Unraveling Silk

“Silk does for the body what diamonds do for the hand.”

— Oscar de la Renta

Silk is the queen of fabrics. No fabric is as luxurious, or as loved. It is elegant, yet classic – shiny, soft, and smooth against the skin. Silk is strong, durable, breathable, fast-drying, and water-, wrinkle- and stain-resistant. Its allure stems from its unparalleled beauty and lustrous sheen. It is expensive and rare, as its production is a meticulous, labor-intensive process. Silk commands a price 20 times that of cotton with a market size of US$17 billion.¹ One yard of silk fabric costs about US$100.

Its origin lies in China, where it was first discovered over 5,000 years ago. The ancient Greek word for China is ‘Seres,’ which literally means ‘the land of silk.’ So prized was silk that its method of production remained a fiercely guarded secret within China for some 3,000 years, with imperial decrees sentencing to death anyone who revealed the process of its production to a foreigner.² Silk trade led to the establishment of an extensive transcontinental network in 130 B.C., later termed ‘The Silk Road’, which stretched over 6,437 kilometers connecting China, the Far East, the Middle East, and Europe. It remained in use for more than 1,500 years to facilitate the trade of both goods and ideas.

The Making of Silk

Silk isn’t so much ‘made’, as it is ‘taken’. Silk is a protein fiber produced by insects to make cocoons or webs. Although they are known as ‘silkworms’, in reality, they are caterpillars who naturally metamorphose into moths. The soft, white domesticated bombyx mori moth makes up approximately 90% of silk production.³

To form its cocoon, a silkworm spins its body in a figure-eight shape up to 300,000 times over eight days. The cocoon, which spans a kilometer of continuous silk filament, is the longest natural fiber on earth. Each silkworm will produce a single strand of silk, which is 10 times finer than a human hair. Multiple silk strands are then entwined to create silk thread. Eventually, that silk thread is woven into a fabric for commercial use. Sericulture, or silk farming, is the process of cultivating silkworms to produce silk.⁴ It typically involves the following steps:⁵

1.      Raising Silkworms: Female moths lay 300 - 500 eggs, which hatch into larvae and feed on mulberry leaves for 6 weeks. Then, they weave their cocoon over 8 days.

2.     Extracting Silk Threads: The cocoons are placed in boiling water to kill the larvae to prevent them from metamorphising, which would rupture the silk thread. Threads are then wound on a reel.

3.    Dyeing: Dyeing methods in commercial silk making have evolved from natural sources to synthetic ones to optimize cost and efficiency.

4.    Spinning & Weaving: The process of spinning unwinds the silk strands onto a spindle or cylinder. Weaving then interlaces them so that they lock around one other to create a strong, uniform piece of fabric.

5.    Printing & Finishing: Textile printers transfer artwork or patterns onto fabric. Finishing applies chemical treatment to add properties such as fire resistance and crease-proofing.

Silk can be harvested from other insects, most notably spiders, whose ‘spider silk’ is stronger than steel, tougher than Kevlar, and more elastic than rubber. Approximately 1,400 spiders are needed to spin 1 ounce of spider silk.⁶ However, spiders have escaped the fate of intensive commercial breeding as they are cannibalistic and territorial animals who, in captivity, also produce less silk and take a longer time to do so. Due to its unique mix of mechanical properties and biocompatibility, spider silk is sought after for varied applications including medicine (artificial materials such as bandages, prostheses, ligaments, and skin), military (body armor, parachutes, shells of tanks aircraft, and satellites), and materials (buildings, bridges, tires).⁷ This has motivated numerous approaches to produce it synthetically.

Image credit: Timekeep/Shutterstock

The Price of Silk

For all its beauty, silk extracts a heavy price on animals, people, and the planet. It is worth a deeper look at each.

Animals

Silk is produced by boiling silkworms alive inside their cocoons so they cannot hatch, which would rupture the silk thread. Approximately 6,600 silkworms are killed to make 1 kilogram of silk. That’s 1,000 animals for one shirt. The result is that 420 billion to 1 trillion silkworms are killed annually for silk production. Disease alone kills between 10% and 47% of worms, largely attributed to the rearing environment with inadequate disinfection and contamination.⁸ To optimize silk production, humans have created a domesticated silk moth that is blind and unable to fly. She lays eggs once and is then killed. Her offspring are killed before they mature and grow into a moth so silk can be harvested from their cocoons. Male moths are discarded after they mate.

Although efforts have been made to create ‘peace silk’ (also known as ‘ahimsa silk’), where silk is harvested from cocoons only after they have hatched, moths are still discarded or crushed after their ‘useful life’ and selectively kept in semi-frozen conditions until breeding season. The process also requires many more silkworms since they produce less silk that is ‘reelable’.⁹ ‘Wild silk’ harvests cocoons of silkworms that live in the wild and is therefore nonviolent, but like peace silk, it produces a rougher variety of silk due to its short fiber length.

The science remains inconclusive when it comes to measuring the sentience of silkworms, yet it is worth noting that they have a brain and a central nervous system, and release endorphins, which are hormones present when pain is experienced.¹⁰ A less scientific explanation is also worth considering – like all insects, silkworms will retreat when they are threatened.

People

The silk industry has faced accusations of human rights violations, human health issues, child labor, and slavery. Although it provides employment in developing countries and rural areas with little opportunity, these jobs often come with meager, unsustainable wages for workers. The industry also offers little transparency to understand and address these issues.¹¹ China, the world’s largest producer of silk, employs approximately 1 million workers in the industry, while India, also a major silk producer, employs 7.9 million largely rural workers. A study by Human Rights Watch in 2003 estimated that 350,000 children in India worked as bonded laborers in the silk industry.¹²

Sericulture workers risk burns and secondary infections as silkworms are placed in boiling water and hot steam. Children who wind silk strands can suffer cuts that may become infected if untreated. A report from the Indian state of Kashmir revealed workers in the industry suffered health problems including headaches, back pain, respiratory issues, and eye irritation.¹³ Further, chemical exposure during processing and dyeing can cause skin irritation and organ damage to the kidneys, liver, and lungs.¹⁴

Environment

According to the Higg Materials Sustainability Index, silk has the highest environmental impact compared to all other fabrics, even synthetic ones.¹⁵ To make one kilogram of silk requires 187kg of mulberry leaves (a source of nutrition for growing silkworms), which utilizes large amounts of land and water, and produces a high volume of greenhouse gases due to the use of manure and fertilizer. Sericulture facilities must be kept at controlled temperatures, which requires energy to boil water and steam, commonly powered by coal.¹⁶ The silk finishing process exacerbates the detrimental impact with the use of acid, metal, and reactive dyes that are added to acidic water in which the silk filament is submerged. Improper waste disposal can pollute the soil and local waterways. Chemical residues in the end product can also lower the material’s biodegradability.

The Evolution of Silk

Silk is utilized not just in fashion, but also in home furnishings such as bedding, curtains, and upholstery, to moisturize and nourish hair, and to create classic and modern art. The quality of silk is measured by momme weight, a more accurate measure of silk quality than thread count. Introduced by the Japanese, it has been adopted as the standard for measuring the density of silk fabric. The higher the momme, the tighter the weave, indicating a higher amount of silk utilized.¹⁷ Silk is prized due to a combination of optical, chemical, mechanical, and thermal properties that make it difficult to replicate:¹⁸

The race to create artificial silk is nothing new. Mercerized Cotton was invented in the mid-1800s, offering higher tensile strength and luster, making it viable as a silk substitute. Rayon, also called viscose, emerged in the late 1800s. It is a semi-synthetic fiber that is comprised of regenerated cellulose from wood and related agricultural products.¹⁹ Nylon and polyester came along in the 1930s as alternatives to silk. Nylon is softer and shinier, while polyester is rougher and duller. Unfortunately, both are petroleum-derived with formulations that contain chemically stable microplastics that poison ecosystems, harm animals, and fuel climate change.²⁰

Innovation is ongoing to produce the next generation of materials to rival silk. While some approaches are biomimetic, able to replicate the structure of silk, others aim to create alternative materials that mimic its properties while minimizing externalities:

Milking Silk

As an industry, silk is a drop in the bucket of the US$1,027 billion textile market.²¹ Yet, as demand for luxury goods grows globally, so will the demand for silk. Unfortunately, when it comes to the impact on animals, people, and the planet, silk punches well above its weight. This heightens the need to create synthetic alternatives that can meet and beat the properties offered by silk at a competitive price while reducing the harm from the industry today. Although the demand for silk will continue rising, synthetic material innovation is marching forward alongside it. This is good news for those that love silk, and our planet too.  

1. Markets and Markets

2. UNESCO

3. The Sustainable Fashion Collective

4. Wikipedia

5. Biddle Sawyer Silks

6. PETA

7. e-Polymers

8. Rethink Priorities

9. Beauty Without Cruelty

10. One Green Planet

11. Collective Fashion Justice

12. Human Rights Watch

13. Wani and Jaiswal (2011), Health Hazards of Rearing Silk Worms and Environmental Impact Assessment of Rearing Households of Kashmir, India, Nature Environment and Pollution Technology Journal

14. Silkworm Mori Blog

15. TreeHugger

16. Silkworm Mori Blog

17. Mayfair Silk

18. Etanna

19. Wikipedia

20. Materials Innovation Initiative

21. IMARC