How Is Silk Made?

The history of silk-making dates back thousands of years. The art of silk production was first discovered in ancient China, home to the silkworm. Today, the silk production process remains mostly the same as it was millennia ago. Silk is made from cocoons that are spun by silkworms. But how do silkworms make silk, and how do we turn these strands of silk into the beautiful silk fabrics that we love to wear?

The Lifecycle of a Silkworm

The process of raising silkworms to produce silk is called sericulture or silk farming. It all starts with the silkworm, or Bombyx Mori as it’s called in Latin. Its Latin name translates to ‘silkworm of the black Mulberry tree’. The creature is also commonly referred to as mulberry silkworm, named after its diet of mulberry leaves.

The mulberry silkworm is responsible for over 95 percent of all the silk produced in the world. It is a completely domesticated species that no longer live in the wild. During sericulture, a silkworm will go through several of the following lifecycle stages:

Stage 1 – The Egg

The lifecycle of a silkworm begins with the eggs of a grown silkmoth. A female silkmoth can lay up to 500 eggs. Within a few days after laying her eggs, the silkmoth will pass away, as its sole purpose in life is to reproduce.

Each of the eggs is about the size of a poppyseed and light yellow in color. Fertile eggs will turn to a dark brown or grey color within a few days. The fertile eggs are incubated under the optimal temperature of about 25 degrees Celsius and a humidity of about 80 to 85 percent. Under these conditions, the eggs are expected to hatch into larvae within approximately 12 days.

Stage 2 – The Larva (Caterpillar)

After hatching, the larvae are carefully transferred from the incubation room to the rearing room, where it’s time for them to feast. The larvae are placed onto rearing trays with great care, as newly hatched silkworms are vulnerable to injuries. Silkworm larvae look like tiny black hairy caterpillars, which will later shed their hair and skin and eventually turn white.

A Diet of Mulberry Leaves

The rearing trays are covered with freshly chopped mulberry leaves, which is the only food a silkworm consumes. The caterpillars get fed fresh leaves twice a day. Care is taken to ensure the mulberry leaves remain moist and the trays remain clean so that the silkworms continue to eat and stay healthy. You will be able to hear the sound of silkworms crunching on mulberry leaves when standing near the trays. The sound of many silkworms simultaneously eating has been described as similar to the sound of falling raindrops.

Shedding of the Skin

The larva stage, also known as the caterpillar stage, is the longest in the lifecycle of a silkworm. As a larva, the silkworm will go through five instars. An instar is a developmental stage between molts, and a molt is when the larva sheds its skin. Each silkworm will shed its skin four times before cocooning.

The silkworm has to molt often as it grows in size exponentially. During every molt, its old skin is shed to make room for a larger one. After its first molt, the silkworm will have already shed its hair to reveal its smooth skin. In the last days of the fifth instar, the larva will be 10.000 times heavier than it was at hatching. After about 24 days as a larva, the silkworm is ready to pupate. At this point, the caterpillar will have eaten its initial bodyweight about 50.000 times over.

Stage 3 – The Pupa

Now that the caterpillars have finished their feast, it is time for them to start spinning their cocoons so that they can pupate and transform into moths. You’ll know the larvae have matured by the signs they show. They’ll no longer eat, they’ll crawl around looking for corners to spin in, and they’ll be creamy white in color. Now the larvae are ready to be transferred from their rearing trays to mountages.

Mounting

The mature silkworms are transferred to mountages by hand, making sure not to overcrowd the mountages and to leave out any diseased caterpillars. This process is also called mounting. A mountage is a device that looks like a frame with cubbyholes, which allows silkworms to comfortable spin their cocoon.

The environmental conditions during mounting should be optimal to ensure that the cocoons are of high quality. The temperature should be no higher than 26 degrees Celsius, and humidity should be between 60 and 70 percent. Care should also be taken not to disturb the silkworms while they’re spinning their cocoons.

Cocoon Spinning

While in the mountages, the larvae start trapping themselves inside a cocoon by spinning silk around their bodies. Each of the cocoons is made out of a single long silk fiber, that the larvae produce using their special silk glands. A cocoon is about the size of a cotton ball, but the silk fiber that makes up the cocoon can measure up to 1.6 kilometers in length.

The color of a cocoon can range from white to yellow, depending on the type of silkworm and its diet. Wild silkworms all make cocoons out of yellow silk, as this blends better with the leaves around them. However, nowadays, most cocoons on silk farms are white. This is because silkworms were domesticated over the centuries and selectively bred to be whiter in appearance. White silk is easier to dye than yellow silk, as you don’t have to bleach it first. This is more convenient for the textile industry, and the reason why white silk is commonly seen in the sericulture industry.

After about 7 or 8 days, the larvae will have transformed into pupae inside their silken cocoons. At this point, their skin has hardened and their color has turned brown.

Stage 4 – The Adult Moth

After about 2 to 3 weeks in its cocoon, the pupa will be ready to metamorphose into a silkmoth. While emerging out of its cocoon, the cocoon will break, making the silk fiber unsuitable for silk fabric production. This is why in sericulture the silkworms will never make it to this stage. Instead, the cocoons are steamed or boiled after around a week since the spinning of its cocoon. This results in the death of the pupae so that the silk cocoon remains intact. At this stage in silk farming, the cocoons are ready to be processed into raw silk.

If the silkworms were allowed to break out of their cocoons, they would have transformed into adult silkmoths. The life of a grown silkmoth is very short, as its only purpose is to reproduce. Domesticated silkmoths are unable to eat or fly. They will find a partner to mate with and then pass away after reproducing. This whole process usually happens within a week. Male silkmoths die after mating, while female silkmoths die after laying their eggs. The lifecycle of the silkworm then starts all over again.

How Silkworms Make Silk

The body of a fully grown silkworm ready to pupate is filled with liquid silk. The liquid silk is created from digested mulberry leaves and is made up of proteins, just like our hair. The caterpillar uses special salivary glands that are located in its head to transform this liquid into a physical silk thread.

The silkworm has two modified salivary glands near its lower jaw called sericteries. These silk-producing glands secrete a clear liquid that hardens into a thin silk thread when it comes into contact with air. This liquid is mostly made up of a type of protein called fibroin. The same glands can give off a second protein at the same time; a gummy substance called sericin. The sericin proteins coat the fibroin proteins, acting as a sort of glue. Thanks to this coating, the two silk filaments from both glands can stick together.

With the use of its silk glands, a silkworm will continue to spin its cocoon until all the liquid silk in its body is used up. The caterpillar will swing its head around while spinning to ensure the silk fiber completely surrounds its body. It does this by instinct to protect itself from predators while pupating. It takes a silkworm about 3 days at a speed of approximately 30 to 40 centimeters of silk a minute to finish its cocoon. The final result will be a cocoon made of a single silk thread measuring up to 1600 meters in length.

How Silk Thread and Silk Fabrics Are Made

After the silkworms transformed into pupae inside their cocoons, the process of making silk yarn can start.

1. Harvesting

First, the cocoons have to be harvested from the mountages. Harvesting happens around 7 to 8 days after the silkworms started spinning their cocoons. A few cocoons can be cut in half to check if the pupae have fully formed. A fully formed pupa is hard and brown in color. The cocoons are carefully picked by hand to ensure that no damage is done to the delicate silk fibers.

2. Stifling & Sorting

The stop the pupa inside the cocoons from hatching and breaking the silk cocoon, the pupa will have to be killed. This process is called stifling and is usually done using hot air or steam. Stifling also dries out the cocoon so that it can be preserved longer. The cocoons can then be sorted based on quality and characteristics such as the length, shape, color, and luster of the silk fiber.

Some cocoons may be deemed unsuitable for further processing and will be thrown out. Examples of cocoon defects include urine stains, mold growth, and perforations.

3. Boiling

After stifling, the cocoons will be exposed to heat once again to prepare them for unreeling. The cocoons are put in boiling water to soften them. Cooking them makes it easier to find the end of the single silk fiber that makes up the cocoons. It also makes it simpler to unwind them.

Another benefit of boiling the cocoons is that it softens the silk. The process of cooking the cocoons sets into motion a degumming process. Degumming is the removal of sericin proteins from the silk fiber. Sericin is a gummy-like protein that coats the other protein in silk, which is called fibroin. Sericin enables two filk filaments to stick together. However, the sericin makes silk feel a little rough, which consequently makes it harder to dye. Cooking the cocoons softens the hard sericin protein and makes the cocoons smoother in texture and feel.

4. Deflossing

After cooking, the surface of the cocoons may still be covered in some loose fiber, making the cocoons look fuzzy. This fuzzy layer consists of broken and uneven silk filaments. The loose fiber is removed from the cocoons in a process called deflossing. Deflossing gives the cocoons a clean look, makes it easier to process the cocoons further, and increases its market value.

5. Reeling

Reeling is the step in the silk production process where silk cocoons are turned into threads of silk yarn. Reeling is the unrolling of the cocoon and the combining of multiple silk filaments into one single strand of silk. Reeling used to be done by hand, but is now mostly automated using machines. During reeling, the revolving brushes of the machine grab the end of a cocoon’s silk filament. The fast-moving reel then unravels the cocoon and dries the silk simultaneously.

A single strand of silk is too thin to use on its own. This is why the filaments of multiple cocoons are reeled together at the same time to create one strand of silk yarn. The number of cocoons reeled together can be anywhere from 2 to 20, depending on the desired thickness of the silk yarn. As silk fibers are so fine and light, you need about 2500 cocoons to produce 1 pound of silk.

After unraveling the cocoons, the remaining silkworm pupae are sometimes saved and sent to countries like China, South Korea, and Thailand, where the pupae are cooked up in meals or eaten as snacks.

6. Twisting & Dying

Now that reeling has been completed, the threads of silk yarn are removed from the reels. The silk is then twisted into spiral circles to form bundles. These bundles of yarn are also called skeins. The twist in a silk thread can be increased further, or more silk threads can be added and twisted together. The amount of twisting needed depends on what kind of fabric the silk will be woven in.

After twisting, the silk yarn is ready to be dyed. You can choose to dye silk before or after weaving the silk thread into fabrics. Silk is easy to dye thanks to the structure of the fibroin proteins that make up most of the silk. The dye is easily absorbed by silk, and the colors will look vibrant. Silk also contains both positive and negative ions, which means that most commercial dyes are effective on silk.

Following twisting and dying, the silk threads are wound onto spools or tubes. The silk yarn is now ready to be sold or to be woven into fabrics.

7. Weaving

Silk yarn is transformed into a silk fabric by weaving the threads. There are many ways to weave silk. One of the most popular methods for weaving silk is called charmeuse, also known as satin. The charmeuse weave is a tight weave that results in a smooth and shiny silk fabric. Silk charmeuse fabrics have a glossy surface and a dull back. This look is achieved by floating the lengthwise thread over three or more transverse threads.

Other popular types of silk weaves include silk chiffon, silk twill, silk crepe, and silk habotai. Each of these weaves has a unique way of layering and weaving the silk yarn, resulting in fabrics with different textures and looks.

The finished silk fabrics can be made into silk scarves, shirts, ties, pocket squares, and more. This is the end of the silk production process.