What Is Silk? Where It Comes From, How It's Made and How It's Graded

Silk is a natural protein fibre that silkworms spin to build the cocoon they pupate in. Almost all commercial silk comes from farmed silkworms, above all those of the domesticated silk moth Bombyx mori, which are fed on mulberry leaves. The cocoons are unwound, the threads of several cocoons are combined into one strand called raw silk, and that yarn is woven into fabric. On an EU fibre label the material is called “silk”, a regulated fibre name. This guide covers what silk is made of, where it comes from, how a cocoon becomes fabric, what figures like momme and 6A mean, and how to tell real silk.

What is silk?

The silkworm makes a liquid protein in a pair of long silk glands that run along its body and releases it through the spinneret on its head; the protein hardens as it meets the air, and the worm lays the thread around itself, layer by layer. EU textile labelling law defines silk as “fibre obtained exclusively from silk-secreting insects”, so silk from moths other than Bombyx mori counts as silk too.

Properties of the fibre:

• Lustre: silk is a fine fibre with a natural sheen.
• Moisture: the fibre's moisture regain, the moisture it holds relative to its dry weight, is about 11%.
• Strength: the silk filament is a continuous thread of great strength.
• Sensitivity: sunlight degrades silk, and even weak alkaline solutions such as soapy water can damage the fibre.

What silk is made of

Silk is made mostly of two proteins. Fibroin forms the thread itself, while sericin, known as silk gum, coats it: each cocoon thread consists of two fibroin strands, one from each silk gland, held together by sericin. Fibroin in turn is built largely from two amino acids, glycine and alanine. Most of the sericin is removed during processing, for example by boiling the silk in soapy water or a dilute alkali, a controlled step called degumming; because over-degumming can make silk fragile, the concentration and boiling time are set. Our guide to whether silk is natural or synthetic covers both proteins, degumming and why silk counts as a natural fibre.

Where does silk come from?

Silk making began in China. According to Chinese records, it existed there before the middle of the third millennium BCE, and Chinese silk later reached Rome along the caravan route across Central Asia known as the Silk Road. The silkworms that stocked Europe's silk farming until the 19th century go back to around 550 CE, when two monks, at the urging of the Byzantine emperor Justinian, smuggled them from China to Constantinople.

Mulberry silk and wild silk

Mulberry silk comes from the domesticated Bombyx mori, which is raised on the leaves of the mulberry tree. Wild silk is a loose umbrella name, often used for silk from the Antheraea species behind tussah. According to the FAO's silk manual, some of these caterpillars feed on oak leaves rather than mulberry, their thread is more irregular and coarser, and the silk is less lustrous than cultivated silk.

On an EU label, silk from any of these insects is called “silk”. Some sellers write “100% mulberry silk”; “silk” is the fibre name, and “mulberry” is the seller's addition to it.

How silk is made: from cocoon to fabric

Each cocoon is a single unbroken thread; a 2016 review puts its length at 900 to 1,500 metres. Before reeling, the cocoons are heated with steam or hot air, and the pupa inside dies in the process. They are then cooked in hot water and steam until the sericin softens. One thread alone is too fine to use, so the threads of several cocoons are reeled together, with a slight twist, into one strand of raw silk.

Short lengths of thread and rejected cocoons are combed and spun into yarn known as spun silk, which the FAO manual describes as soft, but less lustrous, less strong and less elastic than reeled silk. Which other silk moths there are, and which fabrics are woven from silk, is set out in our overview of the types of silk.

Reeled and spun silk are then woven into fabric. Silk is a natural fibre; satin is a weave, a way of constructing fabric that can be done in many fibres. Silk woven in a satin weave is called silk satin, and that is the one place where the two words genuinely meet. Charmeuse is likewise a fabric name, not a fibre: a lightweight fabric in a satin weave that may be made of silk, polyester or viscose (rayon). How silk satin compares with polyester satin is covered in our guide to silk vs satin.

Momme and 6A: weight and grade

Momme (mm) is a unit for the weight of silk fabric. 1 momme equals roughly 4.34 grams per square metre. A fabric at 22 momme therefore weighs about 95 grams per square metre. Most makers state which figures count as entry-level or premium; those bands are a trade convention, not a norm. Our guide to momme explains where the figure comes from and which value suits what.

A grade, by contrast, rates the raw silk thread before it's woven. Grade 6A is the top class in the Chinese raw-silk standard (GB/T 1797). The grade rates the raw silk, momme the weight of the woven fabric. Specifications such as momme (a unit for the weight of silk fabric) or grade, and a higher price, point towards real silk, but they are not proof. Our guide to mulberry silk explains how to read both figures when you buy.

Silk or artificial silk: how to tell real silk

The fibre composition tells you what a fabric is made of. In the EU, only the fibre names listed in Annex I of Regulation (EU) No 1007/2011 may be used for it, and the regulation does not allow those names to be used for other fibres, whether on their own, as a root or as an adjective. Words such as “silky”, “silk touch” or “satin finish” say nothing about the fibre; they describe how a fabric looks or feels.

Artificial silk is not silk: silk-like yarns made from cellulose, such as viscose and acetate, have long been called artificial silk. Our guide to artificial silk covers the fibres behind that name and how they differ from the real thing. Whether a fibre is really silk can only be settled for certain with a lab test, and not even the often-suggested burn test can do that.

Silk at RENESSY

The terms in the table work for any silk product, ours included. The RENESSY silk pillowcases are made of 100% organic mulberry silk at 22 momme, the raw silk is rated Grade 6A, and both faces of each case are silk. The silk is GOTS certified.

Frequently asked questions about silk

What is silk made of?

Silk is made mostly of two proteins, fibroin and sericin: fibroin forms the thread, and sericin coats it as a gum that is largely removed during processing.

How is silk made?

Silk is made by softening silkworm cocoons with heat, water and steam, reeling the threads of several cocoons together into raw silk and weaving the yarn into fabric, with most of the sericin removed along the way.

Is silk a natural fibre?

Yes, silk is a natural fibre: it is a protein fibre that silkworms and other silk-secreting insects spin for their cocoons.

How can you tell if silk is real?

The most reliable guide is the fibre composition on the label or product page; feel, sheen, momme and grade are only hints, and whether a fibre is really silk can only be settled for certain with a lab test.

What do momme and 6A mean for silk?

Momme is the unit for the weight of woven silk fabric (one momme is about 4.34 g/m²), and Grade 6A is the top class in the Chinese raw-silk standard (GB/T 1797); the grade rates the raw silk thread, not the finished fabric.

Sources

• Regulation (EU) No 1007/2011 on textile fibre names, consolidated version: EUR-Lex
• FAO, Silk reeling and testing manual (Agricultural Services Bulletin 136): Contents
• Encyclopaedia Britannica: Silk
• Museum of Fine Arts Boston, CAMEO: Silk, Tussah silk, Charmeuse
• Kunz RI et al. Silkworm Sericin: Properties and Biomedical Applications. BioMed Research International. 2016. PMC5124675
• SAMR national standards platform (China): GB/T 1797-2008, Raw silk
• American Institute for Conservation, Textile Specialty Group: Fiber Identification
• Lee B et al. Species identification of silks by protein mass spectrometry reveals evidence of wild silk use in antiquity. Scientific Reports. 2022. Nature
• University of Georgia Extension: Understand your fibers
• Wikipedia: Art silk; Wikipedia (German): Kunstseide