yarn. Different process, different fabric, different ethics.

Related: what silk is made of · inside the silk gland